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-libpng-manual.txt - A description on how to use and modify libpng
-
- libpng version 1.6.12 - June 12, 2014
- Updated and distributed by Glenn Randers-Pehrson
- <glennrp at users.sourceforge.net>
- Copyright (c) 1998-2014 Glenn Randers-Pehrson
-
- This document is released under the libpng license.
- For conditions of distribution and use, see the disclaimer
- and license in png.h
-
- Based on:
-
- libpng versions 0.97, January 1998, through 1.6.12 - June 12, 2014
- Updated and distributed by Glenn Randers-Pehrson
- Copyright (c) 1998-2014 Glenn Randers-Pehrson
-
- libpng 1.0 beta 6 - version 0.96 - May 28, 1997
- Updated and distributed by Andreas Dilger
- Copyright (c) 1996, 1997 Andreas Dilger
-
- libpng 1.0 beta 2 - version 0.88 - January 26, 1996
- For conditions of distribution and use, see copyright
- notice in png.h. Copyright (c) 1995, 1996 Guy Eric
- Schalnat, Group 42, Inc.
-
- Updated/rewritten per request in the libpng FAQ
- Copyright (c) 1995, 1996 Frank J. T. Wojcik
- December 18, 1995 & January 20, 1996
-
- TABLE OF CONTENTS
-
-    I. Introduction
-   II. Structures
-  III. Reading
-   IV. Writing
-    V. Simplified API
-   VI. Modifying/Customizing libpng
-  VII. MNG support
- VIII. Changes to Libpng from version 0.88
-   IX. Changes to Libpng from version 1.0.x to 1.2.x
-    X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
-   XI. Changes to Libpng from version 1.4.x to 1.5.x
-  XII. Changes to Libpng from version 1.5.x to 1.6.x
- XIII. Detecting libpng
-  XIV. Source code repository
-   XV. Coding style
-  XVI. Y2K Compliance in libpng
-
-I. Introduction
-
-This file describes how to use and modify the PNG reference library
-(known as libpng) for your own use.  In addition to this
-file, example.c is a good starting point for using the library, as
-it is heavily commented and should include everything most people
-will need.  We assume that libpng is already installed; see the
-INSTALL file for instructions on how to configure and install libpng.
-
-For examples of libpng usage, see the files "example.c", "pngtest.c",
-and the files in the "contrib" directory, all of which are included in
-the libpng distribution.
-
-Libpng was written as a companion to the PNG specification, as a way
-of reducing the amount of time and effort it takes to support the PNG
-file format in application programs.
-
-The PNG specification (second edition), November 2003, is available as
-a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
-<http://www.w3.org/TR/2003/REC-PNG-20031110/
-The W3C and ISO documents have identical technical content.
-
-The PNG-1.2 specification is available at
-<http://www.libpng.org/pub/png/documents/>.  It is technically equivalent
-to the PNG specification (second edition) but has some additional material.
-
-The PNG-1.0 specification is available
-as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
-W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
-
-Some additional chunks are described in the special-purpose public chunks
-documents at <http://www.libpng.org/pub/png/documents/>.
-
-Other information
-about PNG, and the latest version of libpng, can be found at the PNG home
-page, <http://www.libpng.org/pub/png/>.
-
-Most users will not have to modify the library significantly; advanced
-users may want to modify it more.  All attempts were made to make it as
-complete as possible, while keeping the code easy to understand.
-Currently, this library only supports C.  Support for other languages
-is being considered.
-
-Libpng has been designed to handle multiple sessions at one time,
-to be easily modifiable, to be portable to the vast majority of
-machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
-to use.  The ultimate goal of libpng is to promote the acceptance of
-the PNG file format in whatever way possible.  While there is still
-work to be done (see the TODO file), libpng should cover the
-majority of the needs of its users.
-
-Libpng uses zlib for its compression and decompression of PNG files.
-Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
-The zlib compression utility is a general purpose utility that is
-useful for more than PNG files, and can be used without libpng.
-See the documentation delivered with zlib for more details.
-You can usually find the source files for the zlib utility wherever you
-find the libpng source files.
-
-Libpng is thread safe, provided the threads are using different
-instances of the structures.  Each thread should have its own
-png_struct and png_info instances, and thus its own image.
-Libpng does not protect itself against two threads using the
-same instance of a structure.
-
-II. Structures
-
-There are two main structures that are important to libpng, png_struct
-and png_info.  Both are internal structures that are no longer exposed
-in the libpng interface (as of libpng 1.5.0).
-
-The png_info structure is designed to provide information about the
-PNG file.  At one time, the fields of png_info were intended to be
-directly accessible to the user.  However, this tended to cause problems
-with applications using dynamically loaded libraries, and as a result
-a set of interface functions for png_info (the png_get_*() and png_set_*()
-functions) was developed, and direct access to the png_info fields was
-deprecated..
-
-The png_struct structure is the object used by the library to decode a
-single image.  As of 1.5.0 this structure is also not exposed.
-
-Almost all libpng APIs require a pointer to a png_struct as the first argument.
-Many (in particular the png_set and png_get APIs) also require a pointer
-to png_info as the second argument.  Some application visible macros
-defined in png.h designed for basic data access (reading and writing
-integers in the PNG format) don't take a png_info pointer, but it's almost
-always safe to assume that a (png_struct*) has to be passed to call an API
-function.
-
-You can have more than one png_info structure associated with an image,
-as illustrated in pngtest.c, one for information valid prior to the
-IDAT chunks and another (called "end_info" below) for things after them.
-
-The png.h header file is an invaluable reference for programming with libpng.
-And while I'm on the topic, make sure you include the libpng header file:
-
-#include <png.h>
-
-and also (as of libpng-1.5.0) the zlib header file, if you need it:
-
-#include <zlib.h>
-
-Types
-
-The png.h header file defines a number of integral types used by the
-APIs.  Most of these are fairly obvious; for example types corresponding
-to integers of particular sizes and types for passing color values.
-
-One exception is how non-integral numbers are handled.  For application
-convenience most APIs that take such numbers have C (double) arguments;
-however, internally PNG, and libpng, use 32 bit signed integers and encode
-the value by multiplying by 100,000.  As of libpng 1.5.0 a convenience
-macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
-which is simply (png_int_32).
-
-All APIs that take (double) arguments also have a matching API that
-takes the corresponding fixed point integer arguments.  The fixed point
-API has the same name as the floating point one with "_fixed" appended.
-The actual range of values permitted in the APIs is frequently less than
-the full range of (png_fixed_point) (-21474 to +21474).  When APIs require
-a non-negative argument the type is recorded as png_uint_32 above.  Consult
-the header file and the text below for more information.
-
-Special care must be take with sCAL chunk handling because the chunk itself
-uses non-integral values encoded as strings containing decimal floating point
-numbers.  See the comments in the header file.
-
-Configuration
-
-The main header file function declarations are frequently protected by C
-preprocessing directives of the form:
-
-    #ifdef PNG_feature_SUPPORTED
-    declare-function
-    #endif
-    ...
-    #ifdef PNG_feature_SUPPORTED
-    use-function
-    #endif
-
-The library can be built without support for these APIs, although a
-standard build will have all implemented APIs.  Application programs
-should check the feature macros before using an API for maximum
-portability.  From libpng 1.5.0 the feature macros set during the build
-of libpng are recorded in the header file "pnglibconf.h" and this file
-is always included by png.h.
-
-If you don't need to change the library configuration from the default, skip to
-the next section ("Reading").
-
-Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
-of the build project files in the 'projects' directory simply copy
-scripts/pnglibconf.h.prebuilt to pnglibconf.h.  This means that these build
-systems do not permit easy auto-configuration of the library - they only
-support the default configuration.
-
-The easiest way to make minor changes to the libpng configuration when
-auto-configuration is supported is to add definitions to the command line
-using (typically) CPPFLAGS.  For example:
-
-CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC
-
-will change the internal libpng math implementation for gamma correction and
-other arithmetic calculations to fixed point, avoiding the need for fast
-floating point support.  The result can be seen in the generated pnglibconf.h -
-make sure it contains the changed feature macro setting.
-
-If you need to make more extensive configuration changes - more than one or two
-feature macro settings - you can either add -DPNG_USER_CONFIG to the build
-command line and put a list of feature macro settings in pngusr.h or you can set
-DFA_XTRA (a makefile variable) to a file containing the same information in the
-form of 'option' settings.
-
-A. Changing pnglibconf.h
-
-A variety of methods exist to build libpng.  Not all of these support
-reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h it must either be
-rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
-
-Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
-pnglibconf.h and changing the lines defining the supported features, paying
-very close attention to the 'option' information in scripts/pnglibconf.dfa
-that describes those features and their requirements.  This is easy to get
-wrong.
-
-B. Configuration using DFA_XTRA
-
-Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
-variant such as 'nawk' or 'gawk', is available.  The configure build will
-automatically find an appropriate awk and build pnglibconf.h.
-The scripts/pnglibconf.mak file contains a set of make rules for doing the
-same thing if configure is not used, and many of the makefiles in the scripts
-directory use this approach.
-
-When rebuilding simply write a new file containing changed options and set
-DFA_XTRA to the name of this file.  This causes the build to append the new file
-to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file should contain lines
-of the following forms:
-
-everything = off
-
-This turns all optional features off.  Include it at the start of pngusr.dfa to
-make it easier to build a minimal configuration.  You will need to turn at least
-some features on afterward to enable either reading or writing code, or both.
-
-option feature on
-option feature off
-
-Enable or disable a single feature.  This will automatically enable other
-features required by a feature that is turned on or disable other features that
-require a feature which is turned off.  Conflicting settings will cause an error
-message to be emitted by awk.
-
-setting feature default value
-
-Changes the default value of setting 'feature' to 'value'.  There are a small
-number of settings listed at the top of pnglibconf.h, they are documented in the
-source code.  Most of these values have performance implications for the library
-but most of them have no visible effect on the API.  Some can also be overridden
-from the API.
-
-This method of building a customized pnglibconf.h is illustrated in
-contrib/pngminim/*.  See the "$(PNGCONF):" target in the makefile and
-pngusr.dfa in these directories.
-
-C. Configuration using PNG_USER_CONFIG
-
-If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
-the file pngusr.h will automatically be included before the options in
-scripts/pnglibconf.dfa are processed.  Your pngusr.h file should contain only
-macro definitions turning features on or off or setting settings.
-
-Apart from the global setting "everything = off" all the options listed above
-can be set using macros in pngusr.h:
-
-#define PNG_feature_SUPPORTED
-
-is equivalent to:
-
-option feature on
-
-#define PNG_NO_feature
-
-is equivalent to:
-
-option feature off
-
-#define PNG_feature value
-
-is equivalent to:
-
-setting feature default value
-
-Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
-pngusr file you supply override the contents of scripts/pnglibconf.dfa
-
-If confusing or incomprehensible behavior results it is possible to
-examine the intermediate file pnglibconf.dfn to find the full set of
-dependency information for each setting and option.  Simply locate the
-feature in the file and read the C comments that precede it.
-
-This method is also illustrated in the contrib/pngminim/* makefiles and
-pngusr.h.
-
-III. Reading
-
-We'll now walk you through the possible functions to call when reading
-in a PNG file sequentially, briefly explaining the syntax and purpose
-of each one.  See example.c and png.h for more detail.  While
-progressive reading is covered in the next section, you will still
-need some of the functions discussed in this section to read a PNG
-file.
-
-Setup
-
-You will want to do the I/O initialization(*) before you get into libpng,
-so if it doesn't work, you don't have much to undo.  Of course, you
-will also want to insure that you are, in fact, dealing with a PNG
-file.  Libpng provides a simple check to see if a file is a PNG file.
-To use it, pass in the first 1 to 8 bytes of the file to the function
-png_sig_cmp(), and it will return 0 (false) if the bytes match the
-corresponding bytes of the PNG signature, or nonzero (true) otherwise.
-Of course, the more bytes you pass in, the greater the accuracy of the
-prediction.
-
-If you are intending to keep the file pointer open for use in libpng,
-you must ensure you don't read more than 8 bytes from the beginning
-of the file, and you also have to make a call to png_set_sig_bytes_read()
-with the number of bytes you read from the beginning.  Libpng will
-then only check the bytes (if any) that your program didn't read.
-
-(*): If you are not using the standard I/O functions, you will need
-to replace them with custom functions.  See the discussion under
-Customizing libpng.
-
-
-    FILE *fp = fopen(file_name, "rb");
-    if (!fp)
-    {
-       return (ERROR);
-    }
-
-    fread(header, 1, number, fp);
-    is_png = !png_sig_cmp(header, 0, number);
-
-    if (!is_png)
-    {
-       return (NOT_PNG);
-    }
-
-
-Next, png_struct and png_info need to be allocated and initialized.  In
-order to ensure that the size of these structures is correct even with a
-dynamically linked libpng, there are functions to initialize and
-allocate the structures.  We also pass the library version, optional
-pointers to error handling functions, and a pointer to a data struct for
-use by the error functions, if necessary (the pointer and functions can
-be NULL if the default error handlers are to be used).  See the section
-on Changes to Libpng below regarding the old initialization functions.
-The structure allocation functions quietly return NULL if they fail to
-create the structure, so your application should check for that.
-
-    png_structp png_ptr = png_create_read_struct
-        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn);
-
-    if (!png_ptr)
-       return (ERROR);
-
-    png_infop info_ptr = png_create_info_struct(png_ptr);
-
-    if (!info_ptr)
-    {
-       png_destroy_read_struct(&png_ptr,
-           (png_infopp)NULL, (png_infopp)NULL);
-       return (ERROR);
-    }
-
-If you want to use your own memory allocation routines,
-use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
-png_create_read_struct_2() instead of png_create_read_struct():
-
-    png_structp png_ptr = png_create_read_struct_2
-        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn, (png_voidp)
-        user_mem_ptr, user_malloc_fn, user_free_fn);
-
-The error handling routines passed to png_create_read_struct()
-and the memory alloc/free routines passed to png_create_struct_2()
-are only necessary if you are not using the libpng supplied error
-handling and memory alloc/free functions.
-
-When libpng encounters an error, it expects to longjmp back
-to your routine.  Therefore, you will need to call setjmp and pass
-your png_jmpbuf(png_ptr).  If you read the file from different
-routines, you will need to update the longjmp buffer every time you enter
-a new routine that will call a png_*() function.
-
-See your documentation of setjmp/longjmp for your compiler for more
-information on setjmp/longjmp.  See the discussion on libpng error
-handling in the Customizing Libpng section below for more information
-on the libpng error handling.  If an error occurs, and libpng longjmp's
-back to your setjmp, you will want to call png_destroy_read_struct() to
-free any memory.
-
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-       png_destroy_read_struct(&png_ptr, &info_ptr,
-           &end_info);
-       fclose(fp);
-       return (ERROR);
-    }
-
-Pass (png_infopp)NULL instead of &end_info if you didn't create
-an end_info structure.
-
-If you would rather avoid the complexity of setjmp/longjmp issues,
-you can compile libpng with PNG_NO_SETJMP, in which case
-errors will result in a call to PNG_ABORT() which defaults to abort().
-
-You can #define PNG_ABORT() to a function that does something
-more useful than abort(), as long as your function does not
-return.
-
-Now you need to set up the input code.  The default for libpng is to
-use the C function fread().  If you use this, you will need to pass a
-valid FILE * in the function png_init_io().  Be sure that the file is
-opened in binary mode.  If you wish to handle reading data in another
-way, you need not call the png_init_io() function, but you must then
-implement the libpng I/O methods discussed in the Customizing Libpng
-section below.
-
-    png_init_io(png_ptr, fp);
-
-If you had previously opened the file and read any of the signature from
-the beginning in order to see if this was a PNG file, you need to let
-libpng know that there are some bytes missing from the start of the file.
-
-    png_set_sig_bytes(png_ptr, number);
-
-You can change the zlib compression buffer size to be used while
-reading compressed data with
-
-    png_set_compression_buffer_size(png_ptr, buffer_size);
-
-where the default size is 8192 bytes.  Note that the buffer size
-is changed immediately and the buffer is reallocated immediately,
-instead of setting a flag to be acted upon later.
-
-If you want CRC errors to be handled in a different manner than
-the default, use
-
-    png_set_crc_action(png_ptr, crit_action, ancil_action);
-
-The values for png_set_crc_action() say how libpng is to handle CRC errors in
-ancillary and critical chunks, and whether to use the data contained
-therein.  Note that it is impossible to "discard" data in a critical
-chunk.
-
-Choices for (int) crit_action are
-   PNG_CRC_DEFAULT      0  error/quit
-   PNG_CRC_ERROR_QUIT   1  error/quit
-   PNG_CRC_WARN_USE     3  warn/use data
-   PNG_CRC_QUIET_USE    4  quiet/use data
-   PNG_CRC_NO_CHANGE    5  use the current value
-
-Choices for (int) ancil_action are
-   PNG_CRC_DEFAULT      0  error/quit
-   PNG_CRC_ERROR_QUIT   1  error/quit
-   PNG_CRC_WARN_DISCARD 2  warn/discard data
-   PNG_CRC_WARN_USE     3  warn/use data
-   PNG_CRC_QUIET_USE    4  quiet/use data
-   PNG_CRC_NO_CHANGE    5  use the current value
-
-Setting up callback code
-
-You can set up a callback function to handle any unknown chunks in the
-input stream. You must supply the function
-
-    read_chunk_callback(png_structp png_ptr,
-         png_unknown_chunkp chunk);
-    {
-       /* The unknown chunk structure contains your
-          chunk data, along with similar data for any other
-          unknown chunks: */
-
-           png_byte name[5];
-           png_byte *data;
-           png_size_t size;
-
-       /* Note that libpng has already taken care of
-          the CRC handling */
-
-       /* put your code here.  Search for your chunk in the
-          unknown chunk structure, process it, and return one
-          of the following: */
-
-       return (-n); /* chunk had an error */
-       return (0); /* did not recognize */
-       return (n); /* success */
-    }
-
-(You can give your function another name that you like instead of
-"read_chunk_callback")
-
-To inform libpng about your function, use
-
-    png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
-        read_chunk_callback);
-
-This names not only the callback function, but also a user pointer that
-you can retrieve with
-
-    png_get_user_chunk_ptr(png_ptr);
-
-If you call the png_set_read_user_chunk_fn() function, then all unknown
-chunks which the callback does not handle will be saved when read.  You can
-cause them to be discarded by returning '1' ("handled") instead of '0'.  This
-behavior will change in libpng 1.7 and the default handling set by the
-png_set_keep_unknown_chunks() function, described below, will be used when the
-callback returns 0.  If you want the existing behavior you should set the global
-default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
-versions of libpng and with 1.7.  Libpng 1.6 issues a warning if you keep the
-default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
-
-At this point, you can set up a callback function that will be
-called after each row has been read, which you can use to control
-a progress meter or the like.  It's demonstrated in pngtest.c.
-You must supply a function
-
-    void read_row_callback(png_structp png_ptr,
-       png_uint_32 row, int pass);
-    {
-      /* put your code here */
-    }
-
-(You can give it another name that you like instead of "read_row_callback")
-
-To inform libpng about your function, use
-
-    png_set_read_status_fn(png_ptr, read_row_callback);
-
-When this function is called the row has already been completely processed and
-the 'row' and 'pass' refer to the next row to be handled.  For the
-non-interlaced case the row that was just handled is simply one less than the
-passed in row number, and pass will always be 0.  For the interlaced case the
-same applies unless the row value is 0, in which case the row just handled was
-the last one from one of the preceding passes.  Because interlacing may skip a
-pass you cannot be sure that the preceding pass is just 'pass-1', if you really
-need to know what the last pass is record (row,pass) from the callback and use
-the last recorded value each time.
-
-As with the user transform you can find the output row using the
-PNG_ROW_FROM_PASS_ROW macro.
-
-Unknown-chunk handling
-
-Now you get to set the way the library processes unknown chunks in the
-input PNG stream. Both known and unknown chunks will be read.  Normal
-behavior is that known chunks will be parsed into information in
-various info_ptr members while unknown chunks will be discarded. This
-behavior can be wasteful if your application will never use some known
-chunk types. To change this, you can call:
-
-    png_set_keep_unknown_chunks(png_ptr, keep,
-        chunk_list, num_chunks);
-
-    keep       - 0: default unknown chunk handling
-                 1: ignore; do not keep
-                 2: keep only if safe-to-copy
-                 3: keep even if unsafe-to-copy
-
-               You can use these definitions:
-                 PNG_HANDLE_CHUNK_AS_DEFAULT   0
-                 PNG_HANDLE_CHUNK_NEVER        1
-                 PNG_HANDLE_CHUNK_IF_SAFE      2
-                 PNG_HANDLE_CHUNK_ALWAYS       3
-
-    chunk_list - list of chunks affected (a byte string,
-                 five bytes per chunk, NULL or '\0' if
-                 num_chunks is positive; ignored if
-                 numchunks <= 0).
-
-    num_chunks - number of chunks affected; if 0, all
-                 unknown chunks are affected.  If positive,
-                 only the chunks in the list are affected,
-                 and if negative all unknown chunks and
-                 all known chunks except for the IHDR,
-                 PLTE, tRNS, IDAT, and IEND chunks are
-                 affected.
-
-Unknown chunks declared in this way will be saved as raw data onto a
-list of png_unknown_chunk structures.  If a chunk that is normally
-known to libpng is named in the list, it will be handled as unknown,
-according to the "keep" directive.  If a chunk is named in successive
-instances of png_set_keep_unknown_chunks(), the final instance will
-take precedence.  The IHDR and IEND chunks should not be named in
-chunk_list; if they are, libpng will process them normally anyway.
-If you know that your application will never make use of some particular
-chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
-
-Here is an example of the usage of png_set_keep_unknown_chunks(),
-where the private "vpAg" chunk will later be processed by a user chunk
-callback function:
-
-    png_byte vpAg[5]={118, 112,  65, 103, (png_byte) '\0'};
-
-    #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
-      png_byte unused_chunks[]=
-      {
-        104,  73,  83,  84, (png_byte) '\0',   /* hIST */
-        105,  84,  88, 116, (png_byte) '\0',   /* iTXt */
-        112,  67,  65,  76, (png_byte) '\0',   /* pCAL */
-        115,  67,  65,  76, (png_byte) '\0',   /* sCAL */
-        115,  80,  76,  84, (png_byte) '\0',   /* sPLT */
-        116,  73,  77,  69, (png_byte) '\0',   /* tIME */
-      };
-    #endif
-
-    ...
-
-    #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
-      /* ignore all unknown chunks
-       * (use global setting "2" for libpng16 and earlier):
-       */
-      png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
-
-      /* except for vpAg: */
-      png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
-
-      /* also ignore unused known chunks: */
-      png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
-         (int)(sizeof unused_chunks)/5);
-    #endif
-
-User limits
-
-The PNG specification allows the width and height of an image to be as
-large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
-Since very few applications really need to process such large images,
-we have imposed an arbitrary 1-million limit on rows and columns.
-Larger images will be rejected immediately with a png_error() call. If
-you wish to change this limit, you can use
-
-   png_set_user_limits(png_ptr, width_max, height_max);
-
-to set your own limits, or use width_max = height_max = 0x7fffffffL
-to allow all valid dimensions (libpng may reject some very large images
-anyway because of potential buffer overflow conditions).
-
-You should put this statement after you create the PNG structure and
-before calling png_read_info(), png_read_png(), or png_process_data().
-
-When writing a PNG datastream, put this statement before calling
-png_write_info() or png_write_png().
-
-If you need to retrieve the limits that are being applied, use
-
-   width_max = png_get_user_width_max(png_ptr);
-   height_max = png_get_user_height_max(png_ptr);
-
-The PNG specification sets no limit on the number of ancillary chunks
-allowed in a PNG datastream.  You can impose a limit on the total number
-of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
-
-   png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
-
-where 0x7fffffffL means unlimited.  You can retrieve this limit with
-
-   chunk_cache_max = png_get_chunk_cache_max(png_ptr);
-
-You can also set a limit on the amount of memory that a compressed chunk
-other than IDAT can occupy, with
-
-   png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
-
-and you can retrieve the limit with
-
-   chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
-
-Any chunks that would cause either of these limits to be exceeded will
-be ignored.
-
-Information about your system
-
-If you intend to display the PNG or to incorporate it in other image data you
-need to tell libpng information about your display or drawing surface so that
-libpng can convert the values in the image to match the display.
-
-From libpng-1.5.4 this information can be set before reading the PNG file
-header.  In earlier versions png_set_gamma() existed but behaved incorrectly if
-called before the PNG file header had been read and png_set_alpha_mode() did not
-exist.
-
-If you need to support versions prior to libpng-1.5.4 test the version number
-as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
-described in the appropriate manual page.
-
-You give libpng the encoding expected by your system expressed as a 'gamma'
-value.  You can also specify a default encoding for the PNG file in
-case the required information is missing from the file.  By default libpng
-assumes that the PNG data matches your system, to keep this default call:
-
-   png_set_gamma(png_ptr, screen_gamma, output_gamma);
-
-or you can use the fixed point equivalent:
-
-   png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
-      PNG_FP_1*output_gamma);
-
-If you don't know the gamma for your system it is probably 2.2 - a good
-approximation to the IEC standard for display systems (sRGB).  If images are
-too contrasty or washed out you got the value wrong - check your system
-documentation!
-
-Many systems permit the system gamma to be changed via a lookup table in the
-display driver, a few systems, including older Macs, change the response by
-default.  As of 1.5.4 three special values are available to handle common
-situations:
-
-   PNG_DEFAULT_sRGB: Indicates that the system conforms to the
-                     IEC 61966-2-1 standard.  This matches almost
-                     all systems.
-   PNG_GAMMA_MAC_18: Indicates that the system is an older
-                     (pre Mac OS 10.6) Apple Macintosh system with
-                     the default settings.
-   PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
-                     that the system expects data with no gamma
-                     encoding.
-
-You would use the linear (unencoded) value if you need to process the pixel
-values further because this avoids the need to decode and re-encode each
-component value whenever arithmetic is performed.  A lot of graphics software
-uses linear values for this reason, often with higher precision component values
-to preserve overall accuracy.
-
-
-The output_gamma value expresses how to decode the output values, not how
-they are encoded.  The values used correspond to the normal numbers used to
-describe the overall gamma of a computer display system; for example 2.2 for
-an sRGB conformant system.  The values are scaled by 100000 in the _fixed
-version of the API (so 220000 for sRGB.)
-
-The inverse of the value is always used to provide a default for the PNG file
-encoding if it has no gAMA chunk and if png_set_gamma() has not been called
-to override the PNG gamma information.
-
-When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
-opaque pixels however pixels with lower alpha values are not encoded,
-regardless of the output gamma setting.
-
-When the standard Porter Duff handling is requested with mode 1 the output
-encoding is set to be linear and the output_gamma value is only relevant
-as a default for input data that has no gamma information.  The linear output
-encoding will be overridden if png_set_gamma() is called - the results may be
-highly unexpected!
-
-The following numbers are derived from the sRGB standard and the research
-behind it.  sRGB is defined to be approximated by a PNG gAMA chunk value of
-0.45455 (1/2.2) for PNG.  The value implicitly includes any viewing
-correction required to take account of any differences in the color
-environment of the original scene and the intended display environment; the
-value expresses how to *decode* the image for display, not how the original
-data was *encoded*.
-
-sRGB provides a peg for the PNG standard by defining a viewing environment.
-sRGB itself, and earlier TV standards, actually use a more complex transform
-(a linear portion then a gamma 2.4 power law) than PNG can express.  (PNG is
-limited to simple power laws.)  By saying that an image for direct display on
-an sRGB conformant system should be stored with a gAMA chunk value of 45455
-(11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification
-makes it possible to derive values for other display systems and
-environments.
-
-The Mac value is deduced from the sRGB based on an assumption that the actual
-extra viewing correction used in early Mac display systems was implemented as
-a power 1.45 lookup table.
-
-Any system where a programmable lookup table is used or where the behavior of
-the final display device characteristics can be changed requires system
-specific code to obtain the current characteristic.  However this can be
-difficult and most PNG gamma correction only requires an approximate value.
-
-By default, if png_set_alpha_mode() is not called, libpng assumes that all
-values are unencoded, linear, values and that the output device also has a
-linear characteristic.  This is only very rarely correct - it is invariably
-better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
-default if you don't know what the right answer is!
-
-The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
-10.6) which used a correction table to implement a somewhat lower gamma on an
-otherwise sRGB system.
-
-Both these values are reserved (not simple gamma values) in order to allow
-more precise correction internally in the future.
-
-NOTE: the values can be passed to either the fixed or floating
-point APIs, but the floating point API will also accept floating point
-values.
-
-The second thing you may need to tell libpng about is how your system handles
-alpha channel information.  Some, but not all, PNG files contain an alpha
-channel.  To display these files correctly you need to compose the data onto a
-suitable background, as described in the PNG specification.
-
-Libpng only supports composing onto a single color (using png_set_background;
-see below).  Otherwise you must do the composition yourself and, in this case,
-you may need to call png_set_alpha_mode:
-
-   #if PNG_LIBPNG_VER >= 10504
-      png_set_alpha_mode(png_ptr, mode, screen_gamma);
-   #else
-      png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
-   #endif
-
-The screen_gamma value is the same as the argument to png_set_gamma; however,
-how it affects the output depends on the mode.  png_set_alpha_mode() sets the
-file gamma default to 1/screen_gamma, so normally you don't need to call
-png_set_gamma.  If you need different defaults call png_set_gamma() before
-png_set_alpha_mode() - if you call it after it will override the settings made
-by png_set_alpha_mode().
-
-The mode is as follows:
-
-    PNG_ALPHA_PNG: The data is encoded according to the PNG
-specification.  Red, green and blue, or gray, components are
-gamma encoded color values and are not premultiplied by the
-alpha value.  The alpha value is a linear measure of the
-contribution of the pixel to the corresponding final output pixel.
-
-You should normally use this format if you intend to perform
-color correction on the color values; most, maybe all, color
-correction software has no handling for the alpha channel and,
-anyway, the math to handle pre-multiplied component values is
-unnecessarily complex.
-
-Before you do any arithmetic on the component values you need
-to remove the gamma encoding and multiply out the alpha
-channel.  See the PNG specification for more detail.  It is
-important to note that when an image with an alpha channel is
-scaled, linear encoded, pre-multiplied component values must
-be used!
-
-The remaining modes assume you don't need to do any further color correction or
-that if you do, your color correction software knows all about alpha (it
-probably doesn't!).  They 'associate' the alpha with the color information by
-storing color channel values that have been scaled by the alpha.  The
-advantage is that the color channels can be resampled (the image can be
-scaled) in this form.  The disadvantage is that normal practice is to store
-linear, not (gamma) encoded, values and this requires 16-bit channels for
-still images rather than the 8-bit channels that are just about sufficient if
-gamma encoding is used.  In addition all non-transparent pixel values,
-including completely opaque ones, must be gamma encoded to produce the final
-image.  These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
-described below (the latter being the two common names for associated alpha
-color channels). Note that PNG files always contain non-associated color
-channels; png_set_alpha_mode() with one of the modes causes the decoder to
-convert the pixels to an associated form before returning them to your
-application. 
-
-Since it is not necessary to perform arithmetic on opaque color values so
-long as they are not to be resampled and are in the final color space it is
-possible to optimize the handling of alpha by storing the opaque pixels in
-the PNG format (adjusted for the output color space) while storing partially
-opaque pixels in the standard, linear, format.  The accuracy required for
-standard alpha composition is relatively low, because the pixels are
-isolated, therefore typically the accuracy loss in storing 8-bit linear
-values is acceptable.  (This is not true if the alpha channel is used to
-simulate transparency over large areas - use 16 bits or the PNG mode in
-this case!)  This is the 'OPTIMIZED' mode.  For this mode a pixel is
-treated as opaque only if the alpha value is equal to the maximum value.
-
-    PNG_ALPHA_STANDARD:  The data libpng produces is encoded in the
-standard way assumed by most correctly written graphics software.
-The gamma encoding will be removed by libpng and the
-linear component values will be pre-multiplied by the
-alpha channel.
-
-With this format the final image must be re-encoded to
-match the display gamma before the image is displayed.
-If your system doesn't do that, yet still seems to
-perform arithmetic on the pixels without decoding them,
-it is broken - check out the modes below.
-
-With PNG_ALPHA_STANDARD libpng always produces linear
-component values, whatever screen_gamma you supply.  The
-screen_gamma value is, however, used as a default for
-the file gamma if the PNG file has no gamma information.
-
-If you call png_set_gamma() after png_set_alpha_mode() you
-will override the linear encoding.  Instead the
-pre-multiplied pixel values will be gamma encoded but
-the alpha channel will still be linear.  This may
-actually match the requirements of some broken software,
-but it is unlikely.
-
-While linear 8-bit data is often used it has
-insufficient precision for any image with a reasonable
-dynamic range.  To avoid problems, and if your software
-supports it, use png_set_expand_16() to force all
-components to 16 bits.
-
-    PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
-except that completely opaque pixels are gamma encoded according to
-the screen_gamma value.  Pixels with alpha less than 1.0
-will still have linear components.
-
-Use this format if you have control over your
-compositing software and so don't do other arithmetic
-(such as scaling) on the data you get from libpng.  Your
-compositing software can simply copy opaque pixels to
-the output but still has linear values for the
-non-opaque pixels.
-
-In normal compositing, where the alpha channel encodes
-partial pixel coverage (as opposed to broad area
-translucency), the inaccuracies of the 8-bit
-representation of non-opaque pixels are irrelevant.
-
-You can also try this format if your software is broken;
-it might look better.
-
-    PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
-values, including the alpha channel are gamma encoded.  This is
-broken because, in practice, no implementation that uses this choice
-correctly undoes the encoding before handling alpha composition.  Use this
-choice only if other serious errors in the software or hardware you use
-mandate it.  In most cases of broken software or hardware the bug in the
-final display manifests as a subtle halo around composited parts of the
-image.  You may not even perceive this as a halo; the composited part of
-the image may simply appear separate from the background, as though it had
-been cut out of paper and pasted on afterward.
-
-If you don't have to deal with bugs in software or hardware, or if you can fix
-them, there are three recommended ways of using png_set_alpha_mode():
-
-   png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
-       screen_gamma);
-
-You can do color correction on the result (libpng does not currently
-support color correction internally).  When you handle the alpha channel
-you need to undo the gamma encoding and multiply out the alpha.
-
-   png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
-       screen_gamma);
-   png_set_expand_16(png_ptr);
-
-If you are using the high level interface, don't call png_set_expand_16();
-instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
-
-With this mode you can't do color correction, but you can do arithmetic,
-including composition and scaling, on the data without further processing.
-
-   png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
-       screen_gamma);
-
-You can avoid the expansion to 16-bit components with this mode, but you
-lose the ability to scale the image or perform other linear arithmetic.
-All you can do is compose the result onto a matching output.  Since this
-mode is libpng-specific you also need to write your own composition
-software.
-
-The following are examples of calls to png_set_alpha_mode to achieve the
-required overall gamma correction and, where necessary, alpha
-premultiplication.
-
-    png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
-
-This is the default libpng handling of the alpha channel - it is not
-pre-multiplied into the color components.  In addition the call states
-that the output is for a sRGB system and causes all PNG files without gAMA
-chunks to be assumed to be encoded using sRGB.
-
-    png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
-
-In this case the output is assumed to be something like an sRGB conformant
-display preceeded by a power-law lookup table of power 1.45.  This is how
-early Mac systems behaved.
-
-    png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
-
-This is the classic Jim Blinn approach and will work in academic
-environments where everything is done by the book.  It has the shortcoming
-of assuming that input PNG data with no gamma information is linear - this
-is unlikely to be correct unless the PNG files where generated locally.
-Most of the time the output precision will be so low as to show
-significant banding in dark areas of the image.
-
-    png_set_expand_16(pp);
-    png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
-
-This is a somewhat more realistic Jim Blinn inspired approach.  PNG files
-are assumed to have the sRGB encoding if not marked with a gamma value and
-the output is always 16 bits per component.  This permits accurate scaling
-and processing of the data.  If you know that your input PNG files were
-generated locally you might need to replace PNG_DEFAULT_sRGB with the
-correct value for your system.
-
-    png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
-
-If you just need to composite the PNG image onto an existing background
-and if you control the code that does this you can use the optimization
-setting.  In this case you just copy completely opaque pixels to the
-output.  For pixels that are not completely transparent (you just skip
-those) you do the composition math using png_composite or png_composite_16
-below then encode the resultant 8-bit or 16-bit values to match the output
-encoding.
-
-    Other cases
-
-If neither the PNG nor the standard linear encoding work for you because
-of the software or hardware you use then you have a big problem.  The PNG
-case will probably result in halos around the image.  The linear encoding
-will probably result in a washed out, too bright, image (it's actually too
-contrasty.)  Try the ALPHA_OPTIMIZED mode above - this will probably
-substantially reduce the halos.  Alternatively try:
-
-    png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
-
-This option will also reduce the halos, but there will be slight dark
-halos round the opaque parts of the image where the background is light.
-In the OPTIMIZED mode the halos will be light halos where the background
-is dark.  Take your pick - the halos are unavoidable unless you can get
-your hardware/software fixed!  (The OPTIMIZED approach is slightly
-faster.)
-
-When the default gamma of PNG files doesn't match the output gamma.
-If you have PNG files with no gamma information png_set_alpha_mode allows
-you to provide a default gamma, but it also sets the ouput gamma to the
-matching value.  If you know your PNG files have a gamma that doesn't
-match the output you can take advantage of the fact that
-png_set_alpha_mode always sets the output gamma but only sets the PNG
-default if it is not already set:
-
-    png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
-    png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
-
-The first call sets both the default and the output gamma values, the
-second call overrides the output gamma without changing the default.  This
-is easier than achieving the same effect with png_set_gamma.  You must use
-PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
-fire if more than one call to png_set_alpha_mode and png_set_background is
-made in the same read operation, however multiple calls with PNG_ALPHA_PNG
-are ignored.
-
-If you don't need, or can't handle, the alpha channel you can call
-png_set_background() to remove it by compositing against a fixed color.  Don't
-call png_set_strip_alpha() to do this - it will leave spurious pixel values in
-transparent parts of this image.
-
-   png_set_background(png_ptr, &background_color,
-       PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);
-
-The background_color is an RGB or grayscale value according to the data format
-libpng will produce for you.  Because you don't yet know the format of the PNG
-file, if you call png_set_background at this point you must arrange for the
-format produced by libpng to always have 8-bit or 16-bit components and then
-store the color as an 8-bit or 16-bit color as appropriate.  The color contains
-separate gray and RGB component values, so you can let libpng produce gray or
-RGB output according to the input format, but low bit depth grayscale images
-must always be converted to at least 8-bit format.  (Even though low bit depth
-grayscale images can't have an alpha channel they can have a transparent
-color!)
-
-You set the transforms you need later, either as flags to the high level
-interface or libpng API calls for the low level interface.  For reference the
-settings and API calls required are:
-
-8-bit values:
-   PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
-   png_set_expand(png_ptr); png_set_scale_16(png_ptr);
-
-   If you must get exactly the same inaccurate results
-   produced by default in versions prior to libpng-1.5.4,
-   use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
-   instead.
-
-16-bit values:
-   PNG_TRANSFORM_EXPAND_16
-   png_set_expand_16(png_ptr);
-
-In either case palette image data will be expanded to RGB.  If you just want
-color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
-to the list.
-
-Calling png_set_background before the PNG file header is read will not work
-prior to libpng-1.5.4.  Because the failure may result in unexpected warnings or
-errors it is therefore much safer to call png_set_background after the head has
-been read.  Unfortunately this means that prior to libpng-1.5.4 it cannot be
-used with the high level interface.
-
-The high-level read interface
-
-At this point there are two ways to proceed; through the high-level
-read interface, or through a sequence of low-level read operations.
-You can use the high-level interface if (a) you are willing to read
-the entire image into memory, and (b) the input transformations
-you want to do are limited to the following set:
-
-    PNG_TRANSFORM_IDENTITY      No transformation
-    PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
-                                8-bit accurately
-    PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
-                                8-bit less accurately
-    PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
-    PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
-                                samples to bytes
-    PNG_TRANSFORM_PACKSWAP      Change order of packed
-                                pixels to LSB first
-    PNG_TRANSFORM_EXPAND        Perform set_expand()
-    PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
-    PNG_TRANSFORM_SHIFT         Normalize pixels to the
-                                sBIT depth
-    PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
-                                to BGRA
-    PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
-                                to AG
-    PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
-                                to transparency
-    PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
-    PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
-                                to RGB (or GA to RGBA)
-    PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits
-
-(This excludes setting a background color, doing gamma transformation,
-quantizing, and setting filler.)  If this is the case, simply do this:
-
-    png_read_png(png_ptr, info_ptr, png_transforms, NULL)
-
-where png_transforms is an integer containing the bitwise OR of some
-set of transformation flags.  This call is equivalent to png_read_info(),
-followed the set of transformations indicated by the transform mask,
-then png_read_image(), and finally png_read_end().
-
-(The final parameter of this call is not yet used.  Someday it might point
-to transformation parameters required by some future input transform.)
-
-You must use png_transforms and not call any png_set_transform() functions
-when you use png_read_png().
-
-After you have called png_read_png(), you can retrieve the image data
-with
-
-   row_pointers = png_get_rows(png_ptr, info_ptr);
-
-where row_pointers is an array of pointers to the pixel data for each row:
-
-   png_bytep row_pointers[height];
-
-If you know your image size and pixel size ahead of time, you can allocate
-row_pointers prior to calling png_read_png() with
-
-   if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
-      png_error (png_ptr,
-          "Image is too tall to process in memory");
-
-   if (width > PNG_UINT_32_MAX/pixel_size)
-      png_error (png_ptr,
-          "Image is too wide to process in memory");
-
-   row_pointers = png_malloc(png_ptr,
-       height*(sizeof (png_bytep)));
-
-   for (int i=0; i<height, i++)
-      row_pointers[i]=NULL;  /* security precaution */
-
-   for (int i=0; i<height, i++)
-      row_pointers[i]=png_malloc(png_ptr,
-          width*pixel_size);
-
-   png_set_rows(png_ptr, info_ptr, &row_pointers);
-
-Alternatively you could allocate your image in one big block and define
-row_pointers[i] to point into the proper places in your block.
-
-If you use png_set_rows(), the application is responsible for freeing
-row_pointers (and row_pointers[i], if they were separately allocated).
-
-If you don't allocate row_pointers ahead of time, png_read_png() will
-do it, and it'll be free'ed by libpng when you call png_destroy_*().
-
-The low-level read interface
-
-If you are going the low-level route, you are now ready to read all
-the file information up to the actual image data.  You do this with a
-call to png_read_info().
-
-    png_read_info(png_ptr, info_ptr);
-
-This will process all chunks up to but not including the image data.
-
-This also copies some of the data from the PNG file into the decode structure
-for use in later transformations.  Important information copied in is:
-
-1) The PNG file gamma from the gAMA chunk.  This overwrites the default value
-provided by an earlier call to png_set_gamma or png_set_alpha_mode.
-
-2) Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
-damages the information provided by an earlier call to png_set_background
-resulting in unexpected behavior.  Libpng-1.5.4 no longer does this.
-
-3) The number of significant bits in each component value.  Libpng uses this to
-optimize gamma handling by reducing the internal lookup table sizes.
-
-4) The transparent color information from a tRNS chunk.  This can be modified by
-a later call to png_set_tRNS.
-
-Querying the info structure
-
-Functions are used to get the information from the info_ptr once it
-has been read.  Note that these fields may not be completely filled
-in until png_read_end() has read the chunk data following the image.
-
-    png_get_IHDR(png_ptr, info_ptr, &width, &height,
-       &bit_depth, &color_type, &interlace_type,
-       &compression_type, &filter_method);
-
-    width          - holds the width of the image
-                     in pixels (up to 2^31).
-
-    height         - holds the height of the image
-                     in pixels (up to 2^31).
-
-    bit_depth      - holds the bit depth of one of the
-                     image channels.  (valid values are
-                     1, 2, 4, 8, 16 and depend also on
-                     the color_type.  See also
-                     significant bits (sBIT) below).
-
-    color_type     - describes which color/alpha channels
-                         are present.
-                     PNG_COLOR_TYPE_GRAY
-                        (bit depths 1, 2, 4, 8, 16)
-                     PNG_COLOR_TYPE_GRAY_ALPHA
-                        (bit depths 8, 16)
-                     PNG_COLOR_TYPE_PALETTE
-                        (bit depths 1, 2, 4, 8)
-                     PNG_COLOR_TYPE_RGB
-                        (bit_depths 8, 16)
-                     PNG_COLOR_TYPE_RGB_ALPHA
-                        (bit_depths 8, 16)
-
-                     PNG_COLOR_MASK_PALETTE
-                     PNG_COLOR_MASK_COLOR
-                     PNG_COLOR_MASK_ALPHA
-
-    interlace_type - (PNG_INTERLACE_NONE or
-                     PNG_INTERLACE_ADAM7)
-
-    compression_type - (must be PNG_COMPRESSION_TYPE_BASE
-                     for PNG 1.0)
-
-    filter_method  - (must be PNG_FILTER_TYPE_BASE
-                     for PNG 1.0, and can also be
-                     PNG_INTRAPIXEL_DIFFERENCING if
-                     the PNG datastream is embedded in
-                     a MNG-1.0 datastream)
-
-    Any or all of interlace_type, compression_type, or
-    filter_method can be NULL if you are
-    not interested in their values.
-
-    Note that png_get_IHDR() returns 32-bit data into
-    the application's width and height variables.
-    This is an unsafe situation if these are 16-bit
-    variables.  In such situations, the
-    png_get_image_width() and png_get_image_height()
-    functions described below are safer.
-
-    width            = png_get_image_width(png_ptr,
-                         info_ptr);
-
-    height           = png_get_image_height(png_ptr,
-                         info_ptr);
-
-    bit_depth        = png_get_bit_depth(png_ptr,
-                         info_ptr);
-
-    color_type       = png_get_color_type(png_ptr,
-                         info_ptr);
-
-    interlace_type   = png_get_interlace_type(png_ptr,
-                         info_ptr);
-
-    compression_type = png_get_compression_type(png_ptr,
-                         info_ptr);
-
-    filter_method    = png_get_filter_type(png_ptr,
-                         info_ptr);
-
-    channels = png_get_channels(png_ptr, info_ptr);
-
-    channels       - number of channels of info for the
-                     color type (valid values are 1 (GRAY,
-                     PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
-                     4 (RGB_ALPHA or RGB + filler byte))
-
-    rowbytes = png_get_rowbytes(png_ptr, info_ptr);
-
-    rowbytes       - number of bytes needed to hold a row
-
-    signature = png_get_signature(png_ptr, info_ptr);
-
-    signature      - holds the signature read from the
-                     file (if any).  The data is kept in
-                     the same offset it would be if the
-                     whole signature were read (i.e. if an
-                     application had already read in 4
-                     bytes of signature before starting
-                     libpng, the remaining 4 bytes would
-                     be in signature[4] through signature[7]
-                     (see png_set_sig_bytes())).
-
-These are also important, but their validity depends on whether the chunk
-has been read.  The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
-png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
-data has been read, or zero if it is missing.  The parameters to the
-png_get_<chunk> are set directly if they are simple data types, or a
-pointer into the info_ptr is returned for any complex types.
-
-The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
-is simply returned to give the application information about how the
-image was encoded.  Libpng itself only does transformations using the file
-gamma when combining semitransparent pixels with the background color, and,
-since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels
-within the simplified API.  Libpng also uses the file gamma when converting
-RGB to gray, beginning with libpng-1.0.5, if the application calls
-png_set_rgb_to_gray()).
-
-    png_get_PLTE(png_ptr, info_ptr, &palette,
-                     &num_palette);
-
-    palette        - the palette for the file
-                     (array of png_color)
-
-    num_palette    - number of entries in the palette
-
-    png_get_gAMA(png_ptr, info_ptr, &file_gamma);
-    png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
-
-    file_gamma     - the gamma at which the file is
-                     written (PNG_INFO_gAMA)
-
-    int_file_gamma - 100,000 times the gamma at which the
-                     file is written
-
-    png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
-                     &red_y, &green_x, &green_y, &blue_x, &blue_y)
-    png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
-                     &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
-                     &blue_Z)
-    png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
-                     &int_white_y, &int_red_x, &int_red_y,
-                     &int_green_x, &int_green_y, &int_blue_x,
-                     &int_blue_y)
-    png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
-                     &int_red_Z, &int_green_X, &int_green_Y,
-                     &int_green_Z, &int_blue_X, &int_blue_Y,
-                     &int_blue_Z)
-
-    {white,red,green,blue}_{x,y}
-                     A color space encoding specified using the
-                     chromaticities of the end points and the
-                     white point. (PNG_INFO_cHRM)
-
-    {red,green,blue}_{X,Y,Z}
-                     A color space encoding specified using the
-                     encoding end points - the CIE tristimulus
-                     specification of the intended color of the red,
-                     green and blue channels in the PNG RGB data.
-                     The white point is simply the sum of the three
-                     end points. (PNG_INFO_cHRM)
-
-    png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
-
-    srgb_intent -    the rendering intent (PNG_INFO_sRGB)
-                     The presence of the sRGB chunk
-                     means that the pixel data is in the
-                     sRGB color space.  This chunk also
-                     implies specific values of gAMA and
-                     cHRM.
-
-    png_get_iCCP(png_ptr, info_ptr, &name,
-       &compression_type, &profile, &proflen);
-
-    name             - The profile name.
-
-    compression_type - The compression type; always
-                       PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
-                       You may give NULL to this argument to
-                       ignore it.
-
-    profile          - International Color Consortium color
-                       profile data. May contain NULs.
-
-    proflen          - length of profile data in bytes.
-
-    png_get_sBIT(png_ptr, info_ptr, &sig_bit);
-
-    sig_bit        - the number of significant bits for
-                     (PNG_INFO_sBIT) each of the gray,
-                     red, green, and blue channels,
-                     whichever are appropriate for the
-                     given color type (png_color_16)
-
-    png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
-                     &num_trans, &trans_color);
-
-    trans_alpha    - array of alpha (transparency)
-                     entries for palette (PNG_INFO_tRNS)
-
-    num_trans      - number of transparent entries
-                     (PNG_INFO_tRNS)
-
-    trans_color    - graylevel or color sample values of
-                     the single transparent color for
-                     non-paletted images (PNG_INFO_tRNS)
-
-    png_get_hIST(png_ptr, info_ptr, &hist);
-                     (PNG_INFO_hIST)
-
-    hist           - histogram of palette (array of
-                     png_uint_16)
-
-    png_get_tIME(png_ptr, info_ptr, &mod_time);
-
-    mod_time       - time image was last modified
-                    (PNG_VALID_tIME)
-
-    png_get_bKGD(png_ptr, info_ptr, &background);
-
-    background     - background color (of type
-                     png_color_16p) (PNG_VALID_bKGD)
-                     valid 16-bit red, green and blue
-                     values, regardless of color_type
-
-    num_comments   = png_get_text(png_ptr, info_ptr,
-                     &text_ptr, &num_text);
-
-    num_comments   - number of comments
-
-    text_ptr       - array of png_text holding image
-                     comments
-
-    text_ptr[i].compression - type of compression used
-                 on "text" PNG_TEXT_COMPRESSION_NONE
-                           PNG_TEXT_COMPRESSION_zTXt
-                           PNG_ITXT_COMPRESSION_NONE
-                           PNG_ITXT_COMPRESSION_zTXt
-
-    text_ptr[i].key   - keyword for comment.  Must contain
-                         1-79 characters.
-
-    text_ptr[i].text  - text comments for current
-                         keyword.  Can be empty.
-
-    text_ptr[i].text_length - length of text string,
-                 after decompression, 0 for iTXt
-
-    text_ptr[i].itxt_length - length of itxt string,
-                 after decompression, 0 for tEXt/zTXt
-
-    text_ptr[i].lang  - language of comment (empty
-                         string for unknown).
-
-    text_ptr[i].lang_key  - keyword in UTF-8
-                         (empty string for unknown).
-
-    Note that the itxt_length, lang, and lang_key
-    members of the text_ptr structure only exist when the
-    library is built with iTXt chunk support.  Prior to
-    libpng-1.4.0 the library was built by default without
-    iTXt support. Also note that when iTXt is supported,
-    they contain NULL pointers when the "compression"
-    field contains PNG_TEXT_COMPRESSION_NONE or
-    PNG_TEXT_COMPRESSION_zTXt.
-
-    num_text       - number of comments (same as
-                     num_comments; you can put NULL here
-                     to avoid the duplication)
-
-    Note while png_set_text() will accept text, language,
-    and translated keywords that can be NULL pointers, the
-    structure returned by png_get_text will always contain
-    regular zero-terminated C strings.  They might be
-    empty strings but they will never be NULL pointers.
-
-    num_spalettes = png_get_sPLT(png_ptr, info_ptr,
-       &palette_ptr);
-
-    num_spalettes  - number of sPLT chunks read.
-
-    palette_ptr    - array of palette structures holding
-                     contents of one or more sPLT chunks
-                     read.
-
-    png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
-       &unit_type);
-
-    offset_x       - positive offset from the left edge
-                     of the screen (can be negative)
-
-    offset_y       - positive offset from the top edge
-                     of the screen (can be negative)
-
-    unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
-
-    png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
-       &unit_type);
-
-    res_x          - pixels/unit physical resolution in
-                     x direction
-
-    res_y          - pixels/unit physical resolution in
-                     x direction
-
-    unit_type      - PNG_RESOLUTION_UNKNOWN,
-                     PNG_RESOLUTION_METER
-
-    png_get_sCAL(png_ptr, info_ptr, &unit, &width,
-       &height)
-
-    unit        - physical scale units (an integer)
-
-    width       - width of a pixel in physical scale units
-
-    height      - height of a pixel in physical scale units
-                 (width and height are doubles)
-
-    png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
-       &height)
-
-    unit        - physical scale units (an integer)
-
-    width       - width of a pixel in physical scale units
-                  (expressed as a string)
-
-    height      - height of a pixel in physical scale units
-                 (width and height are strings like "2.54")
-
-    num_unknown_chunks = png_get_unknown_chunks(png_ptr,
-       info_ptr, &unknowns)
-
-    unknowns          - array of png_unknown_chunk
-                        structures holding unknown chunks
-
-    unknowns[i].name  - name of unknown chunk
-
-    unknowns[i].data  - data of unknown chunk
-
-    unknowns[i].size  - size of unknown chunk's data
-
-    unknowns[i].location - position of chunk in file
-
-    The value of "i" corresponds to the order in which the
-    chunks were read from the PNG file or inserted with the
-    png_set_unknown_chunks() function.
-
-    The value of "location" is a bitwise "or" of
-
-         PNG_HAVE_IHDR  (0x01)
-         PNG_HAVE_PLTE  (0x02)
-         PNG_AFTER_IDAT (0x08)
-
-The data from the pHYs chunk can be retrieved in several convenient
-forms:
-
-    res_x = png_get_x_pixels_per_meter(png_ptr,
-       info_ptr)
-
-    res_y = png_get_y_pixels_per_meter(png_ptr,
-       info_ptr)
-
-    res_x_and_y = png_get_pixels_per_meter(png_ptr,
-       info_ptr)
-
-    res_x = png_get_x_pixels_per_inch(png_ptr,
-       info_ptr)
-
-    res_y = png_get_y_pixels_per_inch(png_ptr,
-       info_ptr)
-
-    res_x_and_y = png_get_pixels_per_inch(png_ptr,
-       info_ptr)
-
-    aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
-       info_ptr)
-
-    Each of these returns 0 [signifying "unknown"] if
-       the data is not present or if res_x is 0;
-       res_x_and_y is 0 if res_x != res_y
-
-    Note that because of the way the resolutions are
-       stored internally, the inch conversions won't
-       come out to exactly even number.  For example,
-       72 dpi is stored as 0.28346 pixels/meter, and
-       when this is retrieved it is 71.9988 dpi, so
-       be sure to round the returned value appropriately
-       if you want to display a reasonable-looking result.
-
-The data from the oFFs chunk can be retrieved in several convenient
-forms:
-
-    x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
-
-    y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
-
-    x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
-
-    y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
-
-    Each of these returns 0 [signifying "unknown" if both
-       x and y are 0] if the data is not present or if the
-       chunk is present but the unit is the pixel.  The
-       remark about inexact inch conversions applies here
-       as well, because a value in inches can't always be
-       converted to microns and back without some loss
-       of precision.
-
-For more information, see the
-PNG specification for chunk contents.  Be careful with trusting
-rowbytes, as some of the transformations could increase the space
-needed to hold a row (expand, filler, gray_to_rgb, etc.).
-See png_read_update_info(), below.
-
-A quick word about text_ptr and num_text.  PNG stores comments in
-keyword/text pairs, one pair per chunk, with no limit on the number
-of text chunks, and a 2^31 byte limit on their size.  While there are
-suggested keywords, there is no requirement to restrict the use to these
-strings.  It is strongly suggested that keywords and text be sensible
-to humans (that's the point), so don't use abbreviations.  Non-printing
-symbols are not allowed.  See the PNG specification for more details.
-There is also no requirement to have text after the keyword.
-
-Keywords should be limited to 79 Latin-1 characters without leading or
-trailing spaces, but non-consecutive spaces are allowed within the
-keyword.  It is possible to have the same keyword any number of times.
-The text_ptr is an array of png_text structures, each holding a
-pointer to a language string, a pointer to a keyword and a pointer to
-a text string.  The text string, language code, and translated
-keyword may be empty or NULL pointers.  The keyword/text
-pairs are put into the array in the order that they are received.
-However, some or all of the text chunks may be after the image, so, to
-make sure you have read all the text chunks, don't mess with these
-until after you read the stuff after the image.  This will be
-mentioned again below in the discussion that goes with png_read_end().
-
-Input transformations
-
-After you've read the header information, you can set up the library
-to handle any special transformations of the image data.  The various
-ways to transform the data will be described in the order that they
-should occur.  This is important, as some of these change the color
-type and/or bit depth of the data, and some others only work on
-certain color types and bit depths.
-
-Transformations you request are ignored if they don't have any meaning for a
-particular input data format.  However some transformations can have an effect
-as a result of a previous transformation.  If you specify a contradictory set of
-transformations, for example both adding and removing the alpha channel, you
-cannot predict the final result.
-
-The color used for the transparency values should be supplied in the same
-format/depth as the current image data.  It is stored in the same format/depth
-as the image data in a tRNS chunk, so this is what libpng expects for this data.
-
-The color used for the background value depends on the need_expand argument as
-described below.
-
-Data will be decoded into the supplied row buffers packed into bytes
-unless the library has been told to transform it into another format.
-For example, 4 bit/pixel paletted or grayscale data will be returned
-2 pixels/byte with the leftmost pixel in the high-order bits of the
-byte, unless png_set_packing() is called.  8-bit RGB data will be stored
-in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
-is called to insert filler bytes, either before or after each RGB triplet.
-16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
-byte of the color value first, unless png_set_scale_16() is called to
-transform it to regular RGB RGB triplets, or png_set_filler() or
-png_set_add alpha() is called to insert filler bytes, either before or
-after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can
-be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
-or png_set_scale_16().
-
-The following code transforms grayscale images of less than 8 to 8 bits,
-changes paletted images to RGB, and adds a full alpha channel if there is
-transparency information in a tRNS chunk.  This is most useful on
-grayscale images with bit depths of 2 or 4 or if there is a multiple-image
-viewing application that wishes to treat all images in the same way.
-
-    if (color_type == PNG_COLOR_TYPE_PALETTE)
-        png_set_palette_to_rgb(png_ptr);
-
-    if (png_get_valid(png_ptr, info_ptr,
-        PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
-
-    if (color_type == PNG_COLOR_TYPE_GRAY &&
-        bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
-
-The first two functions are actually aliases for png_set_expand(), added
-in libpng version 1.0.4, with the function names expanded to improve code
-readability.  In some future version they may actually do different
-things.
-
-As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
-added.  It expands the sample depth without changing tRNS to alpha.
-
-As of libpng version 1.5.2, png_set_expand_16() was added.  It behaves as
-png_set_expand(); however, the resultant channels have 16 bits rather than 8.
-Use this when the output color or gray channels are made linear to avoid fairly
-severe accuracy loss.
-
-   if (bit_depth < 16)
-      png_set_expand_16(png_ptr);
-
-PNG can have files with 16 bits per channel.  If you only can handle
-8 bits per channel, this will strip the pixels down to 8-bit.
-
-    if (bit_depth == 16)
-#if PNG_LIBPNG_VER >= 10504
-       png_set_scale_16(png_ptr);
-#else
-       png_set_strip_16(png_ptr);
-#endif
-
-(The more accurate "png_set_scale_16()" API became available in libpng version
-1.5.4).
-
-If you need to process the alpha channel on the image separately from the image
-data (for example if you convert it to a bitmap mask) it is possible to have
-libpng strip the channel leaving just RGB or gray data:
-
-    if (color_type & PNG_COLOR_MASK_ALPHA)
-       png_set_strip_alpha(png_ptr);
-
-If you strip the alpha channel you need to find some other way of dealing with
-the information.  If, instead, you want to convert the image to an opaque
-version with no alpha channel use png_set_background; see below.
-
-As of libpng version 1.5.2, almost all useful expansions are supported, the
-major ommissions are conversion of grayscale to indexed images (which can be
-done trivially in the application) and conversion of indexed to grayscale (which
-can be done by a trivial manipulation of the palette.)
-
-In the following table, the 01 means grayscale with depth<8, 31 means
-indexed with depth<8, other numerals represent the color type, "T" means
-the tRNS chunk is present, A means an alpha channel is present, and O
-means tRNS or alpha is present but all pixels in the image are opaque.
-
-  FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
-   TO
-   01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
-   31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
-    0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
-   0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
-   0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
-    2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
-   2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
-   2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
-    3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
-   3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
-   3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
-   4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
-   4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
-   6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
-   6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +
-
-Within the matrix,
-     "+" identifies entries where 'from' and 'to' are the same.
-     "-" means the transformation is not supported.
-     "." means nothing is necessary (a tRNS chunk can just be ignored).
-     "t" means the transformation is obtained by png_set_tRNS.
-     "A" means the transformation is obtained by png_set_add_alpha().
-     "X" means the transformation is obtained by png_set_expand().
-     "1" means the transformation is obtained by
-         png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
-         if there is no transparency in the original or the final
-         format).
-     "C" means the transformation is obtained by png_set_gray_to_rgb().
-     "G" means the transformation is obtained by png_set_rgb_to_gray().
-     "P" means the transformation is obtained by
-         png_set_expand_palette_to_rgb().
-     "p" means the transformation is obtained by png_set_packing().
-     "Q" means the transformation is obtained by png_set_quantize().
-     "T" means the transformation is obtained by
-         png_set_tRNS_to_alpha().
-     "B" means the transformation is obtained by
-         png_set_background(), or png_strip_alpha().
-
-When an entry has multiple transforms listed all are required to cause the
-right overall transformation.  When two transforms are separated by a comma
-either will do the job.  When transforms are enclosed in [] the transform should
-do the job but this is currently unimplemented - a different format will result
-if the suggested transformations are used.
-
-In PNG files, the alpha channel in an image
-is the level of opacity.  If you need the alpha channel in an image to
-be the level of transparency instead of opacity, you can invert the
-alpha channel (or the tRNS chunk data) after it's read, so that 0 is
-fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
-images) is fully transparent, with
-
-    png_set_invert_alpha(png_ptr);
-
-PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
-they can, resulting in, for example, 8 pixels per byte for 1 bit
-files.  This code expands to 1 pixel per byte without changing the
-values of the pixels:
-
-    if (bit_depth < 8)
-       png_set_packing(png_ptr);
-
-PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
-stored in a PNG image have been "scaled" or "shifted" up to the next
-higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
-to 8 bits/sample in the range [0, 255]).  However, it is also possible
-to convert the PNG pixel data back to the original bit depth of the
-image.  This call reduces the pixels back down to the original bit depth:
-
-    png_color_8p sig_bit;
-
-    if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
-       png_set_shift(png_ptr, sig_bit);
-
-PNG files store 3-color pixels in red, green, blue order.  This code
-changes the storage of the pixels to blue, green, red:
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-       png_set_bgr(png_ptr);
-
-PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
-into 4 or 8 bytes for windowing systems that need them in this format:
-
-    if (color_type == PNG_COLOR_TYPE_RGB)
-       png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
-
-where "filler" is the 8 or 16-bit number to fill with, and the location is
-either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
-you want the filler before the RGB or after.  This transformation
-does not affect images that already have full alpha channels.  To add an
-opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
-will generate RGBA pixels.
-
-Note that png_set_filler() does not change the color type.  If you want
-to do that, you can add a true alpha channel with
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-       color_type == PNG_COLOR_TYPE_GRAY)
-       png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
-
-where "filler" contains the alpha value to assign to each pixel.
-This function was added in libpng-1.2.7.
-
-If you are reading an image with an alpha channel, and you need the
-data as ARGB instead of the normal PNG format RGBA:
-
-    if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-       png_set_swap_alpha(png_ptr);
-
-For some uses, you may want a grayscale image to be represented as
-RGB.  This code will do that conversion:
-
-    if (color_type == PNG_COLOR_TYPE_GRAY ||
-        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
-       png_set_gray_to_rgb(png_ptr);
-
-Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
-with alpha.
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-       png_set_rgb_to_gray(png_ptr, error_action,
-          double red_weight, double green_weight);
-
-    error_action = 1: silently do the conversion
-
-    error_action = 2: issue a warning if the original
-                      image has any pixel where
-                      red != green or red != blue
-
-    error_action = 3: issue an error and abort the
-                      conversion if the original
-                      image has any pixel where
-                      red != green or red != blue
-
-    red_weight:       weight of red component
-
-    green_weight:     weight of green component
-                      If either weight is negative, default
-                      weights are used.
-
-In the corresponding fixed point API the red_weight and green_weight values are
-simply scaled by 100,000:
-
-    png_set_rgb_to_gray(png_ptr, error_action,
-       png_fixed_point red_weight,
-       png_fixed_point green_weight);
-
-If you have set error_action = 1 or 2, you can
-later check whether the image really was gray, after processing
-the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
-It will return a png_byte that is zero if the image was gray or
-1 if there were any non-gray pixels.  Background and sBIT data
-will be silently converted to grayscale, using the green channel
-data for sBIT, regardless of the error_action setting.
-
-The default values come from the PNG file cHRM chunk if present; otherwise, the
-defaults correspond to the ITU-R recommendation 709, and also the sRGB color
-space, as recommended in the Charles Poynton's Colour FAQ,
-<http://www.poynton.com/>, in section 9:
-
-   <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
-
-    Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
-
-Previous versions of this document, 1998 through 2002, recommended a slightly
-different formula:
-
-    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
-
-Libpng uses an integer approximation:
-
-    Y = (6968 * R + 23434 * G + 2366 * B)/32768
-
-The calculation is done in a linear colorspace, if the image gamma
-can be determined.
-
-The png_set_background() function has been described already; it tells libpng to
-composite images with alpha or simple transparency against the supplied
-background color.  For compatibility with versions of libpng earlier than
-libpng-1.5.4 it is recommended that you call the function after reading the file
-header, even if you don't want to use the color in a bKGD chunk, if one exists.
-
-If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
-you may use this color, or supply another color more suitable for
-the current display (e.g., the background color from a web page).  You
-need to tell libpng how the color is represented, both the format of the
-component values in the color (the number of bits) and the gamma encoding of the
-color.  The function takes two arguments, background_gamma_mode and need_expand
-to convey this information; however, only two combinations are likely to be
-useful:
-
-    png_color_16 my_background;
-    png_color_16p image_background;
-
-    if (png_get_bKGD(png_ptr, info_ptr, &image_background))
-       png_set_background(png_ptr, image_background,
-           PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
-    else
-       png_set_background(png_ptr, &my_background,
-           PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
-
-The second call was described above - my_background is in the format of the
-final, display, output produced by libpng.  Because you now know the format of
-the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
-output and to retain palette images (the palette colors will be modified
-appropriately and the tRNS chunk removed.)  However, if you are doing this,
-take great care not to ask for transformations without checking first that
-they apply!
-
-In the first call the background color has the original bit depth and color type
-of the PNG file.  So, for palette images the color is supplied as a palette
-index and for low bit greyscale images the color is a reduced bit value in
-image_background->gray.
-
-If you didn't call png_set_gamma() before reading the file header, for example
-if you need your code to remain compatible with older versions of libpng prior
-to libpng-1.5.4, this is the place to call it.
-
-Do not call it if you called png_set_alpha_mode(); doing so will damage the
-settings put in place by png_set_alpha_mode().  (If png_set_alpha_mode() is
-supported then you can certainly do png_set_gamma() before reading the PNG
-header.)
-
-This API unconditionally sets the screen and file gamma values, so it will
-override the value in the PNG file unless it is called before the PNG file
-reading starts.  For this reason you must always call it with the PNG file
-value when you call it in this position:
-
-   if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
-      png_set_gamma(png_ptr, screen_gamma, file_gamma);
-
-   else
-      png_set_gamma(png_ptr, screen_gamma, 0.45455);
-
-If you need to reduce an RGB file to a paletted file, or if a paletted
-file has more entries then will fit on your screen, png_set_quantize()
-will do that.  Note that this is a simple match quantization that merely
-finds the closest color available.  This should work fairly well with
-optimized palettes, but fairly badly with linear color cubes.  If you
-pass a palette that is larger than maximum_colors, the file will
-reduce the number of colors in the palette so it will fit into
-maximum_colors.  If there is a histogram, libpng will use it to make
-more intelligent choices when reducing the palette.  If there is no
-histogram, it may not do as good a job.
-
-   if (color_type & PNG_COLOR_MASK_COLOR)
-   {
-      if (png_get_valid(png_ptr, info_ptr,
-          PNG_INFO_PLTE))
-      {
-         png_uint_16p histogram = NULL;
-
-         png_get_hIST(png_ptr, info_ptr,
-             &histogram);
-         png_set_quantize(png_ptr, palette, num_palette,
-            max_screen_colors, histogram, 1);
-      }
-
-      else
-      {
-         png_color std_color_cube[MAX_SCREEN_COLORS] =
-            { ... colors ... };
-
-         png_set_quantize(png_ptr, std_color_cube,
-            MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
-            NULL,0);
-      }
-   }
-
-PNG files describe monochrome as black being zero and white being one.
-The following code will reverse this (make black be one and white be
-zero):
-
-   if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
-      png_set_invert_mono(png_ptr);
-
-This function can also be used to invert grayscale and gray-alpha images:
-
-   if (color_type == PNG_COLOR_TYPE_GRAY ||
-       color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
-      png_set_invert_mono(png_ptr);
-
-PNG files store 16-bit pixels in network byte order (big-endian,
-ie. most significant bits first).  This code changes the storage to the
-other way (little-endian, i.e. least significant bits first, the
-way PCs store them):
-
-    if (bit_depth == 16)
-       png_set_swap(png_ptr);
-
-If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
-need to change the order the pixels are packed into bytes, you can use:
-
-    if (bit_depth < 8)
-       png_set_packswap(png_ptr);
-
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs.  This is done by setting a callback
-with
-
-    png_set_read_user_transform_fn(png_ptr,
-        read_transform_fn);
-
-You must supply the function
-
-    void read_transform_fn(png_structp png_ptr, png_row_infop
-        row_info, png_bytep data)
-
-See pngtest.c for a working example.  Your function will be called
-after all of the other transformations have been processed.  Take care with
-interlaced images if you do the interlace yourself - the width of the row is the
-width in 'row_info', not the overall image width.
-
-If supported, libpng provides two information routines that you can use to find
-where you are in processing the image:
-
-   png_get_current_pass_number(png_structp png_ptr);
-   png_get_current_row_number(png_structp png_ptr);
-
-Don't try using these outside a transform callback - firstly they are only
-supported if user transforms are supported, secondly they may well return
-unexpected results unless the row is actually being processed at the moment they
-are called.
-
-With interlaced
-images the value returned is the row in the input sub-image image.  Use
-PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
-find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
-
-The discussion of interlace handling above contains more information on how to
-use these values.
-
-You can also set up a pointer to a user structure for use by your
-callback function, and you can inform libpng that your transform
-function will change the number of channels or bit depth with the
-function
-
-    png_set_user_transform_info(png_ptr, user_ptr,
-        user_depth, user_channels);
-
-The user's application, not libpng, is responsible for allocating and
-freeing any memory required for the user structure.
-
-You can retrieve the pointer via the function
-png_get_user_transform_ptr().  For example:
-
-    voidp read_user_transform_ptr =
-        png_get_user_transform_ptr(png_ptr);
-
-The last thing to handle is interlacing; this is covered in detail below,
-but you must call the function here if you want libpng to handle expansion
-of the interlaced image.
-
-    number_of_passes = png_set_interlace_handling(png_ptr);
-
-After setting the transformations, libpng can update your png_info
-structure to reflect any transformations you've requested with this
-call.
-
-    png_read_update_info(png_ptr, info_ptr);
-
-This is most useful to update the info structure's rowbytes
-field so you can use it to allocate your image memory.  This function
-will also update your palette with the correct screen_gamma and
-background if these have been given with the calls above.  You may
-only call png_read_update_info() once with a particular info_ptr.
-
-After you call png_read_update_info(), you can allocate any
-memory you need to hold the image.  The row data is simply
-raw byte data for all forms of images.  As the actual allocation
-varies among applications, no example will be given.  If you
-are allocating one large chunk, you will need to build an
-array of pointers to each row, as it will be needed for some
-of the functions below.
-
-Remember: Before you call png_read_update_info(), the png_get_*()
-functions return the values corresponding to the original PNG image.
-After you call png_read_update_info the values refer to the image
-that libpng will output.  Consequently you must call all the png_set_
-functions before you call png_read_update_info().  This is particularly
-important for png_set_interlace_handling() - if you are going to call
-png_read_update_info() you must call png_set_interlace_handling() before
-it unless you want to receive interlaced output.
-
-Reading image data
-
-After you've allocated memory, you can read the image data.
-The simplest way to do this is in one function call.  If you are
-allocating enough memory to hold the whole image, you can just
-call png_read_image() and libpng will read in all the image data
-and put it in the memory area supplied.  You will need to pass in
-an array of pointers to each row.
-
-This function automatically handles interlacing, so you don't
-need to call png_set_interlace_handling() (unless you call
-png_read_update_info()) or call this function multiple times, or any
-of that other stuff necessary with png_read_rows().
-
-   png_read_image(png_ptr, row_pointers);
-
-where row_pointers is:
-
-   png_bytep row_pointers[height];
-
-You can point to void or char or whatever you use for pixels.
-
-If you don't want to read in the whole image at once, you can
-use png_read_rows() instead.  If there is no interlacing (check
-interlace_type == PNG_INTERLACE_NONE), this is simple:
-
-    png_read_rows(png_ptr, row_pointers, NULL,
-        number_of_rows);
-
-where row_pointers is the same as in the png_read_image() call.
-
-If you are doing this just one row at a time, you can do this with
-a single row_pointer instead of an array of row_pointers:
-
-    png_bytep row_pointer = row;
-    png_read_row(png_ptr, row_pointer, NULL);
-
-If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
-get somewhat harder.  The only current (PNG Specification version 1.2)
-interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
-a somewhat complicated 2D interlace scheme, known as Adam7, that
-breaks down an image into seven smaller images of varying size, based
-on an 8x8 grid.  This number is defined (from libpng 1.5) as
-PNG_INTERLACE_ADAM7_PASSES in png.h
-
-libpng can fill out those images or it can give them to you "as is".
-It is almost always better to have libpng handle the interlacing for you.
-If you want the images filled out, there are two ways to do that.  The one
-mentioned in the PNG specification is to expand each pixel to cover
-those pixels that have not been read yet (the "rectangle" method).
-This results in a blocky image for the first pass, which gradually
-smooths out as more pixels are read.  The other method is the "sparkle"
-method, where pixels are drawn only in their final locations, with the
-rest of the image remaining whatever colors they were initialized to
-before the start of the read.  The first method usually looks better,
-but tends to be slower, as there are more pixels to put in the rows.
-
-If, as is likely, you want libpng to expand the images, call this before
-calling png_start_read_image() or png_read_update_info():
-
-    if (interlace_type == PNG_INTERLACE_ADAM7)
-       number_of_passes
-           = png_set_interlace_handling(png_ptr);
-
-This will return the number of passes needed.  Currently, this is seven,
-but may change if another interlace type is added.  This function can be
-called even if the file is not interlaced, where it will return one pass.
-You then need to read the whole image 'number_of_passes' times.  Each time
-will distribute the pixels from the current pass to the correct place in
-the output image, so you need to supply the same rows to png_read_rows in
-each pass.
-
-If you are not going to display the image after each pass, but are
-going to wait until the entire image is read in, use the sparkle
-effect.  This effect is faster and the end result of either method
-is exactly the same.  If you are planning on displaying the image
-after each pass, the "rectangle" effect is generally considered the
-better looking one.
-
-If you only want the "sparkle" effect, just call png_read_rows() as
-normal, with the third parameter NULL.  Make sure you make pass over
-the image number_of_passes times, and you don't change the data in the
-rows between calls.  You can change the locations of the data, just
-not the data.  Each pass only writes the pixels appropriate for that
-pass, and assumes the data from previous passes is still valid.
-
-    png_read_rows(png_ptr, row_pointers, NULL,
-        number_of_rows);
-
-If you only want the first effect (the rectangles), do the same as
-before except pass the row buffer in the third parameter, and leave
-the second parameter NULL.
-
-    png_read_rows(png_ptr, NULL, row_pointers,
-        number_of_rows);
-
-If you don't want libpng to handle the interlacing details, just call
-png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
-Each of the images is a valid image by itself; however, you will almost
-certainly need to distribute the pixels from each sub-image to the
-correct place.  This is where everything gets very tricky.
-
-If you want to retrieve the separate images you must pass the correct
-number of rows to each successive call of png_read_rows().  The calculation
-gets pretty complicated for small images, where some sub-images may
-not even exist because either their width or height ends up zero.
-libpng provides two macros to help you in 1.5 and later versions:
-
-   png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
-   png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
-
-Respectively these tell you the width and height of the sub-image
-corresponding to the numbered pass.  'pass' is in in the range 0 to 6 -
-this can be confusing because the specification refers to the same passes
-as 1 to 7!  Be careful, you must check both the width and height before
-calling png_read_rows() and not call it for that pass if either is zero.
-
-You can, of course, read each sub-image row by row.  If you want to
-produce optimal code to make a pixel-by-pixel transformation of an
-interlaced image this is the best approach; read each row of each pass,
-transform it, and write it out to a new interlaced image.
-
-If you want to de-interlace the image yourself libpng provides further
-macros to help that tell you where to place the pixels in the output image.
-Because the interlacing scheme is rectangular - sub-image pixels are always
-arranged on a rectangular grid - all you need to know for each pass is the
-starting column and row in the output image of the first pixel plus the
-spacing between each pixel.  As of libpng 1.5 there are four macros to
-retrieve this information:
-
-   png_uint_32 x = PNG_PASS_START_COL(pass);
-   png_uint_32 y = PNG_PASS_START_ROW(pass);
-   png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
-   png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
-
-These allow you to write the obvious loop:
-
-   png_uint_32 input_y = 0;
-   png_uint_32 output_y = PNG_PASS_START_ROW(pass);
-
-   while (output_y < output_image_height)
-   {
-      png_uint_32 input_x = 0;
-      png_uint_32 output_x = PNG_PASS_START_COL(pass);
-
-      while (output_x < output_image_width)
-      {
-         image[output_y][output_x] =
-             subimage[pass][input_y][input_x++];
-
-         output_x += xStep;
-      }
-
-      ++input_y;
-      output_y += yStep;
-   }
-
-Notice that the steps between successive output rows and columns are
-returned as shifts.  This is possible because the pixels in the subimages
-are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
-image.  In practice you may need to directly calculate the output coordinate
-given an input coordinate.  libpng provides two further macros for this
-purpose:
-
-   png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
-   png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
-
-Finally a pair of macros are provided to tell you if a particular image
-row or column appears in a given pass:
-
-   int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
-   int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
-
-Bear in mind that you will probably also need to check the width and height
-of the pass in addition to the above to be sure the pass even exists!
-
-With any luck you are convinced by now that you don't want to do your own
-interlace handling.  In reality normally the only good reason for doing this
-is if you are processing PNG files on a pixel-by-pixel basis and don't want
-to load the whole file into memory when it is interlaced.
-
-libpng includes a test program, pngvalid, that illustrates reading and
-writing of interlaced images.  If you can't get interlacing to work in your
-code and don't want to leave it to libpng (the recommended approach), see
-how pngvalid.c does it.
-
-Finishing a sequential read
-
-After you are finished reading the image through the
-low-level interface, you can finish reading the file.
-
-If you want to use a different crc action for handling CRC errors in
-chunks after the image data, you can call png_set_crc_action()
-again at this point.
-
-If you are interested in comments or time, which may be stored either
-before or after the image data, you should pass the separate png_info
-struct if you want to keep the comments from before and after the image
-separate.
-
-    png_infop end_info = png_create_info_struct(png_ptr);
-
-    if (!end_info)
-    {
-       png_destroy_read_struct(&png_ptr, &info_ptr,
-           (png_infopp)NULL);
-       return (ERROR);
-    }
-
-   png_read_end(png_ptr, end_info);
-
-If you are not interested, you should still call png_read_end()
-but you can pass NULL, avoiding the need to create an end_info structure.
-If you do this, libpng will not process any chunks after IDAT other than
-skipping over them and perhaps (depending on whether you have called
-png_set_crc_action) checking their CRCs while looking for the IEND chunk.
-
-   png_read_end(png_ptr, (png_infop)NULL);
-
-If you don't call png_read_end(), then your file pointer will be
-left pointing to the first chunk after the last IDAT, which is probably
-not what you want if you expect to read something beyond the end of
-the PNG datastream.
-
-When you are done, you can free all memory allocated by libpng like this:
-
-   png_destroy_read_struct(&png_ptr, &info_ptr,
-       &end_info);
-
-or, if you didn't create an end_info structure,
-
-   png_destroy_read_struct(&png_ptr, &info_ptr,
-       (png_infopp)NULL);
-
-It is also possible to individually free the info_ptr members that
-point to libpng-allocated storage with the following function:
-
-    png_free_data(png_ptr, info_ptr, mask, seq)
-
-    mask - identifies data to be freed, a mask
-           containing the bitwise OR of one or
-           more of
-             PNG_FREE_PLTE, PNG_FREE_TRNS,
-             PNG_FREE_HIST, PNG_FREE_ICCP,
-             PNG_FREE_PCAL, PNG_FREE_ROWS,
-             PNG_FREE_SCAL, PNG_FREE_SPLT,
-             PNG_FREE_TEXT, PNG_FREE_UNKN,
-           or simply PNG_FREE_ALL
-
-    seq  - sequence number of item to be freed
-           (-1 for all items)
-
-This function may be safely called when the relevant storage has
-already been freed, or has not yet been allocated, or was allocated
-by the user and not by libpng,  and will in those cases do nothing.
-The "seq" parameter is ignored if only one item of the selected data
-type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
-are allowed for the data type identified in the mask, such as text or
-sPLT, only the n'th item in the structure is freed, where n is "seq".
-
-The default behavior is only to free data that was allocated internally
-by libpng.  This can be changed, so that libpng will not free the data,
-or so that it will free data that was allocated by the user with png_malloc()
-or png_calloc() and passed in via a png_set_*() function, with
-
-    png_data_freer(png_ptr, info_ptr, freer, mask)
-
-    freer  - one of
-               PNG_DESTROY_WILL_FREE_DATA
-               PNG_SET_WILL_FREE_DATA
-               PNG_USER_WILL_FREE_DATA
-
-    mask   - which data elements are affected
-             same choices as in png_free_data()
-
-This function only affects data that has already been allocated.
-You can call this function after reading the PNG data but before calling
-any png_set_*() functions, to control whether the user or the png_set_*()
-function is responsible for freeing any existing data that might be present,
-and again after the png_set_*() functions to control whether the user
-or png_destroy_*() is supposed to free the data.  When the user assumes
-responsibility for libpng-allocated data, the application must use
-png_free() to free it, and when the user transfers responsibility to libpng
-for data that the user has allocated, the user must have used png_malloc()
-or png_calloc() to allocate it.
-
-If you allocated your row_pointers in a single block, as suggested above in
-the description of the high level read interface, you must not transfer
-responsibility for freeing it to the png_set_rows or png_read_destroy function,
-because they would also try to free the individual row_pointers[i].
-
-If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
-separately, do not transfer responsibility for freeing text_ptr to libpng,
-because when libpng fills a png_text structure it combines these members with
-the key member, and png_free_data() will free only text_ptr.key.  Similarly,
-if you transfer responsibility for free'ing text_ptr from libpng to your
-application, your application must not separately free those members.
-
-The png_free_data() function will turn off the "valid" flag for anything
-it frees.  If you need to turn the flag off for a chunk that was freed by
-your application instead of by libpng, you can use
-
-    png_set_invalid(png_ptr, info_ptr, mask);
-
-    mask - identifies the chunks to be made invalid,
-           containing the bitwise OR of one or
-           more of
-             PNG_INFO_gAMA, PNG_INFO_sBIT,
-             PNG_INFO_cHRM, PNG_INFO_PLTE,
-             PNG_INFO_tRNS, PNG_INFO_bKGD,
-             PNG_INFO_hIST, PNG_INFO_pHYs,
-             PNG_INFO_oFFs, PNG_INFO_tIME,
-             PNG_INFO_pCAL, PNG_INFO_sRGB,
-             PNG_INFO_iCCP, PNG_INFO_sPLT,
-             PNG_INFO_sCAL, PNG_INFO_IDAT
-
-For a more compact example of reading a PNG image, see the file example.c.
-
-Reading PNG files progressively
-
-The progressive reader is slightly different from the non-progressive
-reader.  Instead of calling png_read_info(), png_read_rows(), and
-png_read_end(), you make one call to png_process_data(), which calls
-callbacks when it has the info, a row, or the end of the image.  You
-set up these callbacks with png_set_progressive_read_fn().  You don't
-have to worry about the input/output functions of libpng, as you are
-giving the library the data directly in png_process_data().  I will
-assume that you have read the section on reading PNG files above,
-so I will only highlight the differences (although I will show
-all of the code).
-
-png_structp png_ptr;
-png_infop info_ptr;
-
- /*  An example code fragment of how you would
-     initialize the progressive reader in your
-     application. */
- int
- initialize_png_reader()
- {
-    png_ptr = png_create_read_struct
-        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-         user_error_fn, user_warning_fn);
-
-    if (!png_ptr)
-        return (ERROR);
-
-    info_ptr = png_create_info_struct(png_ptr);
-
-    if (!info_ptr)
-    {
-       png_destroy_read_struct(&png_ptr,
-          (png_infopp)NULL, (png_infopp)NULL);
-       return (ERROR);
-    }
-
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-       png_destroy_read_struct(&png_ptr, &info_ptr,
-          (png_infopp)NULL);
-       return (ERROR);
-    }
-
-    /* This one's new.  You can provide functions
-       to be called when the header info is valid,
-       when each row is completed, and when the image
-       is finished.  If you aren't using all functions,
-       you can specify NULL parameters.  Even when all
-       three functions are NULL, you need to call
-       png_set_progressive_read_fn().  You can use
-       any struct as the user_ptr (cast to a void pointer
-       for the function call), and retrieve the pointer
-       from inside the callbacks using the function
-
-          png_get_progressive_ptr(png_ptr);
-
-       which will return a void pointer, which you have
-       to cast appropriately.
-     */
-    png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
-        info_callback, row_callback, end_callback);
-
-    return 0;
- }
-
- /* A code fragment that you call as you receive blocks
-   of data */
- int
- process_data(png_bytep buffer, png_uint_32 length)
- {
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-       png_destroy_read_struct(&png_ptr, &info_ptr,
-           (png_infopp)NULL);
-       return (ERROR);
-    }
-
-    /* This one's new also.  Simply give it a chunk
-       of data from the file stream (in order, of
-       course).  On machines with segmented memory
-       models machines, don't give it any more than
-       64K.  The library seems to run fine with sizes
-       of 4K. Although you can give it much less if
-       necessary (I assume you can give it chunks of
-       1 byte, I haven't tried less then 256 bytes
-       yet).  When this function returns, you may
-       want to display any rows that were generated
-       in the row callback if you don't already do
-       so there.
-     */
-    png_process_data(png_ptr, info_ptr, buffer, length);
-
-    /* At this point you can call png_process_data_skip if
-       you want to handle data the library will skip yourself;
-       it simply returns the number of bytes to skip (and stops
-       libpng skipping that number of bytes on the next
-       png_process_data call).
-    return 0;
- }
-
- /* This function is called (as set by
-    png_set_progressive_read_fn() above) when enough data
-    has been supplied so all of the header has been
-    read.
- */
- void
- info_callback(png_structp png_ptr, png_infop info)
- {
-    /* Do any setup here, including setting any of
-       the transformations mentioned in the Reading
-       PNG files section.  For now, you _must_ call
-       either png_start_read_image() or
-       png_read_update_info() after all the
-       transformations are set (even if you don't set
-       any).  You may start getting rows before
-       png_process_data() returns, so this is your
-       last chance to prepare for that.
-
-       This is where you turn on interlace handling,
-       assuming you don't want to do it yourself.
-
-       If you need to you can stop the processing of
-       your original input data at this point by calling
-       png_process_data_pause.  This returns the number
-       of unprocessed bytes from the last png_process_data
-       call - it is up to you to ensure that the next call
-       sees these bytes again.  If you don't want to bother
-       with this you can get libpng to cache the unread
-       bytes by setting the 'save' parameter (see png.h) but
-       then libpng will have to copy the data internally.
-     */
- }
-
- /* This function is called when each row of image
-    data is complete */
- void
- row_callback(png_structp png_ptr, png_bytep new_row,
-    png_uint_32 row_num, int pass)
- {
-    /* If the image is interlaced, and you turned
-       on the interlace handler, this function will
-       be called for every row in every pass.  Some
-       of these rows will not be changed from the
-       previous pass.  When the row is not changed,
-       the new_row variable will be NULL.  The rows
-       and passes are called in order, so you don't
-       really need the row_num and pass, but I'm
-       supplying them because it may make your life
-       easier.
-
-       If you did not turn on interlace handling then
-       the callback is called for each row of each
-       sub-image when the image is interlaced.  In this
-       case 'row_num' is the row in the sub-image, not
-       the row in the output image as it is in all other
-       cases.
-
-       For the non-NULL rows of interlaced images when
-       you have switched on libpng interlace handling,
-       you must call png_progressive_combine_row()
-       passing in the row and the old row.  You can
-       call this function for NULL rows (it will just
-       return) and for non-interlaced images (it just
-       does the memcpy for you) if it will make the
-       code easier.  Thus, you can just do this for
-       all cases if you switch on interlace handling;
-     */
-
-        png_progressive_combine_row(png_ptr, old_row,
-          new_row);
-
-    /* where old_row is what was displayed
-       previously for the row.  Note that the first
-       pass (pass == 0, really) will completely cover
-       the old row, so the rows do not have to be
-       initialized.  After the first pass (and only
-       for interlaced images), you will have to pass
-       the current row, and the function will combine
-       the old row and the new row.
-
-       You can also call png_process_data_pause in this
-       callback - see above.
-    */
- }
-
- void
- end_callback(png_structp png_ptr, png_infop info)
- {
-    /* This function is called after the whole image
-       has been read, including any chunks after the
-       image (up to and including the IEND).  You
-       will usually have the same info chunk as you
-       had in the header, although some data may have
-       been added to the comments and time fields.
-
-       Most people won't do much here, perhaps setting
-       a flag that marks the image as finished.
-     */
- }
-
-
-
-IV. Writing
-
-Much of this is very similar to reading.  However, everything of
-importance is repeated here, so you won't have to constantly look
-back up in the reading section to understand writing.
-
-Setup
-
-You will want to do the I/O initialization before you get into libpng,
-so if it doesn't work, you don't have anything to undo. If you are not
-using the standard I/O functions, you will need to replace them with
-custom writing functions.  See the discussion under Customizing libpng.
-
-    FILE *fp = fopen(file_name, "wb");
-
-    if (!fp)
-       return (ERROR);
-
-Next, png_struct and png_info need to be allocated and initialized.
-As these can be both relatively large, you may not want to store these
-on the stack, unless you have stack space to spare.  Of course, you
-will want to check if they return NULL.  If you are also reading,
-you won't want to name your read structure and your write structure
-both "png_ptr"; you can call them anything you like, such as
-"read_ptr" and "write_ptr".  Look at pngtest.c, for example.
-
-    png_structp png_ptr = png_create_write_struct
-       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn);
-
-    if (!png_ptr)
-       return (ERROR);
-
-    png_infop info_ptr = png_create_info_struct(png_ptr);
-    if (!info_ptr)
-    {
-       png_destroy_write_struct(&png_ptr,
-           (png_infopp)NULL);
-       return (ERROR);
-    }
-
-If you want to use your own memory allocation routines,
-define PNG_USER_MEM_SUPPORTED and use
-png_create_write_struct_2() instead of png_create_write_struct():
-
-    png_structp png_ptr = png_create_write_struct_2
-       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn, (png_voidp)
-        user_mem_ptr, user_malloc_fn, user_free_fn);
-
-After you have these structures, you will need to set up the
-error handling.  When libpng encounters an error, it expects to
-longjmp() back to your routine.  Therefore, you will need to call
-setjmp() and pass the png_jmpbuf(png_ptr).  If you
-write the file from different routines, you will need to update
-the png_jmpbuf(png_ptr) every time you enter a new routine that will
-call a png_*() function.  See your documentation of setjmp/longjmp
-for your compiler for more information on setjmp/longjmp.  See
-the discussion on libpng error handling in the Customizing Libpng
-section below for more information on the libpng error handling.
-
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-    png_destroy_write_struct(&png_ptr, &info_ptr);
-       fclose(fp);
-       return (ERROR);
-    }
-    ...
-    return;
-
-If you would rather avoid the complexity of setjmp/longjmp issues,
-you can compile libpng with PNG_NO_SETJMP, in which case
-errors will result in a call to PNG_ABORT() which defaults to abort().
-
-You can #define PNG_ABORT() to a function that does something
-more useful than abort(), as long as your function does not
-return.
-
-Checking for invalid palette index on write was added at libpng
-1.5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
-a benign error.  This is enabled by default because this condition is an
-error according to the PNG specification, Clause 11.3.2, but the error can
-be ignored in each png_ptr with
-
-   png_set_check_for_invalid_index(png_ptr, 0);
-
-If the error is ignored, or if png_benign_error() treats it as a warning,
-any invalid pixels are written as-is by the encoder, resulting in an
-invalid PNG datastream as output.  In this case the application is
-responsible for ensuring that the pixel indexes are in range when it writes
-a PLTE chunk with fewer entries than the bit depth would allow.
-
-Now you need to set up the output code.  The default for libpng is to
-use the C function fwrite().  If you use this, you will need to pass a
-valid FILE * in the function png_init_io().  Be sure that the file is
-opened in binary mode.  Again, if you wish to handle writing data in
-another way, see the discussion on libpng I/O handling in the Customizing
-Libpng section below.
-
-    png_init_io(png_ptr, fp);
-
-If you are embedding your PNG into a datastream such as MNG, and don't
-want libpng to write the 8-byte signature, or if you have already
-written the signature in your application, use
-
-    png_set_sig_bytes(png_ptr, 8);
-
-to inform libpng that it should not write a signature.
-
-Write callbacks
-
-At this point, you can set up a callback function that will be
-called after each row has been written, which you can use to control
-a progress meter or the like.  It's demonstrated in pngtest.c.
-You must supply a function
-
-    void write_row_callback(png_structp png_ptr, png_uint_32 row,
-       int pass);
-    {
-      /* put your code here */
-    }
-
-(You can give it another name that you like instead of "write_row_callback")
-
-To inform libpng about your function, use
-
-    png_set_write_status_fn(png_ptr, write_row_callback);
-
-When this function is called the row has already been completely processed and
-it has also been written out.  The 'row' and 'pass' refer to the next row to be
-handled.  For the
-non-interlaced case the row that was just handled is simply one less than the
-passed in row number, and pass will always be 0.  For the interlaced case the
-same applies unless the row value is 0, in which case the row just handled was
-the last one from one of the preceding passes.  Because interlacing may skip a
-pass you cannot be sure that the preceding pass is just 'pass-1', if you really
-need to know what the last pass is record (row,pass) from the callback and use
-the last recorded value each time.
-
-As with the user transform you can find the output row using the
-PNG_ROW_FROM_PASS_ROW macro.
-
-You now have the option of modifying how the compression library will
-run.  The following functions are mainly for testing, but may be useful
-in some cases, like if you need to write PNG files extremely fast and
-are willing to give up some compression, or if you want to get the
-maximum possible compression at the expense of slower writing.  If you
-have no special needs in this area, let the library do what it wants by
-not calling this function at all, as it has been tuned to deliver a good
-speed/compression ratio. The second parameter to png_set_filter() is
-the filter method, for which the only valid values are 0 (as of the
-July 1999 PNG specification, version 1.2) or 64 (if you are writing
-a PNG datastream that is to be embedded in a MNG datastream).  The third
-parameter is a flag that indicates which filter type(s) are to be tested
-for each scanline.  See the PNG specification for details on the specific
-filter types.
-
-
-    /* turn on or off filtering, and/or choose
-       specific filters.  You can use either a single
-       PNG_FILTER_VALUE_NAME or the bitwise OR of one
-       or more PNG_FILTER_NAME masks.
-     */
-    png_set_filter(png_ptr, 0,
-       PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
-       PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
-       PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
-       PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
-       PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
-       PNG_ALL_FILTERS);
-
-If an application wants to start and stop using particular filters during
-compression, it should start out with all of the filters (to ensure that
-the previous row of pixels will be stored in case it's needed later),
-and then add and remove them after the start of compression.
-
-If you are writing a PNG datastream that is to be embedded in a MNG
-datastream, the second parameter can be either 0 or 64.
-
-The png_set_compression_*() functions interface to the zlib compression
-library, and should mostly be ignored unless you really know what you are
-doing.  The only generally useful call is png_set_compression_level()
-which changes how much time zlib spends on trying to compress the image
-data.  See the Compression Library (zlib.h and algorithm.txt, distributed
-with zlib) for details on the compression levels.
-
-    #include zlib.h
-
-    /* Set the zlib compression level */
-    png_set_compression_level(png_ptr,
-        Z_BEST_COMPRESSION);
-
-    /* Set other zlib parameters for compressing IDAT */
-    png_set_compression_mem_level(png_ptr, 8);
-    png_set_compression_strategy(png_ptr,
-        Z_DEFAULT_STRATEGY);
-    png_set_compression_window_bits(png_ptr, 15);
-    png_set_compression_method(png_ptr, 8);
-    png_set_compression_buffer_size(png_ptr, 8192)
-
-    /* Set zlib parameters for text compression
-     * If you don't call these, the parameters
-     * fall back on those defined for IDAT chunks
-     */
-    png_set_text_compression_mem_level(png_ptr, 8);
-    png_set_text_compression_strategy(png_ptr,
-        Z_DEFAULT_STRATEGY);
-    png_set_text_compression_window_bits(png_ptr, 15);
-    png_set_text_compression_method(png_ptr, 8);
-
-Setting the contents of info for output
-
-You now need to fill in the png_info structure with all the data you
-wish to write before the actual image.  Note that the only thing you
-are allowed to write after the image is the text chunks and the time
-chunk (as of PNG Specification 1.2, anyway).  See png_write_end() and
-the latest PNG specification for more information on that.  If you
-wish to write them before the image, fill them in now, and flag that
-data as being valid.  If you want to wait until after the data, don't
-fill them until png_write_end().  For all the fields in png_info and
-their data types, see png.h.  For explanations of what the fields
-contain, see the PNG specification.
-
-Some of the more important parts of the png_info are:
-
-    png_set_IHDR(png_ptr, info_ptr, width, height,
-       bit_depth, color_type, interlace_type,
-       compression_type, filter_method)
-
-    width          - holds the width of the image
-                     in pixels (up to 2^31).
-
-    height         - holds the height of the image
-                     in pixels (up to 2^31).
-
-    bit_depth      - holds the bit depth of one of the
-                     image channels.
-                     (valid values are 1, 2, 4, 8, 16
-                     and depend also on the
-                     color_type.  See also significant
-                     bits (sBIT) below).
-
-    color_type     - describes which color/alpha
-                     channels are present.
-                     PNG_COLOR_TYPE_GRAY
-                        (bit depths 1, 2, 4, 8, 16)
-                     PNG_COLOR_TYPE_GRAY_ALPHA
-                        (bit depths 8, 16)
-                     PNG_COLOR_TYPE_PALETTE
-                        (bit depths 1, 2, 4, 8)
-                     PNG_COLOR_TYPE_RGB
-                        (bit_depths 8, 16)
-                     PNG_COLOR_TYPE_RGB_ALPHA
-                        (bit_depths 8, 16)
-
-                     PNG_COLOR_MASK_PALETTE
-                     PNG_COLOR_MASK_COLOR
-                     PNG_COLOR_MASK_ALPHA
-
-    interlace_type - PNG_INTERLACE_NONE or
-                     PNG_INTERLACE_ADAM7
-
-    compression_type - (must be
-                     PNG_COMPRESSION_TYPE_DEFAULT)
-
-    filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
-                     or, if you are writing a PNG to
-                     be embedded in a MNG datastream,
-                     can also be
-                     PNG_INTRAPIXEL_DIFFERENCING)
-
-If you call png_set_IHDR(), the call must appear before any of the
-other png_set_*() functions, because they might require access to some of
-the IHDR settings.  The remaining png_set_*() functions can be called
-in any order.
-
-If you wish, you can reset the compression_type, interlace_type, or
-filter_method later by calling png_set_IHDR() again; if you do this, the
-width, height, bit_depth, and color_type must be the same in each call.
-
-    png_set_PLTE(png_ptr, info_ptr, palette,
-       num_palette);
-
-    palette        - the palette for the file
-                     (array of png_color)
-    num_palette    - number of entries in the palette
-
-    png_set_gAMA(png_ptr, info_ptr, file_gamma);
-    png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
-
-    file_gamma     - the gamma at which the image was
-                     created (PNG_INFO_gAMA)
-
-    int_file_gamma - 100,000 times the gamma at which
-                     the image was created
-
-    png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
-                     green_x, green_y, blue_x, blue_y)
-    png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
-                     green_Y, green_Z, blue_X, blue_Y, blue_Z)
-    png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
-                     int_red_x, int_red_y, int_green_x, int_green_y,
-                     int_blue_x, int_blue_y)
-    png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
-                     int_red_Z, int_green_X, int_green_Y, int_green_Z,
-                     int_blue_X, int_blue_Y, int_blue_Z)
-
-    {white,red,green,blue}_{x,y}
-                     A color space encoding specified using the chromaticities
-                     of the end points and the white point.
-
-    {red,green,blue}_{X,Y,Z}
-                     A color space encoding specified using the encoding end
-                     points - the CIE tristimulus specification of the intended
-                     color of the red, green and blue channels in the PNG RGB
-                     data.  The white point is simply the sum of the three end
-                     points.
-
-    png_set_sRGB(png_ptr, info_ptr, srgb_intent);
-
-    srgb_intent    - the rendering intent
-                     (PNG_INFO_sRGB) The presence of
-                     the sRGB chunk means that the pixel
-                     data is in the sRGB color space.
-                     This chunk also implies specific
-                     values of gAMA and cHRM.  Rendering
-                     intent is the CSS-1 property that
-                     has been defined by the International
-                     Color Consortium
-                     (http://www.color.org).
-                     It can be one of
-                     PNG_sRGB_INTENT_SATURATION,
-                     PNG_sRGB_INTENT_PERCEPTUAL,
-                     PNG_sRGB_INTENT_ABSOLUTE, or
-                     PNG_sRGB_INTENT_RELATIVE.
-
-
-    png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
-       srgb_intent);
-
-    srgb_intent    - the rendering intent
-                     (PNG_INFO_sRGB) The presence of the
-                     sRGB chunk means that the pixel
-                     data is in the sRGB color space.
-                     This function also causes gAMA and
-                     cHRM chunks with the specific values
-                     that are consistent with sRGB to be
-                     written.
-
-    png_set_iCCP(png_ptr, info_ptr, name, compression_type,
-                       profile, proflen);
-
-    name             - The profile name.
-
-    compression_type - The compression type; always
-                       PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
-                       You may give NULL to this argument to
-                       ignore it.
-
-    profile          - International Color Consortium color
-                       profile data. May contain NULs.
-
-    proflen          - length of profile data in bytes.
-
-    png_set_sBIT(png_ptr, info_ptr, sig_bit);
-
-    sig_bit        - the number of significant bits for
-                     (PNG_INFO_sBIT) each of the gray, red,
-                     green, and blue channels, whichever are
-                     appropriate for the given color type
-                     (png_color_16)
-
-    png_set_tRNS(png_ptr, info_ptr, trans_alpha,
-       num_trans, trans_color);
-
-    trans_alpha    - array of alpha (transparency)
-                     entries for palette (PNG_INFO_tRNS)
-
-    num_trans      - number of transparent entries
-                     (PNG_INFO_tRNS)
-
-    trans_color    - graylevel or color sample values
-                     (in order red, green, blue) of the
-                     single transparent color for
-                     non-paletted images (PNG_INFO_tRNS)
-
-    png_set_hIST(png_ptr, info_ptr, hist);
-
-    hist           - histogram of palette (array of
-                     png_uint_16) (PNG_INFO_hIST)
-
-    png_set_tIME(png_ptr, info_ptr, mod_time);
-
-    mod_time       - time image was last modified
-                     (PNG_VALID_tIME)
-
-    png_set_bKGD(png_ptr, info_ptr, background);
-
-    background     - background color (of type
-                     png_color_16p) (PNG_VALID_bKGD)
-
-    png_set_text(png_ptr, info_ptr, text_ptr, num_text);
-
-    text_ptr       - array of png_text holding image
-                     comments
-
-    text_ptr[i].compression - type of compression used
-                 on "text" PNG_TEXT_COMPRESSION_NONE
-                           PNG_TEXT_COMPRESSION_zTXt
-                           PNG_ITXT_COMPRESSION_NONE
-                           PNG_ITXT_COMPRESSION_zTXt
-    text_ptr[i].key   - keyword for comment.  Must contain
-                 1-79 characters.
-    text_ptr[i].text  - text comments for current
-                         keyword.  Can be NULL or empty.
-    text_ptr[i].text_length - length of text string,
-                 after decompression, 0 for iTXt
-    text_ptr[i].itxt_length - length of itxt string,
-                 after decompression, 0 for tEXt/zTXt
-    text_ptr[i].lang  - language of comment (NULL or
-                         empty for unknown).
-    text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
-                         or empty for unknown).
-
-    Note that the itxt_length, lang, and lang_key
-    members of the text_ptr structure only exist when the
-    library is built with iTXt chunk support.  Prior to
-    libpng-1.4.0 the library was built by default without
-    iTXt support. Also note that when iTXt is supported,
-    they contain NULL pointers when the "compression"
-    field contains PNG_TEXT_COMPRESSION_NONE or
-    PNG_TEXT_COMPRESSION_zTXt.
-
-    num_text       - number of comments
-
-    png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
-       num_spalettes);
-
-    palette_ptr    - array of png_sPLT_struct structures
-                     to be added to the list of palettes
-                     in the info structure.
-    num_spalettes  - number of palette structures to be
-                     added.
-
-    png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
-        unit_type);
-
-    offset_x  - positive offset from the left
-                     edge of the screen
-
-    offset_y  - positive offset from the top
-                     edge of the screen
-
-    unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
-
-    png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
-        unit_type);
-
-    res_x       - pixels/unit physical resolution
-                  in x direction
-
-    res_y       - pixels/unit physical resolution
-                  in y direction
-
-    unit_type   - PNG_RESOLUTION_UNKNOWN,
-                  PNG_RESOLUTION_METER
-
-    png_set_sCAL(png_ptr, info_ptr, unit, width, height)
-
-    unit        - physical scale units (an integer)
-
-    width       - width of a pixel in physical scale units
-
-    height      - height of a pixel in physical scale units
-                  (width and height are doubles)
-
-    png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
-
-    unit        - physical scale units (an integer)
-
-    width       - width of a pixel in physical scale units
-                  expressed as a string
-
-    height      - height of a pixel in physical scale units
-                 (width and height are strings like "2.54")
-
-    png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
-       num_unknowns)
-
-    unknowns          - array of png_unknown_chunk
-                        structures holding unknown chunks
-    unknowns[i].name  - name of unknown chunk
-    unknowns[i].data  - data of unknown chunk
-    unknowns[i].size  - size of unknown chunk's data
-    unknowns[i].location - position to write chunk in file
-                           0: do not write chunk
-                           PNG_HAVE_IHDR: before PLTE
-                           PNG_HAVE_PLTE: before IDAT
-                           PNG_AFTER_IDAT: after IDAT
-
-The "location" member is set automatically according to
-what part of the output file has already been written.
-You can change its value after calling png_set_unknown_chunks()
-as demonstrated in pngtest.c.  Within each of the "locations",
-the chunks are sequenced according to their position in the
-structure (that is, the value of "i", which is the order in which
-the chunk was either read from the input file or defined with
-png_set_unknown_chunks).
-
-A quick word about text and num_text.  text is an array of png_text
-structures.  num_text is the number of valid structures in the array.
-Each png_text structure holds a language code, a keyword, a text value,
-and a compression type.
-
-The compression types have the same valid numbers as the compression
-types of the image data.  Currently, the only valid number is zero.
-However, you can store text either compressed or uncompressed, unlike
-images, which always have to be compressed.  So if you don't want the
-text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
-Because tEXt and zTXt chunks don't have a language field, if you
-specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
-any language code or translated keyword will not be written out.
-
-Until text gets around a few hundred bytes, it is not worth compressing it.
-After the text has been written out to the file, the compression type
-is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
-so that it isn't written out again at the end (in case you are calling
-png_write_end() with the same struct).
-
-The keywords that are given in the PNG Specification are:
-
-    Title            Short (one line) title or
-                     caption for image
-
-    Author           Name of image's creator
-
-    Description      Description of image (possibly long)
-
-    Copyright        Copyright notice
-
-    Creation Time    Time of original image creation
-                     (usually RFC 1123 format, see below)
-
-    Software         Software used to create the image
-
-    Disclaimer       Legal disclaimer
-
-    Warning          Warning of nature of content
-
-    Source           Device used to create the image
-
-    Comment          Miscellaneous comment; conversion
-                     from other image format
-
-The keyword-text pairs work like this.  Keywords should be short
-simple descriptions of what the comment is about.  Some typical
-keywords are found in the PNG specification, as is some recommendations
-on keywords.  You can repeat keywords in a file.  You can even write
-some text before the image and some after.  For example, you may want
-to put a description of the image before the image, but leave the
-disclaimer until after, so viewers working over modem connections
-don't have to wait for the disclaimer to go over the modem before
-they start seeing the image.  Finally, keywords should be full
-words, not abbreviations.  Keywords and text are in the ISO 8859-1
-(Latin-1) character set (a superset of regular ASCII) and can not
-contain NUL characters, and should not contain control or other
-unprintable characters.  To make the comments widely readable, stick
-with basic ASCII, and avoid machine specific character set extensions
-like the IBM-PC character set.  The keyword must be present, but
-you can leave off the text string on non-compressed pairs.
-Compressed pairs must have a text string, as only the text string
-is compressed anyway, so the compression would be meaningless.
-
-PNG supports modification time via the png_time structure.  Two
-conversion routines are provided, png_convert_from_time_t() for
-time_t and png_convert_from_struct_tm() for struct tm.  The
-time_t routine uses gmtime().  You don't have to use either of
-these, but if you wish to fill in the png_time structure directly,
-you should provide the time in universal time (GMT) if possible
-instead of your local time.  Note that the year number is the full
-year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
-that months start with 1.
-
-If you want to store the time of the original image creation, you should
-use a plain tEXt chunk with the "Creation Time" keyword.  This is
-necessary because the "creation time" of a PNG image is somewhat vague,
-depending on whether you mean the PNG file, the time the image was
-created in a non-PNG format, a still photo from which the image was
-scanned, or possibly the subject matter itself.  In order to facilitate
-machine-readable dates, it is recommended that the "Creation Time"
-tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
-although this isn't a requirement.  Unlike the tIME chunk, the
-"Creation Time" tEXt chunk is not expected to be automatically changed
-by the software.  To facilitate the use of RFC 1123 dates, a function
-png_convert_to_rfc1123_buffer(png_ptr, buffer, png_timep) is provided to
-convert from PNG time to an RFC 1123 format string.  The caller must provide
-a writeable buffer of at least 29 bytes.
-
-Writing unknown chunks
-
-You can use the png_set_unknown_chunks function to queue up private chunks
-for writing.  You give it a chunk name, location, raw data, and a size.  You
-also must use png_set_keep_unknown_chunks() to ensure that libpng will
-handle them.  That's all there is to it.  The chunks will be written by the
-next following png_write_info_before_PLTE, png_write_info, or png_write_end
-function, depending upon the specified location.  Any chunks previously
-read into the info structure's unknown-chunk list will also be written out
-in a sequence that satisfies the PNG specification's ordering rules.
-
-Here is an example of writing two private chunks, prVt and miNE:
-
-    #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
-    /* Set unknown chunk data */
-    png_unknown_chunk unk_chunk[2];
-    strcpy((char *) unk_chunk[0].name, "prVt";
-    unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
-    unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
-    unk_chunk[0].location = PNG_HAVE_IHDR;
-    strcpy((char *) unk_chunk[1].name, "miNE";
-    unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
-    unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
-    unk_chunk[1].location = PNG_AFTER_IDAT;
-    png_set_unknown_chunks(write_ptr, write_info_ptr,
-        unk_chunk, 2);
-    /* Needed because miNE is not safe-to-copy */
-    png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
-       (png_bytep) "miNE", 1);
-    # if PNG_LIBPNG_VER < 10600
-      /* Deal with unknown chunk location bug in 1.5.x and earlier */
-      png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
-      png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
-    # endif
-    # if PNG_LIBPNG_VER < 10500
-      /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0,
-       * one before IDAT and another after IDAT, so don't use it; only use
-       * PNG_HAVE_IHDR location.  This call resets the location previously
-       * set by assignment and png_set_unknown_chunk_location() for chunk 1.
-       */
-      png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
-    # endif
-    #endif
-
-The high-level write interface
-
-At this point there are two ways to proceed; through the high-level
-write interface, or through a sequence of low-level write operations.
-You can use the high-level interface if your image data is present
-in the info structure.  All defined output
-transformations are permitted, enabled by the following masks.
-
-    PNG_TRANSFORM_IDENTITY      No transformation
-    PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
-    PNG_TRANSFORM_PACKSWAP      Change order of packed
-                                pixels to LSB first
-    PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
-    PNG_TRANSFORM_SHIFT         Normalize pixels to the
-                                sBIT depth
-    PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
-                                to BGRA
-    PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
-                                to AG
-    PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
-                                to transparency
-    PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
-    PNG_TRANSFORM_STRIP_FILLER        Strip out filler
-                                      bytes (deprecated).
-    PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
-                                      filler bytes
-    PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
-                                      filler bytes
-
-If you have valid image data in the info structure (you can use
-png_set_rows() to put image data in the info structure), simply do this:
-
-    png_write_png(png_ptr, info_ptr, png_transforms, NULL)
-
-where png_transforms is an integer containing the bitwise OR of some set of
-transformation flags.  This call is equivalent to png_write_info(),
-followed the set of transformations indicated by the transform mask,
-then png_write_image(), and finally png_write_end().
-
-(The final parameter of this call is not yet used.  Someday it might point
-to transformation parameters required by some future output transform.)
-
-You must use png_transforms and not call any png_set_transform() functions
-when you use png_write_png().
-
-The low-level write interface
-
-If you are going the low-level route instead, you are now ready to
-write all the file information up to the actual image data.  You do
-this with a call to png_write_info().
-
-    png_write_info(png_ptr, info_ptr);
-
-Note that there is one transformation you may need to do before
-png_write_info().  In PNG files, the alpha channel in an image is the
-level of opacity.  If your data is supplied as a level of transparency,
-you can invert the alpha channel before you write it, so that 0 is
-fully transparent and 255 (in 8-bit or paletted images) or 65535
-(in 16-bit images) is fully opaque, with
-
-    png_set_invert_alpha(png_ptr);
-
-This must appear before png_write_info() instead of later with the
-other transformations because in the case of paletted images the tRNS
-chunk data has to be inverted before the tRNS chunk is written.  If
-your image is not a paletted image, the tRNS data (which in such cases
-represents a single color to be rendered as transparent) won't need to
-be changed, and you can safely do this transformation after your
-png_write_info() call.
-
-If you need to write a private chunk that you want to appear before
-the PLTE chunk when PLTE is present, you can write the PNG info in
-two steps, and insert code to write your own chunk between them:
-
-    png_write_info_before_PLTE(png_ptr, info_ptr);
-    png_set_unknown_chunks(png_ptr, info_ptr, ...);
-    png_write_info(png_ptr, info_ptr);
-
-After you've written the file information, you can set up the library
-to handle any special transformations of the image data.  The various
-ways to transform the data will be described in the order that they
-should occur.  This is important, as some of these change the color
-type and/or bit depth of the data, and some others only work on
-certain color types and bit depths.  Even though each transformation
-checks to see if it has data that it can do something with, you should
-make sure to only enable a transformation if it will be valid for the
-data.  For example, don't swap red and blue on grayscale data.
-
-PNG files store RGB pixels packed into 3 or 6 bytes.  This code tells
-the library to strip input data that has 4 or 8 bytes per pixel down
-to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
-bytes per pixel).
-
-    png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
-
-where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
-PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
-is stored XRGB or RGBX.
-
-PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
-they can, resulting in, for example, 8 pixels per byte for 1 bit files.
-If the data is supplied at 1 pixel per byte, use this code, which will
-correctly pack the pixels into a single byte:
-
-    png_set_packing(png_ptr);
-
-PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.  If your
-data is of another bit depth, you can write an sBIT chunk into the
-file so that decoders can recover the original data if desired.
-
-    /* Set the true bit depth of the image data */
-    if (color_type & PNG_COLOR_MASK_COLOR)
-    {
-       sig_bit.red = true_bit_depth;
-       sig_bit.green = true_bit_depth;
-       sig_bit.blue = true_bit_depth;
-    }
-
-    else
-    {
-       sig_bit.gray = true_bit_depth;
-    }
-
-    if (color_type & PNG_COLOR_MASK_ALPHA)
-    {
-       sig_bit.alpha = true_bit_depth;
-    }
-
-    png_set_sBIT(png_ptr, info_ptr, &sig_bit);
-
-If the data is stored in the row buffer in a bit depth other than
-one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
-this will scale the values to appear to be the correct bit depth as
-is required by PNG.
-
-    png_set_shift(png_ptr, &sig_bit);
-
-PNG files store 16-bit pixels in network byte order (big-endian,
-ie. most significant bits first).  This code would be used if they are
-supplied the other way (little-endian, i.e. least significant bits
-first, the way PCs store them):
-
-    if (bit_depth > 8)
-       png_set_swap(png_ptr);
-
-If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
-need to change the order the pixels are packed into bytes, you can use:
-
-    if (bit_depth < 8)
-       png_set_packswap(png_ptr);
-
-PNG files store 3 color pixels in red, green, blue order.  This code
-would be used if they are supplied as blue, green, red:
-
-    png_set_bgr(png_ptr);
-
-PNG files describe monochrome as black being zero and white being
-one. This code would be used if the pixels are supplied with this reversed
-(black being one and white being zero):
-
-    png_set_invert_mono(png_ptr);
-
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs.  This is done by setting a callback
-with
-
-    png_set_write_user_transform_fn(png_ptr,
-       write_transform_fn);
-
-You must supply the function
-
-    void write_transform_fn(png_structp png_ptr, png_row_infop
-       row_info, png_bytep data)
-
-See pngtest.c for a working example.  Your function will be called
-before any of the other transformations are processed.  If supported
-libpng also supplies an information routine that may be called from
-your callback:
-
-   png_get_current_row_number(png_ptr);
-   png_get_current_pass_number(png_ptr);
-
-This returns the current row passed to the transform.  With interlaced
-images the value returned is the row in the input sub-image image.  Use
-PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
-find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
-
-The discussion of interlace handling above contains more information on how to
-use these values.
-
-You can also set up a pointer to a user structure for use by your
-callback function.
-
-    png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
-
-The user_channels and user_depth parameters of this function are ignored
-when writing; you can set them to zero as shown.
-
-You can retrieve the pointer via the function png_get_user_transform_ptr().
-For example:
-
-    voidp write_user_transform_ptr =
-       png_get_user_transform_ptr(png_ptr);
-
-It is possible to have libpng flush any pending output, either manually,
-or automatically after a certain number of lines have been written.  To
-flush the output stream a single time call:
-
-    png_write_flush(png_ptr);
-
-and to have libpng flush the output stream periodically after a certain
-number of scanlines have been written, call:
-
-    png_set_flush(png_ptr, nrows);
-
-Note that the distance between rows is from the last time png_write_flush()
-was called, or the first row of the image if it has never been called.
-So if you write 50 lines, and then png_set_flush 25, it will flush the
-output on the next scanline, and every 25 lines thereafter, unless
-png_write_flush() is called before 25 more lines have been written.
-If nrows is too small (less than about 10 lines for a 640 pixel wide
-RGB image) the image compression may decrease noticeably (although this
-may be acceptable for real-time applications).  Infrequent flushing will
-only degrade the compression performance by a few percent over images
-that do not use flushing.
-
-Writing the image data
-
-That's it for the transformations.  Now you can write the image data.
-The simplest way to do this is in one function call.  If you have the
-whole image in memory, you can just call png_write_image() and libpng
-will write the image.  You will need to pass in an array of pointers to
-each row.  This function automatically handles interlacing, so you don't
-need to call png_set_interlace_handling() or call this function multiple
-times, or any of that other stuff necessary with png_write_rows().
-
-    png_write_image(png_ptr, row_pointers);
-
-where row_pointers is:
-
-    png_byte *row_pointers[height];
-
-You can point to void or char or whatever you use for pixels.
-
-If you don't want to write the whole image at once, you can
-use png_write_rows() instead.  If the file is not interlaced,
-this is simple:
-
-    png_write_rows(png_ptr, row_pointers,
-       number_of_rows);
-
-row_pointers is the same as in the png_write_image() call.
-
-If you are just writing one row at a time, you can do this with
-a single row_pointer instead of an array of row_pointers:
-
-    png_bytep row_pointer = row;
-
-    png_write_row(png_ptr, row_pointer);
-
-When the file is interlaced, things can get a good deal more complicated.
-The only currently (as of the PNG Specification version 1.2, dated July
-1999) defined interlacing scheme for PNG files is the "Adam7" interlace
-scheme, that breaks down an image into seven smaller images of varying
-size.  libpng will build these images for you, or you can do them
-yourself.  If you want to build them yourself, see the PNG specification
-for details of which pixels to write when.
-
-If you don't want libpng to handle the interlacing details, just
-use png_set_interlace_handling() and call png_write_rows() the
-correct number of times to write all the sub-images
-(png_set_interlace_handling() returns the number of sub-images.)
-
-If you want libpng to build the sub-images, call this before you start
-writing any rows:
-
-    number_of_passes = png_set_interlace_handling(png_ptr);
-
-This will return the number of passes needed.  Currently, this is seven,
-but may change if another interlace type is added.
-
-Then write the complete image number_of_passes times.
-
-    png_write_rows(png_ptr, row_pointers, number_of_rows);
-
-Think carefully before you write an interlaced image.  Typically code that
-reads such images reads all the image data into memory, uncompressed, before
-doing any processing.  Only code that can display an image on the fly can
-take advantage of the interlacing and even then the image has to be exactly
-the correct size for the output device, because scaling an image requires
-adjacent pixels and these are not available until all the passes have been
-read.
-
-If you do write an interlaced image you will hardly ever need to handle
-the interlacing yourself.  Call png_set_interlace_handling() and use the
-approach described above.
-
-The only time it is conceivable that you will really need to write an
-interlaced image pass-by-pass is when you have read one pass by pass and
-made some pixel-by-pixel transformation to it, as described in the read
-code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
-to determine the size of each sub-image in turn and simply write the rows
-you obtained from the read code.
-
-Finishing a sequential write
-
-After you are finished writing the image, you should finish writing
-the file.  If you are interested in writing comments or time, you should
-pass an appropriately filled png_info pointer.  If you are not interested,
-you can pass NULL.
-
-    png_write_end(png_ptr, info_ptr);
-
-When you are done, you can free all memory used by libpng like this:
-
-    png_destroy_write_struct(&png_ptr, &info_ptr);
-
-It is also possible to individually free the info_ptr members that
-point to libpng-allocated storage with the following function:
-
-    png_free_data(png_ptr, info_ptr, mask, seq)
-
-    mask  - identifies data to be freed, a mask
-            containing the bitwise OR of one or
-            more of
-              PNG_FREE_PLTE, PNG_FREE_TRNS,
-              PNG_FREE_HIST, PNG_FREE_ICCP,
-              PNG_FREE_PCAL, PNG_FREE_ROWS,
-              PNG_FREE_SCAL, PNG_FREE_SPLT,
-              PNG_FREE_TEXT, PNG_FREE_UNKN,
-            or simply PNG_FREE_ALL
-
-    seq   - sequence number of item to be freed
-            (-1 for all items)
-
-This function may be safely called when the relevant storage has
-already been freed, or has not yet been allocated, or was allocated
-by the user  and not by libpng,  and will in those cases do nothing.
-The "seq" parameter is ignored if only one item of the selected data
-type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
-are allowed for the data type identified in the mask, such as text or
-sPLT, only the n'th item in the structure is freed, where n is "seq".
-
-If you allocated data such as a palette that you passed in to libpng
-with png_set_*, you must not free it until just before the call to
-png_destroy_write_struct().
-
-The default behavior is only to free data that was allocated internally
-by libpng.  This can be changed, so that libpng will not free the data,
-or so that it will free data that was allocated by the user with png_malloc()
-or png_calloc() and passed in via a png_set_*() function, with
-
-    png_data_freer(png_ptr, info_ptr, freer, mask)
-
-    freer  - one of
-               PNG_DESTROY_WILL_FREE_DATA
-               PNG_SET_WILL_FREE_DATA
-               PNG_USER_WILL_FREE_DATA
-
-    mask   - which data elements are affected
-             same choices as in png_free_data()
-
-For example, to transfer responsibility for some data from a read structure
-to a write structure, you could use
-
-    png_data_freer(read_ptr, read_info_ptr,
-       PNG_USER_WILL_FREE_DATA,
-       PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
-
-    png_data_freer(write_ptr, write_info_ptr,
-       PNG_DESTROY_WILL_FREE_DATA,
-       PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
-
-thereby briefly reassigning responsibility for freeing to the user but
-immediately afterwards reassigning it once more to the write_destroy
-function.  Having done this, it would then be safe to destroy the read
-structure and continue to use the PLTE, tRNS, and hIST data in the write
-structure.
-
-This function only affects data that has already been allocated.
-You can call this function before calling after the png_set_*() functions
-to control whether the user or png_destroy_*() is supposed to free the data.
-When the user assumes responsibility for libpng-allocated data, the
-application must use
-png_free() to free it, and when the user transfers responsibility to libpng
-for data that the user has allocated, the user must have used png_malloc()
-or png_calloc() to allocate it.
-
-If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
-separately, do not transfer responsibility for freeing text_ptr to libpng,
-because when libpng fills a png_text structure it combines these members with
-the key member, and png_free_data() will free only text_ptr.key.  Similarly,
-if you transfer responsibility for free'ing text_ptr from libpng to your
-application, your application must not separately free those members.
-For a more compact example of writing a PNG image, see the file example.c.
-
-V. Simplified API
-
-The simplified API, which became available in libpng-1.6.0, hides the details
-of both libpng and the PNG file format itself.
-It allows PNG files to be read into a very limited number of
-in-memory bitmap formats or to be written from the same formats.  If these
-formats do not accommodate your needs then you can, and should, use the more
-sophisticated APIs above - these support a wide variety of in-memory formats
-and a wide variety of sophisticated transformations to those formats as well
-as a wide variety of APIs to manipulate ancilliary information.
-
-To read a PNG file using the simplified API:
-
-  1) Declare a 'png_image' structure (see below) on the
-     stack and memset() it to all zero.
-
-  2) Call the appropriate png_image_begin_read... function.
-
-  3) Set the png_image 'format' member to the required
-     format and allocate a buffer for the image.
-
-  4) Call png_image_finish_read to read the image into
-     your buffer.
-
-There are no restrictions on the format of the PNG input itself; all valid
-color types, bit depths, and interlace methods are acceptable, and the
-input image is transformed as necessary to the requested in-memory format
-during the png_image_finish_read() step.
-
-To write a PNG file using the simplified API:
-
-  1) Declare a 'png_image' structure on the stack and memset()
-     it to all zero.
-
-  2) Initialize the members of the structure that describe the
-     image, setting the 'format' member to the format of the
-     image in memory.
-
-  3) Call the appropriate png_image_write... function with a
-     pointer to the image to write the PNG data.
-
-png_image is a structure that describes the in-memory format of an image
-when it is being read or define the in-memory format of an image that you
-need to write.  The "png_image" structure contains the following members:
-
-   png_uint_32  version Set to PNG_IMAGE_VERSION
-   png_uint_32  width   Image width in pixels (columns)
-   png_uint_32  height  Image height in pixels (rows)
-   png_uint_32  format  Image format as defined below
-   png_uint_32  flags   A bit mask containing informational flags
-   png_controlp opaque  Initialize to NULL, free with png_image_free
-   png_uint_32  colormap_entries; Number of entries in the color-map
-   png_uint_32  warning_or_error;
-   char         message[64];
-
-In the event of an error or warning the following field warning_or_error
-field will be set to a non-zero value and the 'message' field will contain
-a '\0' terminated string with the libpng error or warning message.  If both
-warnings and an error were encountered, only the error is recorded.  If there
-are multiple warnings, only the first one is recorded.
-
-The upper 30 bits of this value are reserved; the low two bits contain
-a two bit code such that a value more than 1 indicates a failure in the API
-just called:
-
-   0 - no warning or error
-   1 - warning
-   2 - error
-   3 - error preceded by warning
-
-The pixels (samples) of the image have one to four channels whose components
-have original values in the range 0 to 1.0:
-
-  1: A single gray or luminance channel (G).
-  2: A gray/luminance channel and an alpha channel (GA).
-  3: Three red, green, blue color channels (RGB).
-  4: Three color channels and an alpha channel (RGBA).
-
-The channels are encoded in one of two ways:
-
-  a) As a small integer, value 0..255, contained in a single byte.  For the
-alpha channel the original value is simply value/255.  For the color or
-luminance channels the value is encoded according to the sRGB specification
-and matches the 8-bit format expected by typical display devices.
-
-The color/gray channels are not scaled (pre-multiplied) by the alpha
-channel and are suitable for passing to color management software.
-
-  b) As a value in the range 0..65535, contained in a 2-byte integer, in
-the native byte order of the platform on which the application is running.
-All channels can be converted to the original value by dividing by 65535; all
-channels are linear.  Color channels use the RGB encoding (RGB end-points) of
-the sRGB specification.  This encoding is identified by the
-PNG_FORMAT_FLAG_LINEAR flag below.
-
-When an alpha channel is present it is expected to denote pixel coverage
-of the color or luminance channels and is returned as an associated alpha
-channel: the color/gray channels are scaled (pre-multiplied) by the alpha
-value.
-
-When a color-mapped image is used as a result of calling
-png_image_read_colormap or png_image_write_colormap the channels are encoded
-in the color-map and the descriptions above apply to the color-map entries.
-The image data is encoded as small integers, value 0..255, that index the
-entries in the color-map.  One integer (one byte) is stored for each pixel.
-
-PNG_FORMAT_*
-
-The #defines to be used in png_image::format.  Each #define identifies a
-particular layout of channel data and, if present, alpha values.  There are
-separate defines for each of the two channel encodings.
-
-A format is built up using single bit flag values.  Not all combinations are
-valid: use the bit flag values below for testing a format returned by the
-read APIs, but set formats from the derived values.
-
-When reading or writing color-mapped images the format should be set to the
-format of the entries in the color-map then png_image_{read,write}_colormap
-called to read or write the color-map and set the format correctly for the
-image data.  Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!
-
-NOTE: libpng can be built with particular features disabled, if you see
-compiler errors because the definition of one of the following flags has been
-compiled out it is because libpng does not have the required support.  It is
-possible, however, for the libpng configuration to enable the format on just
-read or just write; in that case you may see an error at run time.  You can
-guard against this by checking for the definition of:
-
-   PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED
-
-   PNG_FORMAT_FLAG_ALPHA    0x01 format with an alpha channel
-   PNG_FORMAT_FLAG_COLOR    0x02 color format: otherwise grayscale
-   PNG_FORMAT_FLAG_LINEAR   0x04 png_uint_16 channels else png_byte
-   PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
-   PNG_FORMAT_FLAG_BGR      0x10 BGR colors, else order is RGB
-   PNG_FORMAT_FLAG_AFIRST   0x20 alpha channel comes first
-
-Supported formats are as follows.  Future versions of libpng may support more
-formats; for compatibility with older versions simply check if the format
-macro is defined using #ifdef.  These defines describe the in-memory layout
-of the components of the pixels of the image.
-
-First the single byte formats:
-
-   PNG_FORMAT_GRAY 0
-   PNG_FORMAT_GA   PNG_FORMAT_FLAG_ALPHA
-   PNG_FORMAT_AG   (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
-   PNG_FORMAT_RGB  PNG_FORMAT_FLAG_COLOR
-   PNG_FORMAT_BGR  (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
-   PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
-   PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
-   PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
-   PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)
-
-Then the linear 2-byte formats.  When naming these "Y" is used to
-indicate a luminance (gray) channel.  The component order within the pixel
-is always the same - there is no provision for swapping the order of the
-components in the linear format.  The components are 16-bit integers in
-the native byte order for your platform, and there is no provision for
-swapping the bytes to a different endian condition.
-
-   PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
-   PNG_FORMAT_LINEAR_Y_ALPHA
-      (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
-   PNG_FORMAT_LINEAR_RGB
-      (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
-   PNG_FORMAT_LINEAR_RGB_ALPHA
-      (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
-      PNG_FORMAT_FLAG_ALPHA)
-
-Color-mapped formats are obtained by calling png_image_{read,write}_colormap,
-as appropriate after setting png_image::format to the format of the color-map
-to be read or written.  Applications may check the value of
-PNG_FORMAT_FLAG_COLORMAP to see if they have called the colormap API.  The
-format of the color-map may be extracted using the following macro.
-
-   PNG_FORMAT_OF_COLORMAP(fmt) ((fmt) & ~PNG_FORMAT_FLAG_COLORMAP)
-
-PNG_IMAGE macros
-
-These are convenience macros to derive information from a png_image
-structure.  The PNG_IMAGE_SAMPLE_ macros return values appropriate to the
-actual image sample values - either the entries in the color-map or the
-pixels in the image.  The PNG_IMAGE_PIXEL_ macros return corresponding values
-for the pixels and will always return 1 after a call to
-png_image_{read,write}_colormap.  The remaining macros return information
-about the rows in the image and the complete image.
-
-NOTE: All the macros that take a png_image::format parameter are compile time
-constants if the format parameter is, itself, a constant.  Therefore these
-macros can be used in array declarations and case labels where required.
-Similarly the macros are also pre-processor constants (sizeof is not used) so
-they can be used in #if tests.
-
-First the information about the samples.
-
-  PNG_IMAGE_SAMPLE_CHANNELS(fmt)
-    Returns the total number of channels in a given format: 1..4
-
-  PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
-    Returns the size in bytes of a single component of a pixel or color-map
-    entry (as appropriate) in the image.
-
-  PNG_IMAGE_SAMPLE_SIZE(fmt)
-    This is the size of the sample data for one sample.  If the image is
-    color-mapped it is the size of one color-map entry (and image pixels are
-    one byte in size), otherwise it is the size of one image pixel.
-
-  PNG_IMAGE_COLORMAP_SIZE(fmt)
-   The size of the color-map required by the format; this is the size of the
-   color-map buffer passed to the png_image_{read,write}_colormap APIs, it is
-   a fixed number determined by the format so can easily be allocated on the
-   stack if necessary.
-
-#define PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)\
-   (PNG_IMAGE_SAMPLE_CHANNELS(fmt) * 256)
-   /* The maximum size of the color-map required by the format expressed in a
-    * count of components.  This can be used to compile-time allocate a
-    * color-map:
-    *
-    * png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
-    *
-    * png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
-    *
-    * Alternatively, use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
-    * information from one of the png_image_begin_read_ APIs and dynamically
-    * allocate the required memory.
-    */
-
-
-Corresponding information about the pixels
-
-  PNG_IMAGE_PIXEL_(test,fmt)
-
-  PNG_IMAGE_PIXEL_CHANNELS(fmt)
-   The number of separate channels (components) in a pixel; 1 for a
-   color-mapped image.
-
-  PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)\
-   The size, in bytes, of each component in a pixel; 1 for a color-mapped
-   image.
-
-  PNG_IMAGE_PIXEL_SIZE(fmt)
-   The size, in bytes, of a complete pixel; 1 for a color-mapped image.
-
-Information about the whole row, or whole image
-
-  PNG_IMAGE_ROW_STRIDE(image)
-   Returns the total number of components in a single row of the image; this
-   is the minimum 'row stride', the minimum count of components between each
-   row.  For a color-mapped image this is the minimum number of bytes in a
-   row.
-
-   If you need the stride measured in bytes, row_stride_bytes is
-   PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
-   plus any padding bytes that your application might need, for example
-   to start the next row on a 4-byte boundary.
-
-  PNG_IMAGE_BUFFER_SIZE(image, row_stride)
-    Returns the size, in bytes, of an image buffer given a png_image and a row
-    stride - the number of components to leave space for in each row.  This
-    macro takes care of multiplying row_stride by PNG_IMAGE_PIXEL_COMONENT_SIZE
-    when the image has 2-byte components.
-
-  PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
-    This indicates the the RGB values of the in-memory bitmap do not
-    correspond to the red, green and blue end-points defined by sRGB.
-
-  PNG_IMAGE_FLAG_COLORMAP == 0x02
-    The PNG is color-mapped.  If this flag is set png_image_read_colormap
-    can be used without further loss of image information.  If it is not set
-    png_image_read_colormap will cause significant loss if the image has any
-
-READ APIs
-
-   The png_image passed to the read APIs must have been initialized by setting
-   the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.)
-
-   int png_image_begin_read_from_file( png_imagep image,
-     const char *file_name)
-
-     The named file is opened for read and the image header
-     is filled in from the PNG header in the file.
-
-   int png_image_begin_read_from_stdio (png_imagep image,
-     FILE* file)
-
-      The PNG header is read from the stdio FILE object.
-
-   int png_image_begin_read_from_memory(png_imagep image,
-      png_const_voidp memory, png_size_t size)
-
-      The PNG header is read from the given memory buffer.
-
-   int png_image_finish_read(png_imagep image,
-      png_colorp background, void *buffer,
-      png_int_32 row_stride, void *colormap));
-
-      Finish reading the image into the supplied buffer and
-      clean up the png_image structure.
-
-      row_stride is the step, in png_byte or png_uint_16 units
-      as appropriate, between adjacent rows.  A positive stride
-      indicates that the top-most row is first in the buffer -
-      the normal top-down arrangement.  A negative stride
-      indicates that the bottom-most row is first in the buffer.
-
-      background need only be supplied if an alpha channel must
-      be removed from a png_byte format and the removal is to be
-      done by compositing on a solid color; otherwise it may be
-      NULL and any composition will be done directly onto the
-      buffer.  The value is an sRGB color to use for the
-      background, for grayscale output the green channel is used.
-
-      For linear output removing the alpha channel is always done
-      by compositing on black.
-
-   void png_image_free(png_imagep image)
-
-      Free any data allocated by libpng in image->opaque,
-      setting the pointer to NULL.  May be called at any time
-      after the structure is initialized.
-
-When the simplified API needs to convert between sRGB and linear colorspaces,
-the actual sRGB transfer curve defined in the sRGB specification (see the
-article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
-approximation used elsewhere in libpng.
-
-WRITE APIS
-
-For write you must initialize a png_image structure to describe the image to
-be written:
-
-   version: must be set to PNG_IMAGE_VERSION
-   opaque: must be initialized to NULL
-   width: image width in pixels
-   height: image height in rows
-   format: the format of the data you wish to write
-   flags: set to 0 unless one of the defined flags applies; set
-      PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
-      where the RGB values do not correspond to the colors in sRGB.
-   colormap_entries: set to the number of entries in the color-map (0 to 256)
-
-   int png_image_write_to_file, (png_imagep image,
-      const char *file, int convert_to_8bit, const void *buffer,
-      png_int_32 row_stride, const void *colormap));
-
-      Write the image to the named file.
-
-   int png_image_write_to_stdio(png_imagep image, FILE *file,
-      int convert_to_8_bit, const void *buffer,
-      png_int_32 row_stride, const void *colormap)
-
-      Write the image to the given (FILE*).
-
-With all write APIs if image is in one of the linear formats with
-(png_uint_16) data then setting convert_to_8_bit will cause the output to be
-a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise
-a 16-bit linear encoded PNG file is written.
-
-With all APIs row_stride is handled as in the read APIs - it is the spacing
-from one row to the next in component sized units (float) and if negative
-indicates a bottom-up row layout in the buffer.
-
-Note that the write API does not support interlacing, sub-8-bit pixels,
-and indexed (paletted) images.
-
-VI. Modifying/Customizing libpng
-
-There are two issues here.  The first is changing how libpng does
-standard things like memory allocation, input/output, and error handling.
-The second deals with more complicated things like adding new chunks,
-adding new transformations, and generally changing how libpng works.
-Both of those are compile-time issues; that is, they are generally
-determined at the time the code is written, and there is rarely a need
-to provide the user with a means of changing them.
-
-Memory allocation, input/output, and error handling
-
-All of the memory allocation, input/output, and error handling in libpng
-goes through callbacks that are user-settable.  The default routines are
-in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively.  To change
-these functions, call the appropriate png_set_*_fn() function.
-
-Memory allocation is done through the functions png_malloc(), png_calloc(),
-and png_free().  The png_malloc() and png_free() functions currently just
-call the standard C functions and png_calloc() calls png_malloc() and then
-clears the newly allocated memory to zero; note that png_calloc(png_ptr, size)
-is not the same as the calloc(number, size) function provided by stdlib.h.
-There is limited support for certain systems with segmented memory
-architectures and the types of pointers declared by png.h match this; you
-will have to use appropriate pointers in your application.  If you prefer
-to use a different method of allocating and freeing data, you can use
-png_create_read_struct_2() or png_create_write_struct_2() to register your
-own functions as described above.  These functions also provide a void
-pointer that can be retrieved via
-
-    mem_ptr=png_get_mem_ptr(png_ptr);
-
-Your replacement memory functions must have prototypes as follows:
-
-    png_voidp malloc_fn(png_structp png_ptr,
-       png_alloc_size_t size);
-
-    void free_fn(png_structp png_ptr, png_voidp ptr);
-
-Your malloc_fn() must return NULL in case of failure.  The png_malloc()
-function will normally call png_error() if it receives a NULL from the
-system memory allocator or from your replacement malloc_fn().
-
-Your free_fn() will never be called with a NULL ptr, since libpng's
-png_free() checks for NULL before calling free_fn().
-
-Input/Output in libpng is done through png_read() and png_write(),
-which currently just call fread() and fwrite().  The FILE * is stored in
-png_struct and is initialized via png_init_io().  If you wish to change
-the method of I/O, the library supplies callbacks that you can set
-through the function png_set_read_fn() and png_set_write_fn() at run
-time, instead of calling the png_init_io() function.  These functions
-also provide a void pointer that can be retrieved via the function
-png_get_io_ptr().  For example:
-
-    png_set_read_fn(png_structp read_ptr,
-        voidp read_io_ptr, png_rw_ptr read_data_fn)
-
-    png_set_write_fn(png_structp write_ptr,
-        voidp write_io_ptr, png_rw_ptr write_data_fn,
-        png_flush_ptr output_flush_fn);
-
-    voidp read_io_ptr = png_get_io_ptr(read_ptr);
-    voidp write_io_ptr = png_get_io_ptr(write_ptr);
-
-The replacement I/O functions must have prototypes as follows:
-
-    void user_read_data(png_structp png_ptr,
-        png_bytep data, png_size_t length);
-
-    void user_write_data(png_structp png_ptr,
-        png_bytep data, png_size_t length);
-
-    void user_flush_data(png_structp png_ptr);
-
-The user_read_data() function is responsible for detecting and
-handling end-of-data errors.
-
-Supplying NULL for the read, write, or flush functions sets them back
-to using the default C stream functions, which expect the io_ptr to
-point to a standard *FILE structure.  It is probably a mistake
-to use NULL for one of write_data_fn and output_flush_fn but not both
-of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
-It is an error to read from a write stream, and vice versa.
-
-Error handling in libpng is done through png_error() and png_warning().
-Errors handled through png_error() are fatal, meaning that png_error()
-should never return to its caller.  Currently, this is handled via
-setjmp() and longjmp() (unless you have compiled libpng with
-PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
-but you could change this to do things like exit() if you should wish,
-as long as your function does not return.
-
-On non-fatal errors, png_warning() is called
-to print a warning message, and then control returns to the calling code.
-By default png_error() and png_warning() print a message on stderr via
-fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
-(because you don't want the messages) or PNG_NO_STDIO defined (because
-fprintf() isn't available).  If you wish to change the behavior of the error
-functions, you will need to set up your own message callbacks.  These
-functions are normally supplied at the time that the png_struct is created.
-It is also possible to redirect errors and warnings to your own replacement
-functions after png_create_*_struct() has been called by calling:
-
-    png_set_error_fn(png_structp png_ptr,
-        png_voidp error_ptr, png_error_ptr error_fn,
-        png_error_ptr warning_fn);
-
-    png_voidp error_ptr = png_get_error_ptr(png_ptr);
-
-If NULL is supplied for either error_fn or warning_fn, then the libpng
-default function will be used, calling fprintf() and/or longjmp() if a
-problem is encountered.  The replacement error functions should have
-parameters as follows:
-
-    void user_error_fn(png_structp png_ptr,
-        png_const_charp error_msg);
-
-    void user_warning_fn(png_structp png_ptr,
-        png_const_charp warning_msg);
-
-The motivation behind using setjmp() and longjmp() is the C++ throw and
-catch exception handling methods.  This makes the code much easier to write,
-as there is no need to check every return code of every function call.
-However, there are some uncertainties about the status of local variables
-after a longjmp, so the user may want to be careful about doing anything
-after setjmp returns non-zero besides returning itself.  Consult your
-compiler documentation for more details.  For an alternative approach, you
-may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net),
-which is illustrated in pngvalid.c and in contrib/visupng.
-
-Beginning in libpng-1.4.0, the png_set_benign_errors() API became available.
-You can use this to handle certain errors (normally handled as errors)
-as warnings.
-
-    png_set_benign_errors (png_ptr, int allowed);
-
-    allowed: 0: treat png_benign_error() as an error.
-             1: treat png_benign_error() as a warning.
-
-As of libpng-1.6.0, the default condition is to treat benign errors as
-warnings while reading and as errors while writing.
-
-Custom chunks
-
-If you need to read or write custom chunks, you may need to get deeper
-into the libpng code.  The library now has mechanisms for storing
-and writing chunks of unknown type; you can even declare callbacks
-for custom chunks.  However, this may not be good enough if the
-library code itself needs to know about interactions between your
-chunk and existing `intrinsic' chunks.
-
-If you need to write a new intrinsic chunk, first read the PNG
-specification. Acquire a first level of understanding of how it works.
-Pay particular attention to the sections that describe chunk names,
-and look at how other chunks were designed, so you can do things
-similarly.  Second, check out the sections of libpng that read and
-write chunks.  Try to find a chunk that is similar to yours and use
-it as a template.  More details can be found in the comments inside
-the code.  It is best to handle private or unknown chunks in a generic method,
-via callback functions, instead of by modifying libpng functions. This
-is illustrated in pngtest.c, which uses a callback function to handle a
-private "vpAg" chunk and the new "sTER" chunk, which are both unknown to
-libpng.
-
-If you wish to write your own transformation for the data, look through
-the part of the code that does the transformations, and check out some of
-the simpler ones to get an idea of how they work.  Try to find a similar
-transformation to the one you want to add and copy off of it.  More details
-can be found in the comments inside the code itself.
-
-Configuring for gui/windowing platforms:
-
-You will need to write new error and warning functions that use the GUI
-interface, as described previously, and set them to be the error and
-warning functions at the time that png_create_*_struct() is called,
-in order to have them available during the structure initialization.
-They can be changed later via png_set_error_fn().  On some compilers,
-you may also have to change the memory allocators (png_malloc, etc.).
-
-Configuring zlib:
-
-There are special functions to configure the compression.  Perhaps the
-most useful one changes the compression level, which currently uses
-input compression values in the range 0 - 9.  The library normally
-uses the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests
-have shown that for a large majority of images, compression values in
-the range 3-6 compress nearly as well as higher levels, and do so much
-faster.  For online applications it may be desirable to have maximum speed
-(Z_BEST_SPEED = 1).  With versions of zlib after v0.99, you can also
-specify no compression (Z_NO_COMPRESSION = 0), but this would create
-files larger than just storing the raw bitmap.  You can specify the
-compression level by calling:
-
-    #include zlib.h
-    png_set_compression_level(png_ptr, level);
-
-Another useful one is to reduce the memory level used by the library.
-The memory level defaults to 8, but it can be lowered if you are
-short on memory (running DOS, for example, where you only have 640K).
-Note that the memory level does have an effect on compression; among
-other things, lower levels will result in sections of incompressible
-data being emitted in smaller stored blocks, with a correspondingly
-larger relative overhead of up to 15% in the worst case.
-
-    #include zlib.h
-    png_set_compression_mem_level(png_ptr, level);
-
-The other functions are for configuring zlib.  They are not recommended
-for normal use and may result in writing an invalid PNG file.  See
-zlib.h for more information on what these mean.
-
-    #include zlib.h
-    png_set_compression_strategy(png_ptr,
-        strategy);
-
-    png_set_compression_window_bits(png_ptr,
-        window_bits);
-
-    png_set_compression_method(png_ptr, method);
-
-This controls the size of the IDAT chunks (default 8192):
-
-    png_set_compression_buffer_size(png_ptr, size);
-
-As of libpng version 1.5.4, additional APIs became
-available to set these separately for non-IDAT
-compressed chunks such as zTXt, iTXt, and iCCP:
-
-    #include zlib.h
-    #if PNG_LIBPNG_VER >= 10504
-    png_set_text_compression_level(png_ptr, level);
-
-    png_set_text_compression_mem_level(png_ptr, level);
-
-    png_set_text_compression_strategy(png_ptr,
-        strategy);
-
-    png_set_text_compression_window_bits(png_ptr,
-        window_bits);
-
-    png_set_text_compression_method(png_ptr, method);
-    #endif
-
-Controlling row filtering
-
-If you want to control whether libpng uses filtering or not, which
-filters are used, and how it goes about picking row filters, you
-can call one of these functions.  The selection and configuration
-of row filters can have a significant impact on the size and
-encoding speed and a somewhat lesser impact on the decoding speed
-of an image.  Filtering is enabled by default for RGB and grayscale
-images (with and without alpha), but not for paletted images nor
-for any images with bit depths less than 8 bits/pixel.
-
-The 'method' parameter sets the main filtering method, which is
-currently only '0' in the PNG 1.2 specification.  The 'filters'
-parameter sets which filter(s), if any, should be used for each
-scanline.  Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
-to turn filtering on and off, respectively.
-
-Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
-PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
-ORed together with '|' to specify one or more filters to use.
-These filters are described in more detail in the PNG specification.
-If you intend to change the filter type during the course of writing
-the image, you should start with flags set for all of the filters
-you intend to use so that libpng can initialize its internal
-structures appropriately for all of the filter types.  (Note that this
-means the first row must always be adaptively filtered, because libpng
-currently does not allocate the filter buffers until png_write_row()
-is called for the first time.)
-
-    filters = PNG_FILTER_NONE | PNG_FILTER_SUB
-              PNG_FILTER_UP | PNG_FILTER_AVG |
-              PNG_FILTER_PAETH | PNG_ALL_FILTERS;
-
-    png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
-       filters);
-              The second parameter can also be
-              PNG_INTRAPIXEL_DIFFERENCING if you are
-              writing a PNG to be embedded in a MNG
-              datastream.  This parameter must be the
-              same as the value of filter_method used
-              in png_set_IHDR().
-
-It is also possible to influence how libpng chooses from among the
-available filters.  This is done in one or both of two ways - by
-telling it how important it is to keep the same filter for successive
-rows, and by telling it the relative computational costs of the filters.
-
-    double weights[3] = {1.5, 1.3, 1.1},
-       costs[PNG_FILTER_VALUE_LAST] =
-       {1.0, 1.3, 1.3, 1.5, 1.7};
-
-    png_set_filter_heuristics(png_ptr,
-       PNG_FILTER_HEURISTIC_WEIGHTED, 3,
-       weights, costs);
-
-The weights are multiplying factors that indicate to libpng that the
-row filter should be the same for successive rows unless another row filter
-is that many times better than the previous filter.  In the above example,
-if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
-"sum of absolute differences" 1.5 x 1.3 times higher than other filters
-and still be chosen, while the NONE filter could have a sum 1.1 times
-higher than other filters and still be chosen.  Unspecified weights are
-taken to be 1.0, and the specified weights should probably be declining
-like those above in order to emphasize recent filters over older filters.
-
-The filter costs specify for each filter type a relative decoding cost
-to be considered when selecting row filters.  This means that filters
-with higher costs are less likely to be chosen over filters with lower
-costs, unless their "sum of absolute differences" is that much smaller.
-The costs do not necessarily reflect the exact computational speeds of
-the various filters, since this would unduly influence the final image
-size.
-
-Note that the numbers above were invented purely for this example and
-are given only to help explain the function usage.  Little testing has
-been done to find optimum values for either the costs or the weights.
-
-Requesting debug printout
-
-The macro definition PNG_DEBUG can be used to request debugging
-printout.  Set it to an integer value in the range 0 to 3.  Higher
-numbers result in increasing amounts of debugging information.  The
-information is printed to the "stderr" file, unless another file
-name is specified in the PNG_DEBUG_FILE macro definition.
-
-When PNG_DEBUG > 0, the following functions (macros) become available:
-
-   png_debug(level, message)
-   png_debug1(level, message, p1)
-   png_debug2(level, message, p1, p2)
-
-in which "level" is compared to PNG_DEBUG to decide whether to print
-the message, "message" is the formatted string to be printed,
-and p1 and p2 are parameters that are to be embedded in the string
-according to printf-style formatting directives.  For example,
-
-   png_debug1(2, "foo=%d", foo);
-
-is expanded to
-
-   if (PNG_DEBUG > 2)
-      fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
-
-When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
-can still use PNG_DEBUG to control your own debugging:
-
-   #ifdef PNG_DEBUG
-       fprintf(stderr, ...
-   #endif
-
-When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
-having level = 0 will be printed.  There aren't any such statements in
-this version of libpng, but if you insert some they will be printed.
-
-VII.  MNG support
-
-The MNG specification (available at http://www.libpng.org/pub/mng) allows
-certain extensions to PNG for PNG images that are embedded in MNG datastreams.
-Libpng can support some of these extensions.  To enable them, use the
-png_permit_mng_features() function:
-
-   feature_set = png_permit_mng_features(png_ptr, mask)
-
-   mask is a png_uint_32 containing the bitwise OR of the
-        features you want to enable.  These include
-        PNG_FLAG_MNG_EMPTY_PLTE
-        PNG_FLAG_MNG_FILTER_64
-        PNG_ALL_MNG_FEATURES
-
-   feature_set is a png_uint_32 that is the bitwise AND of
-      your mask with the set of MNG features that is
-      supported by the version of libpng that you are using.
-
-It is an error to use this function when reading or writing a standalone
-PNG file with the PNG 8-byte signature.  The PNG datastream must be wrapped
-in a MNG datastream.  As a minimum, it must have the MNG 8-byte signature
-and the MHDR and MEND chunks.  Libpng does not provide support for these
-or any other MNG chunks; your application must provide its own support for
-them.  You may wish to consider using libmng (available at
-http://www.libmng.com) instead.
-
-VIII.  Changes to Libpng from version 0.88
-
-It should be noted that versions of libpng later than 0.96 are not
-distributed by the original libpng author, Guy Schalnat, nor by
-Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
-distributed versions 0.89 through 0.96, but rather by another member
-of the original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas are
-still alive and well, but they have moved on to other things.
-
-The old libpng functions png_read_init(), png_write_init(),
-png_info_init(), png_read_destroy(), and png_write_destroy() have been
-moved to PNG_INTERNAL in version 0.95 to discourage their use.  These
-functions will be removed from libpng version 1.4.0.
-
-The preferred method of creating and initializing the libpng structures is
-via the png_create_read_struct(), png_create_write_struct(), and
-png_create_info_struct() because they isolate the size of the structures
-from the application, allow version error checking, and also allow the
-use of custom error handling routines during the initialization, which
-the old functions do not.  The functions png_read_destroy() and
-png_write_destroy() do not actually free the memory that libpng
-allocated for these structs, but just reset the data structures, so they
-can be used instead of png_destroy_read_struct() and
-png_destroy_write_struct() if you feel there is too much system overhead
-allocating and freeing the png_struct for each image read.
-
-Setting the error callbacks via png_set_message_fn() before
-png_read_init() as was suggested in libpng-0.88 is no longer supported
-because this caused applications that do not use custom error functions
-to fail if the png_ptr was not initialized to zero.  It is still possible
-to set the error callbacks AFTER png_read_init(), or to change them with
-png_set_error_fn(), which is essentially the same function, but with a new
-name to force compilation errors with applications that try to use the old
-method.
-
-Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6;
-however, iTXt support was not enabled by default.
-
-Starting with version 1.0.7, you can find out which version of the library
-you are using at run-time:
-
-   png_uint_32 libpng_vn = png_access_version_number();
-
-The number libpng_vn is constructed from the major version, minor
-version with leading zero, and release number with leading zero,
-(e.g., libpng_vn for version 1.0.7 is 10007).
-
-Note that this function does not take a png_ptr, so you can call it
-before you've created one.
-
-You can also check which version of png.h you used when compiling your
-application:
-
-   png_uint_32 application_vn = PNG_LIBPNG_VER;
-
-IX.  Changes to Libpng from version 1.0.x to 1.2.x
-
-Support for user memory management was enabled by default.  To
-accomplish this, the functions png_create_read_struct_2(),
-png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
-png_malloc_default(), and png_free_default() were added.
-
-Support for the iTXt chunk has been enabled by default as of
-version 1.2.41.
-
-Support for certain MNG features was enabled.
-
-Support for numbered error messages was added.  However, we never got
-around to actually numbering the error messages.  The function
-png_set_strip_error_numbers() was added (Note: the prototype for this
-function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
-builds of libpng-1.2.15.  It was restored in libpng-1.2.36).
-
-The png_malloc_warn() function was added at libpng-1.2.3.  This issues
-a png_warning and returns NULL instead of aborting when it fails to
-acquire the requested memory allocation.
-
-Support for setting user limits on image width and height was enabled
-by default.  The functions png_set_user_limits(), png_get_user_width_max(),
-and png_get_user_height_max() were added at libpng-1.2.6.
-
-The png_set_add_alpha() function was added at libpng-1.2.7.
-
-The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
-Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
-tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
-deprecated.
-
-A number of macro definitions in support of runtime selection of
-assembler code features (especially Intel MMX code support) were
-added at libpng-1.2.0:
-
-    PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
-    PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
-    PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
-    PNG_ASM_FLAG_MMX_READ_INTERLACE
-    PNG_ASM_FLAG_MMX_READ_FILTER_SUB
-    PNG_ASM_FLAG_MMX_READ_FILTER_UP
-    PNG_ASM_FLAG_MMX_READ_FILTER_AVG
-    PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
-    PNG_ASM_FLAGS_INITIALIZED
-    PNG_MMX_READ_FLAGS
-    PNG_MMX_FLAGS
-    PNG_MMX_WRITE_FLAGS
-    PNG_MMX_FLAGS
-
-We added the following functions in support of runtime
-selection of assembler code features:
-
-    png_get_mmx_flagmask()
-    png_set_mmx_thresholds()
-    png_get_asm_flags()
-    png_get_mmx_bitdepth_threshold()
-    png_get_mmx_rowbytes_threshold()
-    png_set_asm_flags()
-
-We replaced all of these functions with simple stubs in libpng-1.2.20,
-when the Intel assembler code was removed due to a licensing issue.
-
-These macros are deprecated:
-
-    PNG_READ_TRANSFORMS_NOT_SUPPORTED
-    PNG_PROGRESSIVE_READ_NOT_SUPPORTED
-    PNG_NO_SEQUENTIAL_READ_SUPPORTED
-    PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
-    PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
-    PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
-
-They have been replaced, respectively, by:
-
-    PNG_NO_READ_TRANSFORMS
-    PNG_NO_PROGRESSIVE_READ
-    PNG_NO_SEQUENTIAL_READ
-    PNG_NO_WRITE_TRANSFORMS
-    PNG_NO_READ_ANCILLARY_CHUNKS
-    PNG_NO_WRITE_ANCILLARY_CHUNKS
-
-PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been
-deprecated since libpng-1.0.16 and libpng-1.2.6.
-
-The function
-    png_check_sig(sig, num)
-was replaced with
-    !png_sig_cmp(sig, 0, num)
-It has been deprecated since libpng-0.90.
-
-The function
-    png_set_gray_1_2_4_to_8()
-which also expands tRNS to alpha was replaced with
-    png_set_expand_gray_1_2_4_to_8()
-which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
-
-X.  Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
-
-Private libpng prototypes and macro definitions were moved from
-png.h and pngconf.h into a new pngpriv.h header file.
-
-Functions png_set_benign_errors(), png_benign_error(), and
-png_chunk_benign_error() were added.
-
-Support for setting the maximum amount of memory that the application
-will allocate for reading chunks was added, as a security measure.
-The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
-were added to the library.
-
-We implemented support for I/O states by adding png_ptr member io_state
-and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
-
-We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
-input transforms.
-
-Checking for and reporting of errors in the IHDR chunk is more thorough.
-
-Support for global arrays was removed, to improve thread safety.
-
-Some obsolete/deprecated macros and functions have been removed.
-
-Typecasted NULL definitions such as
-   #define png_voidp_NULL            (png_voidp)NULL
-were eliminated.  If you used these in your application, just use
-NULL instead.
-
-The png_struct and info_struct members "trans" and "trans_values" were
-changed to "trans_alpha" and "trans_color", respectively.
-
-The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
-were removed.
-
-The PNG_1_0_X and PNG_1_2_X macros were eliminated.
-
-The PNG_LEGACY_SUPPORTED macro was eliminated.
-
-Many WIN32_WCE #ifdefs were removed.
-
-The functions png_read_init(info_ptr), png_write_init(info_ptr),
-png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
-have been removed.  They have been deprecated since libpng-0.95.
-
-The png_permit_empty_plte() was removed. It has been deprecated
-since libpng-1.0.9.  Use png_permit_mng_features() instead.
-
-We removed the obsolete stub functions png_get_mmx_flagmask(),
-png_set_mmx_thresholds(), png_get_asm_flags(),
-png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
-png_set_asm_flags(), and png_mmx_supported()
-
-We removed the obsolete png_check_sig(), png_memcpy_check(), and
-png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
-and memset(), respectively.
-
-The function png_set_gray_1_2_4_to_8() was removed. It has been
-deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
-png_set_expand_gray_1_2_4_to_8() because the former function also
-expanded any tRNS chunk to an alpha channel.
-
-Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32
-were added and are used by default instead of the corresponding
-functions. Unfortunately,
-from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
-function) incorrectly returned a value of type png_uint_32.
-
-We changed the prototype for png_malloc() from
-    png_malloc(png_structp png_ptr, png_uint_32 size)
-to
-    png_malloc(png_structp png_ptr, png_alloc_size_t size)
-
-This also applies to the prototype for the user replacement malloc_fn().
-
-The png_calloc() function was added and is used in place of
-of "png_malloc(); memset();" except in the case in png_read_png()
-where the array consists of pointers; in this case a "for" loop is used
-after the png_malloc() to set the pointers to NULL, to give robust.
-behavior in case the application runs out of memory part-way through
-the process.
-
-We changed the prototypes of png_get_compression_buffer_size() and
-png_set_compression_buffer_size() to work with png_size_t instead of
-png_uint_32.
-
-Support for numbered error messages was removed by default, since we
-never got around to actually numbering the error messages. The function
-png_set_strip_error_numbers() was removed from the library by default.
-
-The png_zalloc() and png_zfree() functions are no longer exported.
-The png_zalloc() function no longer zeroes out the memory that it
-allocates.  Applications that called png_zalloc(png_ptr, number, size)
-can call png_calloc(png_ptr, number*size) instead, and can call
-png_free() instead of png_zfree().
-
-Support for dithering was disabled by default in libpng-1.4.0, because
-it has not been well tested and doesn't actually "dither".
-The code was not
-removed, however, and could be enabled by building libpng with
-PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this support
-was re-enabled, but the function was renamed png_set_quantize() to
-reflect more accurately what it actually does.  At the same time,
-the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
-PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED
-was renamed to PNG_READ_QUANTIZE_SUPPORTED.
-
-We removed the trailing '.' from the warning and error messages.
-
-XI.  Changes to Libpng from version 1.4.x to 1.5.x
-
-From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
-function) incorrectly returned a value of type png_uint_32.
-The incorrect macro was removed from libpng-1.4.5.
-
-Checking for invalid palette index on write was added at libpng
-1.5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
-a benign error.  This is enabled by default because this condition is an
-error according to the PNG specification, Clause 11.3.2, but the error can
-be ignored in each png_ptr with
-
-   png_set_check_for_invalid_index(png_ptr, allowed);
-
-      allowed  - one of
-                 0: disable benign error (accept the
-                    invalid data without warning).
-                 1: enable benign error (treat the
-                    invalid data as an error or a
-                    warning).
-
-If the error is ignored, or if png_benign_error() treats it as a warning,
-any invalid pixels are decoded as opaque black by the decoder and written
-as-is by the encoder.
-
-Retrieving the maximum palette index found was added at libpng-1.5.15.
-This statement must appear after png_read_png() or png_read_image() while
-reading, and after png_write_png() or png_write_image() while writing.
-
-   int max_palette = png_get_palette_max(png_ptr, info_ptr);
-
-This will return the maximum palette index found in the image, or "-1" if
-the palette was not checked, or "0" if no palette was found.  Note that this
-does not account for any palette index used by ancillary chunks such as the
-bKGD chunk; you must check those separately to determine the maximum
-palette index actually used.
-
-There are no substantial API changes between the non-deprecated parts of
-the 1.4.5 API and the 1.5.0 API; however, the ability to directly access
-members of the main libpng control structures, png_struct and png_info,
-deprecated in earlier versions of libpng, has been completely removed from
-libpng 1.5.
-
-We no longer include zlib.h in png.h.  The include statement has been moved
-to pngstruct.h, where it is not accessible by applications. Applications that
-need access to information in zlib.h will need to add the '#include "zlib.h"'
-directive.  It does not matter whether this is placed prior to or after
-the '"#include png.h"' directive.
-
-The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used
-and were removed.
-
-We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
-macros into a private header file (pngpriv.h) that is not accessible to
-applications.
-
-In png_get_iCCP, the type of "profile" was changed from png_charpp
-to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.
-
-There are changes of form in png.h, including new and changed macros to
-declare parts of the API.  Some API functions with arguments that are
-pointers to data not modified within the function have been corrected to
-declare these arguments with PNG_CONST.
-
-Much of the internal use of C macros to control the library build has also
-changed and some of this is visible in the exported header files, in
-particular the use of macros to control data and API elements visible
-during application compilation may require significant revision to
-application code.  (It is extremely rare for an application to do this.)
-
-Any program that compiled against libpng 1.4 and did not use deprecated
-features or access internal library structures should compile and work
-against libpng 1.5, except for the change in the prototype for
-png_get_iCCP() and png_set_iCCP() API functions mentioned above.
-
-libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of
-interlaced images.  The macros return the number of rows and columns in
-each pass and information that can be used to de-interlace and (if
-absolutely necessary) interlace an image.
-
-libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This API calls
-the application-provided png_longjmp_ptr on the internal, but application
-initialized, longjmp buffer.  It is provided as a convenience to avoid
-the need to use the png_jmpbuf macro, which had the unnecessary side
-effect of resetting the internal png_longjmp_ptr value.
-
-libpng 1.5.0 includes a complete fixed point API.  By default this is
-present along with the corresponding floating point API.  In general the
-fixed point API is faster and smaller than the floating point one because
-the PNG file format used fixed point, not floating point.  This applies
-even if the library uses floating point in internal calculations.  A new
-macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library
-uses floating point arithmetic (the default) or fixed point arithmetic
-internally for performance critical calculations such as gamma correction.
-In some cases, the gamma calculations may produce slightly different
-results.  This has changed the results in png_rgb_to_gray and in alpha
-composition (png_set_background for example). This applies even if the
-original image was already linear (gamma == 1.0) and, therefore, it is
-not necessary to linearize the image.  This is because libpng has *not*
-been changed to optimize that case correctly, yet.
-
-Fixed point support for the sCAL chunk comes with an important caveat;
-the sCAL specification uses a decimal encoding of floating point values
-and the accuracy of PNG fixed point values is insufficient for
-representation of these values. Consequently a "string" API
-(png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading
-arbitrary sCAL chunks in the absence of either the floating point API or
-internal floating point calculations.  Starting with libpng-1.5.0, both
-of these functions are present when PNG_sCAL_SUPPORTED is defined.  Prior
-to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED
-being defined and PNG_FLOATING_POINT_SUPPORTED not being defined.
-
-Applications no longer need to include the optional distribution header
-file pngusr.h or define the corresponding macros during application
-build in order to see the correct variant of the libpng API.  From 1.5.0
-application code can check for the corresponding _SUPPORTED macro:
-
-#ifdef PNG_INCH_CONVERSIONS_SUPPORTED
-   /* code that uses the inch conversion APIs. */
-#endif
-
-This macro will only be defined if the inch conversion functions have been
-compiled into libpng.  The full set of macros, and whether or not support
-has been compiled in, are available in the header file pnglibconf.h.
-This header file is specific to the libpng build.  Notice that prior to
-1.5.0 the _SUPPORTED macros would always have the default definition unless
-reset by pngusr.h or by explicit settings on the compiler command line.
-These settings may produce compiler warnings or errors in 1.5.0 because
-of macro redefinition.
-
-Applications can now choose whether to use these macros or to call the
-corresponding function by defining PNG_USE_READ_MACROS or
-PNG_NO_USE_READ_MACROS before including png.h.  Notice that this is
-only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0
-will lead to a link failure.
-
-Prior to libpng-1.5.4, the zlib compressor used the same set of parameters
-when compressing the IDAT data and textual data such as zTXt and iCCP.
-In libpng-1.5.4 we reinitialized the zlib stream for each type of data.
-We added five png_set_text_*() functions for setting the parameters to
-use with textual data.
-
-Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
-option was off by default, and slightly inaccurate scaling occurred.
-This option can no longer be turned off, and the choice of accurate
-or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8()
-API for accurate scaling or the old png_set_strip_16_to_8() API for simple
-chopping.  In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
-macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8
-macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two
-png_set_*_16_to_8() functions separately.
-
-Prior to libpng-1.5.4, the png_set_user_limits() function could only be
-used to reduce the width and height limits from the value of
-PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
-that it could be used to override them.  Now this function will reduce or
-increase the limits.
-
-Starting in libpng-1.5.10, the user limits can be set en masse with the
-configuration option PNG_SAFE_LIMITS_SUPPORTED.  If this option is enabled,
-a set of "safe" limits is applied in pngpriv.h.  These can be overridden by
-application calls to png_set_user_limits(), png_set_user_chunk_cache_max(),
-and/or png_set_user_malloc_max() that increase or decrease the limits.  Also,
-in libpng-1.5.10 the default width and height limits were increased
-from 1,000,000 to 0x7ffffff (i.e., made unlimited).  Therefore, the
-limits are now
-                               default      safe
-   png_user_width_max        0x7fffffff    1,000,000
-   png_user_height_max       0x7fffffff    1,000,000
-   png_user_chunk_cache_max  0 (unlimited)   128
-   png_user_chunk_malloc_max 0 (unlimited) 8,000,000
-
-The png_set_option() function (and the "options" member of the png struct) was
-added to libpng-1.5.15.
-
-The library now supports a complete fixed point implementation and can
-thus be used on systems that have no floating point support or very
-limited or slow support.  Previously gamma correction, an essential part
-of complete PNG support, required reasonably fast floating point.
-
-As part of this the choice of internal implementation has been made
-independent of the choice of fixed versus floating point APIs and all the
-missing fixed point APIs have been implemented.
-
-The exact mechanism used to control attributes of API functions has
-changed, as described in the INSTALL file.
-
-A new test program, pngvalid, is provided in addition to pngtest.
-pngvalid validates the arithmetic accuracy of the gamma correction
-calculations and includes a number of validations of the file format.
-A subset of the full range of tests is run when "make check" is done
-(in the 'configure' build.)  pngvalid also allows total allocated memory
-usage to be evaluated and performs additional memory overwrite validation.
-
-Many changes to individual feature macros have been made. The following
-are the changes most likely to be noticed by library builders who
-configure libpng:
-
-1) All feature macros now have consistent naming:
-
-#define PNG_NO_feature turns the feature off
-#define PNG_feature_SUPPORTED turns the feature on
-
-pnglibconf.h contains one line for each feature macro which is either:
-
-#define PNG_feature_SUPPORTED
-
-if the feature is supported or:
-
-/*#undef PNG_feature_SUPPORTED*/
-
-if it is not.  Library code consistently checks for the 'SUPPORTED' macro.
-It does not, and libpng applications should not, check for the 'NO' macro
-which will not normally be defined even if the feature is not supported.
-The 'NO' macros are only used internally for setting or not setting the
-corresponding 'SUPPORTED' macros.
-
-Compatibility with the old names is provided as follows:
-
-PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED
-
-And the following definitions disable the corresponding feature:
-
-PNG_SETJMP_NOT_SUPPORTED disables SETJMP
-PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
-PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
-PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
-PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS
-PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS
-
-Library builders should remove use of the above, inconsistent, names.
-
-2) Warning and error message formatting was previously conditional on
-the STDIO feature. The library has been changed to use the
-CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled
-the library no longer uses the printf(3) functions, even though the
-default read/write implementations use (FILE) style stdio.h functions.
-
-3) Three feature macros now control the fixed/floating point decisions:
-
-PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs
-
-PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in
-practice these are normally required internally anyway (because the PNG
-file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
-merely stops the function from being exported.
-
-PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
-point implementation or the fixed point one.  Typically the fixed point
-implementation is larger and slower than the floating point implementation
-on a system that supports floating point; however, it may be faster on a
-system which lacks floating point hardware and therefore uses a software
-emulation.
-
-4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.  This allows the
-functions to read and write ints to be disabled independently of
-PNG_USE_READ_MACROS, which allows libpng to be built with the functions
-even though the default is to use the macros - this allows applications
-to choose at app buildtime whether or not to use macros (previously
-impossible because the functions weren't in the default build.)
-
-XII.  Changes to Libpng from version 1.5.x to 1.6.x
-
-A "simplified API" has been added (see documentation in png.h and a simple
-example in contrib/examples/pngtopng.c).  The new publicly visible API
-includes the following:
-
-   macros:
-     PNG_FORMAT_*
-     PNG_IMAGE_*
-   structures:
-     png_control
-     png_image
-   read functions
-     png_image_begin_read_from_file()
-     png_image_begin_read_from_stdio()
-     png_image_begin_read_from_memory()
-     png_image_finish_read()
-     png_image_free()
-   write functions
-     png_image_write_to_file()
-     png_image_write_to_stdio()
-
-Starting with libpng-1.6.0, you can configure libpng to prefix all exported
-symbols, using the PNG_PREFIX macro.
-
-We no longer include string.h in png.h.  The include statement has been moved
-to pngpriv.h, where it is not accessible by applications.  Applications that
-need access to information in string.h must add an '#include <string.h>'
-directive.  It does not matter whether this is placed prior to or after
-the '#include "png.h"' directive.
-
-The following API are now DEPRECATED:
-   png_info_init_3()
-   png_convert_to_rfc1123() which has been replaced
-     with png_convert_to_rfc1123_buffer()
-   png_malloc_default()
-   png_free_default()
-   png_reset_zstream()
-
-The following have been removed:
-   png_get_io_chunk_name(), which has been replaced
-     with png_get_io_chunk_type().  The new
-     function returns a 32-bit integer instead of
-     a string.
-   The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
-     png_memset() macros are no longer used in the libpng sources and
-     have been removed.  These had already been made invisible to applications
-     (i.e., defined in the private pngpriv.h header file) since libpng-1.5.0.
-
-The signatures of many exported functions were changed, such that
-   png_structp became png_structrp or png_const_structrp
-   png_infop became png_inforp or png_const_inforp
-where "rp" indicates a "restricted pointer".
-
-Error detection in some chunks has improved; in particular the iCCP chunk
-reader now does pretty complete validation of the basic format.  Some bad
-profiles that were previously accepted are now accepted with a warning or
-rejected, depending upon the png_set_benign_errors() setting, in particular the
-very old broken Microsoft/HP 3144-byte sRGB profile.  Starting with
-libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by
-means of
-
-    #ifdef PNG_SKIP_sRGB_CHECK_PROFILE
-       png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
-           PNG_OPTION_ON);
-    #endif
-
-It's not a good idea to do this if you are using the "simplified API",
-which needs to be able to recognize an sRGB profile conveyed via the iCCP
-chunk.
-
-The PNG spec requirement that only grayscale profiles may appear in images
-with color type 0 or 4 and that even if the image only contains gray pixels,
-only RGB profiles may appear in images with color type 2, 3, or 6, is now
-enforced.  The sRGB chunk is allowed to appear in images with any color type
-and is interpreted by libpng to convey a one-tracer-curve profile or a
-three-tracer-curve profile as appropriate.
-
-Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained
-an empty language field or an empty translated keyword.  Both of these
-are allowed by the PNG specification, so these warnings are no longer issued.
-
-The library now issues an error if the application attempts to set a
-transform after it calls png_read_update_info() or if it attempts to call
-both png_read_update_info() and png_start_read_image() or to call either
-of them more than once.
-
-The default condition for benign_errors is now to treat benign errors as
-warnings while reading and as errors while writing.
-
-The library now issues a warning if both background processing and RGB to
-gray are used when gamma correction happens. As with previous versions of
-the library the results are numerically very incorrect in this case.
-
-There are some minor arithmetic changes in some transforms such as
-png_set_background(), that might be detected by certain regression tests.
-
-Unknown chunk handling has been improved internally, without any API change.
-This adds more correct option control of the unknown handling, corrects
-a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes
-it possible to skip IDAT chunks in the sequential reader.
-
-The machine-generated configure files are no longer included in branches
-libpng16 and later of the GIT repository.  They continue to be included
-in the tarball releases, however.
-
-Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT
-stream to set the size of the sliding window for reading instead of using the
-default 32-kbyte sliding window size.  It was discovered that there are
-hundreds of PNG files in the wild that have incorrect CMF bytes that caused
-zlib to issue the "invalid distance too far back" error and reject the file.
-Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions,
-provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes
-and using a 32-kbyte sliding window), by using
-
-    png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
-        PNG_OPTION_ON);
-
-and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while
-optimizing the CMF bytes in its IDAT chunk correctly.
-
-Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong
-length, which resulted in PNG files that cannot be read beyond the bad iTXt
-chunk.  This error was fixed in libpng-1.6.3, and a tool (called
-contrib/tools/png-fix-itxt) has been added to the libpng distribution.
-
-XIII.  Detecting libpng
-
-The png_get_io_ptr() function has been present since libpng-0.88, has never
-changed, and is unaffected by conditional compilation macros.  It is the
-best choice for use in configure scripts for detecting the presence of any
-libpng version since 0.88.  In an autoconf "configure.in" you could use
-
-    AC_CHECK_LIB(png, png_get_io_ptr, ...
-
-XV. Source code repository
-
-Since about February 2009, version 1.2.34, libpng has been under "git" source
-control.  The git repository was built from old libpng-x.y.z.tar.gz files
-going back to version 0.70.  You can access the git repository (read only)
-at
-
-    git://git.code.sf.net/p/libpng/code
-
-or you can browse it with a web browser by selecting the "code" button at
-
-    https://sourceforge.net/projects/libpng
-
-Patches can be sent to glennrp at users.sourceforge.net or to
-png-mng-implement at lists.sourceforge.net or you can upload them to
-the libpng bug tracker at
-
-    http://libpng.sourceforge.net
-
-We also accept patches built from the tar or zip distributions, and
-simple verbal discriptions of bug fixes, reported either to the
-SourceForge bug tracker, to the png-mng-implement at lists.sf.net
-mailing list, or directly to glennrp.
-
-XV. Coding style
-
-Our coding style is similar to the "Allman" style
-(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
-braces on separate lines:
-
-    if (condition)
-    {
-       action;
-    }
-
-    else if (another condition)
-    {
-       another action;
-    }
-
-The braces can be omitted from simple one-line actions:
-
-    if (condition)
-       return (0);
-
-We use 3-space indentation, except for continued statements which
-are usually indented the same as the first line of the statement
-plus four more spaces.
-
-For macro definitions we use 2-space indentation, always leaving the "#"
-in the first column.
-
-    #ifndef PNG_NO_FEATURE
-    #  ifndef PNG_FEATURE_SUPPORTED
-    #    define PNG_FEATURE_SUPPORTED
-    #  endif
-    #endif
-
-Comments appear with the leading "/*" at the same indentation as
-the statement that follows the comment:
-
-    /* Single-line comment */
-    statement;
-
-    /* This is a multiple-line
-     * comment.
-     */
-    statement;
-
-Very short comments can be placed after the end of the statement
-to which they pertain:
-
-    statement;    /* comment */
-
-We don't use C++ style ("//") comments. We have, however,
-used them in the past in some now-abandoned MMX assembler
-code.
-
-Functions and their curly braces are not indented, and
-exported functions are marked with PNGAPI:
-
- /* This is a public function that is visible to
-  * application programmers. It does thus-and-so.
-  */
- void PNGAPI
- png_exported_function(png_ptr, png_info, foo)
- {
-    body;
- }
-
-The return type and decorations are placed on a separate line
-ahead of the function name, as illustrated above.
-
-The prototypes for all exported functions appear in png.h,
-above the comment that says
-
-    /* Maintainer: Put new public prototypes here ... */
-
-We mark all non-exported functions with "/* PRIVATE */"":
-
- void /* PRIVATE */
- png_non_exported_function(png_ptr, png_info, foo)
- {
-    body;
- }
-
-The prototypes for non-exported functions (except for those in
-pngtest) appear in
-pngpriv.h
-above the comment that says
-
-  /* Maintainer: Put new private prototypes here ^ */
-
-We put a space after the "sizeof" operator and we omit the
-optional parentheses around its argument when the argument
-is an expression, not a type name, and we always enclose the
-sizeof operator, with its argument, in parentheses:
-
-  (sizeof (png_uint_32))
-  (sizeof array)
-
-Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as
-though it were a function.
-
-To avoid polluting the global namespace, the names of all exported
-functions and variables begin with "png_", and all publicly visible C
-preprocessor macros begin with "PNG".  We request that applications that
-use libpng *not* begin any of their own symbols with either of these strings.
-
-We put a space after each comma and after each semicolon
-in "for" statements, and we put spaces before and after each
-C binary operator and after "for" or "while", and before
-"?".  We don't put a space between a typecast and the expression
-being cast, nor do we put one between a function name and the
-left parenthesis that follows it:
-
-    for (i = 2; i > 0; --i)
-       y[i] = a(x) + (int)b;
-
-We prefer #ifdef and #ifndef to #if defined() and #if !defined()
-when there is only one macro being tested.  We always use parentheses
-with "defined".
-
-We prefer to express integers that are used as bit masks in hex format,
-with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100).
-
-We prefer to use underscores in variable names rather than camelCase, except
-for a few type names that we inherit from zlib.h.
-
-We prefer "if (something != 0)" and "if (something == 0)"
-over "if (something)" and if "(!something)", respectively.
-
-We do not use the TAB character for indentation in the C sources.
-
-Lines do not exceed 80 characters.
-
-Other rules can be inferred by inspecting the libpng source.
-
-XVI. Y2K Compliance in libpng
-
-June 12, 2014
-
-Since the PNG Development group is an ad-hoc body, we can't make
-an official declaration.
-
-This is your unofficial assurance that libpng from version 0.71 and
-upward through 1.6.12 are Y2K compliant.  It is my belief that earlier
-versions were also Y2K compliant.
-
-Libpng only has two year fields.  One is a 2-byte unsigned integer
-that will hold years up to 65535.  The other, which is deprecated,
-holds the date in text format, and will hold years up to 9999.
-
-The integer is
-    "png_uint_16 year" in png_time_struct.
-
-The string is
-    "char time_buffer[29]" in png_struct.  This is no longer used
-in libpng-1.6.x and will be removed from libpng-1.7.0.
-
-There are seven time-related functions:
-
-    png_convert_to_rfc_1123() in png.c
-      (formerly png_convert_to_rfc_1152() in error)
-    png_convert_from_struct_tm() in pngwrite.c, called
-      in pngwrite.c
-    png_convert_from_time_t() in pngwrite.c
-    png_get_tIME() in pngget.c
-    png_handle_tIME() in pngrutil.c, called in pngread.c
-    png_set_tIME() in pngset.c
-    png_write_tIME() in pngwutil.c, called in pngwrite.c
-
-All appear to handle dates properly in a Y2K environment.  The
-png_convert_from_time_t() function calls gmtime() to convert from system
-clock time, which returns (year - 1900), which we properly convert to
-the full 4-digit year.  There is a possibility that applications using
-libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
-function, or that they are incorrectly passing only a 2-digit year
-instead of "year - 1900" into the png_convert_from_struct_tm() function,
-but this is not under our control.  The libpng documentation has always
-stated that it works with 4-digit years, and the APIs have been
-documented as such.
-
-The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
-integer to hold the year, and can hold years as large as 65535.
-
-zlib, upon which libpng depends, is also Y2K compliant.  It contains
-no date-related code.
-
-
-   Glenn Randers-Pehrson
-   libpng maintainer
-   PNG Development Group