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diff --git a/png/libpng-manual.txt b/png/libpng-manual.txt deleted file mode 100644 index f79ca7e..0000000 --- a/png/libpng-manual.txt +++ /dev/null @@ -1,5330 +0,0 @@ -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 |