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diff --git a/jpg/structure.txt b/jpg/structure.txt deleted file mode 100644 index 44e48ca..0000000 --- a/jpg/structure.txt +++ /dev/null @@ -1,941 +0,0 @@ -IJG JPEG LIBRARY: SYSTEM ARCHITECTURE - -Copyright (C) 1991-2012, Thomas G. Lane, Guido Vollbeding. -This file is part of the Independent JPEG Group's software. -For conditions of distribution and use, see the accompanying README file. - - -This file provides an overview of the architecture of the IJG JPEG software; -that is, the functions of the various modules in the system and the interfaces -between modules. For more precise details about any data structure or calling -convention, see the include files and comments in the source code. - -We assume that the reader is already somewhat familiar with the JPEG standard. -The README file includes references for learning about JPEG. The file -libjpeg.txt describes the library from the viewpoint of an application -programmer using the library; it's best to read that file before this one. -Also, the file coderules.txt describes the coding style conventions we use. - -In this document, JPEG-specific terminology follows the JPEG standard: - A "component" means a color channel, e.g., Red or Luminance. - A "sample" is a single component value (i.e., one number in the image data). - A "coefficient" is a frequency coefficient (a DCT transform output number). - A "block" is an array of samples or coefficients. - An "MCU" (minimum coded unit) is an interleaved set of blocks of size - determined by the sampling factors, or a single block in a - noninterleaved scan. -We do not use the terms "pixel" and "sample" interchangeably. When we say -pixel, we mean an element of the full-size image, while a sample is an element -of the downsampled image. Thus the number of samples may vary across -components while the number of pixels does not. (This terminology is not used -rigorously throughout the code, but it is used in places where confusion would -otherwise result.) - - -*** System features *** - -The IJG distribution contains two parts: - * A subroutine library for JPEG compression and decompression. - * cjpeg/djpeg, two sample applications that use the library to transform - JFIF JPEG files to and from several other image formats. -cjpeg/djpeg are of no great intellectual complexity: they merely add a simple -command-line user interface and I/O routines for several uncompressed image -formats. This document concentrates on the library itself. - -We desire the library to be capable of supporting all JPEG baseline, extended -sequential, and progressive DCT processes. The library does not support the -hierarchical or lossless processes defined in the standard. - -Within these limits, any set of compression parameters allowed by the JPEG -spec should be readable for decompression. (We can be more restrictive about -what formats we can generate.) Although the system design allows for all -parameter values, some uncommon settings are not yet implemented and may -never be; nonintegral sampling ratios are the prime example. Furthermore, -we treat 8-bit vs. 12-bit data precision as a compile-time switch, not a -run-time option, because most machines can store 8-bit pixels much more -compactly than 12-bit. - -By itself, the library handles only interchange JPEG datastreams --- in -particular the widely used JFIF file format. The library can be used by -surrounding code to process interchange or abbreviated JPEG datastreams that -are embedded in more complex file formats. (For example, libtiff uses this -library to implement JPEG compression within the TIFF file format.) - -The library includes a substantial amount of code that is not covered by the -JPEG standard but is necessary for typical applications of JPEG. These -functions preprocess the image before JPEG compression or postprocess it after -decompression. They include colorspace conversion, downsampling/upsampling, -and color quantization. This code can be omitted if not needed. - -A wide range of quality vs. speed tradeoffs are possible in JPEG processing, -and even more so in decompression postprocessing. The decompression library -provides multiple implementations that cover most of the useful tradeoffs, -ranging from very-high-quality down to fast-preview operation. On the -compression side we have generally not provided low-quality choices, since -compression is normally less time-critical. It should be understood that the -low-quality modes may not meet the JPEG standard's accuracy requirements; -nonetheless, they are useful for viewers. - - -*** Portability issues *** - -Portability is an essential requirement for the library. The key portability -issues that show up at the level of system architecture are: - -1. Memory usage. We want the code to be able to run on PC-class machines -with limited memory. Images should therefore be processed sequentially (in -strips), to avoid holding the whole image in memory at once. Where a -full-image buffer is necessary, we should be able to use either virtual memory -or temporary files. - -2. Near/far pointer distinction. To run efficiently on 80x86 machines, the -code should distinguish "small" objects (kept in near data space) from -"large" ones (kept in far data space). This is an annoying restriction, but -fortunately it does not impact code quality for less brain-damaged machines, -and the source code clutter turns out to be minimal with sufficient use of -pointer typedefs. - -3. Data precision. We assume that "char" is at least 8 bits, "short" and -"int" at least 16, "long" at least 32. The code will work fine with larger -data sizes, although memory may be used inefficiently in some cases. However, -the JPEG compressed datastream must ultimately appear on external storage as a -sequence of 8-bit bytes if it is to conform to the standard. This may pose a -problem on machines where char is wider than 8 bits. The library represents -compressed data as an array of values of typedef JOCTET. If no data type -exactly 8 bits wide is available, custom data source and data destination -modules must be written to unpack and pack the chosen JOCTET datatype into -8-bit external representation. - - -*** System overview *** - -The compressor and decompressor are each divided into two main sections: -the JPEG compressor or decompressor proper, and the preprocessing or -postprocessing functions. The interface between these two sections is the -image data that the official JPEG spec regards as its input or output: this -data is in the colorspace to be used for compression, and it is downsampled -to the sampling factors to be used. The preprocessing and postprocessing -steps are responsible for converting a normal image representation to or from -this form. (Those few applications that want to deal with YCbCr downsampled -data can skip the preprocessing or postprocessing step.) - -Looking more closely, the compressor library contains the following main -elements: - - Preprocessing: - * Color space conversion (e.g., RGB to YCbCr). - * Edge expansion and downsampling. Optionally, this step can do simple - smoothing --- this is often helpful for low-quality source data. - JPEG proper: - * MCU assembly, DCT, quantization. - * Entropy coding (sequential or progressive, Huffman or arithmetic). - -In addition to these modules we need overall control, marker generation, -and support code (memory management & error handling). There is also a -module responsible for physically writing the output data --- typically -this is just an interface to fwrite(), but some applications may need to -do something else with the data. - -The decompressor library contains the following main elements: - - JPEG proper: - * Entropy decoding (sequential or progressive, Huffman or arithmetic). - * Dequantization, inverse DCT, MCU disassembly. - Postprocessing: - * Upsampling. Optionally, this step may be able to do more general - rescaling of the image. - * Color space conversion (e.g., YCbCr to RGB). This step may also - provide gamma adjustment [ currently it does not ]. - * Optional color quantization (e.g., reduction to 256 colors). - * Optional color precision reduction (e.g., 24-bit to 15-bit color). - [This feature is not currently implemented.] - -We also need overall control, marker parsing, and a data source module. -The support code (memory management & error handling) can be shared with -the compression half of the library. - -There may be several implementations of each of these elements, particularly -in the decompressor, where a wide range of speed/quality tradeoffs is very -useful. It must be understood that some of the best speedups involve -merging adjacent steps in the pipeline. For example, upsampling, color space -conversion, and color quantization might all be done at once when using a -low-quality ordered-dither technique. The system architecture is designed to -allow such merging where appropriate. - - -Note: it is convenient to regard edge expansion (padding to block boundaries) -as a preprocessing/postprocessing function, even though the JPEG spec includes -it in compression/decompression. We do this because downsampling/upsampling -can be simplified a little if they work on padded data: it's not necessary to -have special cases at the right and bottom edges. Therefore the interface -buffer is always an integral number of blocks wide and high, and we expect -compression preprocessing to pad the source data properly. Padding will occur -only to the next block (N-sample) boundary. In an interleaved-scan situation, -additional dummy blocks may be used to fill out MCUs, but the MCU assembly and -disassembly logic will create or discard these blocks internally. (This is -advantageous for speed reasons, since we avoid DCTing the dummy blocks. -It also permits a small reduction in file size, because the compressor can -choose dummy block contents so as to minimize their size in compressed form. -Finally, it makes the interface buffer specification independent of whether -the file is actually interleaved or not.) Applications that wish to deal -directly with the downsampled data must provide similar buffering and padding -for odd-sized images. - - -*** Poor man's object-oriented programming *** - -It should be clear by now that we have a lot of quasi-independent processing -steps, many of which have several possible behaviors. To avoid cluttering the -code with lots of switch statements, we use a simple form of object-style -programming to separate out the different possibilities. - -For example, two different color quantization algorithms could be implemented -as two separate modules that present the same external interface; at runtime, -the calling code will access the proper module indirectly through an "object". - -We can get the limited features we need while staying within portable C. -The basic tool is a function pointer. An "object" is just a struct -containing one or more function pointer fields, each of which corresponds to -a method name in real object-oriented languages. During initialization we -fill in the function pointers with references to whichever module we have -determined we need to use in this run. Then invocation of the module is done -by indirecting through a function pointer; on most machines this is no more -expensive than a switch statement, which would be the only other way of -making the required run-time choice. The really significant benefit, of -course, is keeping the source code clean and well structured. - -We can also arrange to have private storage that varies between different -implementations of the same kind of object. We do this by making all the -module-specific object structs be separately allocated entities, which will -be accessed via pointers in the master compression or decompression struct. -The "public" fields or methods for a given kind of object are specified by -a commonly known struct. But a module's initialization code can allocate -a larger struct that contains the common struct as its first member, plus -additional private fields. With appropriate pointer casting, the module's -internal functions can access these private fields. (For a simple example, -see jdatadst.c, which implements the external interface specified by struct -jpeg_destination_mgr, but adds extra fields.) - -(Of course this would all be a lot easier if we were using C++, but we are -not yet prepared to assume that everyone has a C++ compiler.) - -An important benefit of this scheme is that it is easy to provide multiple -versions of any method, each tuned to a particular case. While a lot of -precalculation might be done to select an optimal implementation of a method, -the cost per invocation is constant. For example, the upsampling step might -have a "generic" method, plus one or more "hardwired" methods for the most -popular sampling factors; the hardwired methods would be faster because they'd -use straight-line code instead of for-loops. The cost to determine which -method to use is paid only once, at startup, and the selection criteria are -hidden from the callers of the method. - -This plan differs a little bit from usual object-oriented structures, in that -only one instance of each object class will exist during execution. The -reason for having the class structure is that on different runs we may create -different instances (choose to execute different modules). You can think of -the term "method" as denoting the common interface presented by a particular -set of interchangeable functions, and "object" as denoting a group of related -methods, or the total shared interface behavior of a group of modules. - - -*** Overall control structure *** - -We previously mentioned the need for overall control logic in the compression -and decompression libraries. In IJG implementations prior to v5, overall -control was mostly provided by "pipeline control" modules, which proved to be -large, unwieldy, and hard to understand. To improve the situation, the -control logic has been subdivided into multiple modules. The control modules -consist of: - -1. Master control for module selection and initialization. This has two -responsibilities: - - 1A. Startup initialization at the beginning of image processing. - The individual processing modules to be used in this run are selected - and given initialization calls. - - 1B. Per-pass control. This determines how many passes will be performed - and calls each active processing module to configure itself - appropriately at the beginning of each pass. End-of-pass processing, - where necessary, is also invoked from the master control module. - - Method selection is partially distributed, in that a particular processing - module may contain several possible implementations of a particular method, - which it will select among when given its initialization call. The master - control code need only be concerned with decisions that affect more than - one module. - -2. Data buffering control. A separate control module exists for each - inter-processing-step data buffer. This module is responsible for - invoking the processing steps that write or read that data buffer. - -Each buffer controller sees the world as follows: - -input data => processing step A => buffer => processing step B => output data - | | | - ------------------ controller ------------------ - -The controller knows the dataflow requirements of steps A and B: how much data -they want to accept in one chunk and how much they output in one chunk. Its -function is to manage its buffer and call A and B at the proper times. - -A data buffer control module may itself be viewed as a processing step by a -higher-level control module; thus the control modules form a binary tree with -elementary processing steps at the leaves of the tree. - -The control modules are objects. A considerable amount of flexibility can -be had by replacing implementations of a control module. For example: -* Merging of adjacent steps in the pipeline is done by replacing a control - module and its pair of processing-step modules with a single processing- - step module. (Hence the possible merges are determined by the tree of - control modules.) -* In some processing modes, a given interstep buffer need only be a "strip" - buffer large enough to accommodate the desired data chunk sizes. In other - modes, a full-image buffer is needed and several passes are required. - The control module determines which kind of buffer is used and manipulates - virtual array buffers as needed. One or both processing steps may be - unaware of the multi-pass behavior. - -In theory, we might be able to make all of the data buffer controllers -interchangeable and provide just one set of implementations for all. In -practice, each one contains considerable special-case processing for its -particular job. The buffer controller concept should be regarded as an -overall system structuring principle, not as a complete description of the -task performed by any one controller. - - -*** Compression object structure *** - -Here is a sketch of the logical structure of the JPEG compression library: - - |-- Colorspace conversion - |-- Preprocessing controller --| - | |-- Downsampling -Main controller --| - | |-- Forward DCT, quantize - |-- Coefficient controller --| - |-- Entropy encoding - -This sketch also describes the flow of control (subroutine calls) during -typical image data processing. Each of the components shown in the diagram is -an "object" which may have several different implementations available. One -or more source code files contain the actual implementation(s) of each object. - -The objects shown above are: - -* Main controller: buffer controller for the subsampled-data buffer, which - holds the preprocessed input data. This controller invokes preprocessing to - fill the subsampled-data buffer, and JPEG compression to empty it. There is - usually no need for a full-image buffer here; a strip buffer is adequate. - -* Preprocessing controller: buffer controller for the downsampling input data - buffer, which lies between colorspace conversion and downsampling. Note - that a unified conversion/downsampling module would probably replace this - controller entirely. - -* Colorspace conversion: converts application image data into the desired - JPEG color space; also changes the data from pixel-interleaved layout to - separate component planes. Processes one pixel row at a time. - -* Downsampling: performs reduction of chroma components as required. - Optionally may perform pixel-level smoothing as well. Processes a "row - group" at a time, where a row group is defined as Vmax pixel rows of each - component before downsampling, and Vk sample rows afterwards (remember Vk - differs across components). Some downsampling or smoothing algorithms may - require context rows above and below the current row group; the - preprocessing controller is responsible for supplying these rows via proper - buffering. The downsampler is responsible for edge expansion at the right - edge (i.e., extending each sample row to a multiple of N samples); but the - preprocessing controller is responsible for vertical edge expansion (i.e., - duplicating the bottom sample row as needed to make a multiple of N rows). - -* Coefficient controller: buffer controller for the DCT-coefficient data. - This controller handles MCU assembly, including insertion of dummy DCT - blocks when needed at the right or bottom edge. When performing - Huffman-code optimization or emitting a multiscan JPEG file, this - controller is responsible for buffering the full image. The equivalent of - one fully interleaved MCU row of subsampled data is processed per call, - even when the JPEG file is noninterleaved. - -* Forward DCT and quantization: Perform DCT, quantize, and emit coefficients. - Works on one or more DCT blocks at a time. (Note: the coefficients are now - emitted in normal array order, which the entropy encoder is expected to - convert to zigzag order as necessary. Prior versions of the IJG code did - the conversion to zigzag order within the quantization step.) - -* Entropy encoding: Perform Huffman or arithmetic entropy coding and emit the - coded data to the data destination module. Works on one MCU per call. - For progressive JPEG, the same DCT blocks are fed to the entropy coder - during each pass, and the coder must emit the appropriate subset of - coefficients. - -In addition to the above objects, the compression library includes these -objects: - -* Master control: determines the number of passes required, controls overall - and per-pass initialization of the other modules. - -* Marker writing: generates JPEG markers (except for RSTn, which is emitted - by the entropy encoder when needed). - -* Data destination manager: writes the output JPEG datastream to its final - destination (e.g., a file). The destination manager supplied with the - library knows how to write to a stdio stream or to a memory buffer; - for other behaviors, the surrounding application may provide its own - destination manager. - -* Memory manager: allocates and releases memory, controls virtual arrays - (with backing store management, where required). - -* Error handler: performs formatting and output of error and trace messages; - determines handling of nonfatal errors. The surrounding application may - override some or all of this object's methods to change error handling. - -* Progress monitor: supports output of "percent-done" progress reports. - This object represents an optional callback to the surrounding application: - if wanted, it must be supplied by the application. - -The error handler, destination manager, and progress monitor objects are -defined as separate objects in order to simplify application-specific -customization of the JPEG library. A surrounding application may override -individual methods or supply its own all-new implementation of one of these -objects. The object interfaces for these objects are therefore treated as -part of the application interface of the library, whereas the other objects -are internal to the library. - -The error handler and memory manager are shared by JPEG compression and -decompression; the progress monitor, if used, may be shared as well. - - -*** Decompression object structure *** - -Here is a sketch of the logical structure of the JPEG decompression library: - - |-- Entropy decoding - |-- Coefficient controller --| - | |-- Dequantize, Inverse DCT -Main controller --| - | |-- Upsampling - |-- Postprocessing controller --| |-- Colorspace conversion - |-- Color quantization - |-- Color precision reduction - -As before, this diagram also represents typical control flow. The objects -shown are: - -* Main controller: buffer controller for the subsampled-data buffer, which - holds the output of JPEG decompression proper. This controller's primary - task is to feed the postprocessing procedure. Some upsampling algorithms - may require context rows above and below the current row group; when this - is true, the main controller is responsible for managing its buffer so as - to make context rows available. In the current design, the main buffer is - always a strip buffer; a full-image buffer is never required. - -* Coefficient controller: buffer controller for the DCT-coefficient data. - This controller handles MCU disassembly, including deletion of any dummy - DCT blocks at the right or bottom edge. When reading a multiscan JPEG - file, this controller is responsible for buffering the full image. - (Buffering DCT coefficients, rather than samples, is necessary to support - progressive JPEG.) The equivalent of one fully interleaved MCU row of - subsampled data is processed per call, even when the source JPEG file is - noninterleaved. - -* Entropy decoding: Read coded data from the data source module and perform - Huffman or arithmetic entropy decoding. Works on one MCU per call. - For progressive JPEG decoding, the coefficient controller supplies the prior - coefficients of each MCU (initially all zeroes), which the entropy decoder - modifies in each scan. - -* Dequantization and inverse DCT: like it says. Note that the coefficients - buffered by the coefficient controller have NOT been dequantized; we - merge dequantization and inverse DCT into a single step for speed reasons. - When scaled-down output is asked for, simplified DCT algorithms may be used - that need fewer coefficients and emit fewer samples per DCT block, not the - full 8x8. Works on one DCT block at a time. - -* Postprocessing controller: buffer controller for the color quantization - input buffer, when quantization is in use. (Without quantization, this - controller just calls the upsampler.) For two-pass quantization, this - controller is responsible for buffering the full-image data. - -* Upsampling: restores chroma components to full size. (May support more - general output rescaling, too. Note that if undersized DCT outputs have - been emitted by the DCT module, this module must adjust so that properly - sized outputs are created.) Works on one row group at a time. This module - also calls the color conversion module, so its top level is effectively a - buffer controller for the upsampling->color conversion buffer. However, in - all but the highest-quality operating modes, upsampling and color - conversion are likely to be merged into a single step. - -* Colorspace conversion: convert from JPEG color space to output color space, - and change data layout from separate component planes to pixel-interleaved. - Works on one pixel row at a time. - -* Color quantization: reduce the data to colormapped form, using either an - externally specified colormap or an internally generated one. This module - is not used for full-color output. Works on one pixel row at a time; may - require two passes to generate a color map. Note that the output will - always be a single component representing colormap indexes. In the current - design, the output values are JSAMPLEs, so an 8-bit compilation cannot - quantize to more than 256 colors. This is unlikely to be a problem in - practice. - -* Color reduction: this module handles color precision reduction, e.g., - generating 15-bit color (5 bits/primary) from JPEG's 24-bit output. - Not quite clear yet how this should be handled... should we merge it with - colorspace conversion??? - -Note that some high-speed operating modes might condense the entire -postprocessing sequence to a single module (upsample, color convert, and -quantize in one step). - -In addition to the above objects, the decompression library includes these -objects: - -* Master control: determines the number of passes required, controls overall - and per-pass initialization of the other modules. This is subdivided into - input and output control: jdinput.c controls only input-side processing, - while jdmaster.c handles overall initialization and output-side control. - -* Marker reading: decodes JPEG markers (except for RSTn). - -* Data source manager: supplies the input JPEG datastream. The source - manager supplied with the library knows how to read from a stdio stream - or from a memory buffer; for other behaviors, the surrounding application - may provide its own source manager. - -* Memory manager: same as for compression library. - -* Error handler: same as for compression library. - -* Progress monitor: same as for compression library. - -As with compression, the data source manager, error handler, and progress -monitor are candidates for replacement by a surrounding application. - - -*** Decompression input and output separation *** - -To support efficient incremental display of progressive JPEG files, the -decompressor is divided into two sections that can run independently: - -1. Data input includes marker parsing, entropy decoding, and input into the - coefficient controller's DCT coefficient buffer. Note that this - processing is relatively cheap and fast. - -2. Data output reads from the DCT coefficient buffer and performs the IDCT - and all postprocessing steps. - -For a progressive JPEG file, the data input processing is allowed to get -arbitrarily far ahead of the data output processing. (This occurs only -if the application calls jpeg_consume_input(); otherwise input and output -run in lockstep, since the input section is called only when the output -section needs more data.) In this way the application can avoid making -extra display passes when data is arriving faster than the display pass -can run. Furthermore, it is possible to abort an output pass without -losing anything, since the coefficient buffer is read-only as far as the -output section is concerned. See libjpeg.txt for more detail. - -A full-image coefficient array is only created if the JPEG file has multiple -scans (or if the application specifies buffered-image mode anyway). When -reading a single-scan file, the coefficient controller normally creates only -a one-MCU buffer, so input and output processing must run in lockstep in this -case. jpeg_consume_input() is effectively a no-op in this situation. - -The main impact of dividing the decompressor in this fashion is that we must -be very careful with shared variables in the cinfo data structure. Each -variable that can change during the course of decompression must be -classified as belonging to data input or data output, and each section must -look only at its own variables. For example, the data output section may not -depend on any of the variables that describe the current scan in the JPEG -file, because these may change as the data input section advances into a new -scan. - -The progress monitor is (somewhat arbitrarily) defined to treat input of the -file as one pass when buffered-image mode is not used, and to ignore data -input work completely when buffered-image mode is used. Note that the -library has no reliable way to predict the number of passes when dealing -with a progressive JPEG file, nor can it predict the number of output passes -in buffered-image mode. So the work estimate is inherently bogus anyway. - -No comparable division is currently made in the compression library, because -there isn't any real need for it. - - -*** Data formats *** - -Arrays of pixel sample values use the following data structure: - - typedef something JSAMPLE; a pixel component value, 0..MAXJSAMPLE - typedef JSAMPLE *JSAMPROW; ptr to a row of samples - typedef JSAMPROW *JSAMPARRAY; ptr to a list of rows - typedef JSAMPARRAY *JSAMPIMAGE; ptr to a list of color-component arrays - -The basic element type JSAMPLE will typically be one of unsigned char, -(signed) char, or short. Short will be used if samples wider than 8 bits are -to be supported (this is a compile-time option). Otherwise, unsigned char is -used if possible. If the compiler only supports signed chars, then it is -necessary to mask off the value when reading. Thus, all reads of JSAMPLE -values must be coded as "GETJSAMPLE(value)", where the macro will be defined -as "((value) & 0xFF)" on signed-char machines and "((int) (value))" elsewhere. - -With these conventions, JSAMPLE values can be assumed to be >= 0. This helps -simplify correct rounding during downsampling, etc. The JPEG standard's -specification that sample values run from -128..127 is accommodated by -subtracting 128 from the sample value in the DCT step. Similarly, during -decompression the output of the IDCT step will be immediately shifted back to -0..255. (NB: different values are required when 12-bit samples are in use. -The code is written in terms of MAXJSAMPLE and CENTERJSAMPLE, which will be -defined as 255 and 128 respectively in an 8-bit implementation, and as 4095 -and 2048 in a 12-bit implementation.) - -We use a pointer per row, rather than a two-dimensional JSAMPLE array. This -choice costs only a small amount of memory and has several benefits: -* Code using the data structure doesn't need to know the allocated width of - the rows. This simplifies edge expansion/compression, since we can work - in an array that's wider than the logical picture width. -* Indexing doesn't require multiplication; this is a performance win on many - machines. -* Arrays with more than 64K total elements can be supported even on machines - where malloc() cannot allocate chunks larger than 64K. -* The rows forming a component array may be allocated at different times - without extra copying. This trick allows some speedups in smoothing steps - that need access to the previous and next rows. - -Note that each color component is stored in a separate array; we don't use the -traditional layout in which the components of a pixel are stored together. -This simplifies coding of modules that work on each component independently, -because they don't need to know how many components there are. Furthermore, -we can read or write each component to a temporary file independently, which -is helpful when dealing with noninterleaved JPEG files. - -In general, a specific sample value is accessed by code such as - GETJSAMPLE(image[colorcomponent][row][col]) -where col is measured from the image left edge, but row is measured from the -first sample row currently in memory. Either of the first two indexings can -be precomputed by copying the relevant pointer. - - -Since most image-processing applications prefer to work on images in which -the components of a pixel are stored together, the data passed to or from the -surrounding application uses the traditional convention: a single pixel is -represented by N consecutive JSAMPLE values, and an image row is an array of -(# of color components)*(image width) JSAMPLEs. One or more rows of data can -be represented by a pointer of type JSAMPARRAY in this scheme. This scheme is -converted to component-wise storage inside the JPEG library. (Applications -that want to skip JPEG preprocessing or postprocessing will have to contend -with component-wise storage.) - - -Arrays of DCT-coefficient values use the following data structure: - - typedef short JCOEF; a 16-bit signed integer - typedef JCOEF JBLOCK[DCTSIZE2]; an 8x8 block of coefficients - typedef JBLOCK *JBLOCKROW; ptr to one horizontal row of 8x8 blocks - typedef JBLOCKROW *JBLOCKARRAY; ptr to a list of such rows - typedef JBLOCKARRAY *JBLOCKIMAGE; ptr to a list of color component arrays - -The underlying type is at least a 16-bit signed integer; while "short" is big -enough on all machines of interest, on some machines it is preferable to use -"int" for speed reasons, despite the storage cost. Coefficients are grouped -into 8x8 blocks (but we always use #defines DCTSIZE and DCTSIZE2 rather than -"8" and "64"). - -The contents of a coefficient block may be in either "natural" or zigzagged -order, and may be true values or divided by the quantization coefficients, -depending on where the block is in the processing pipeline. In the current -library, coefficient blocks are kept in natural order everywhere; the entropy -codecs zigzag or dezigzag the data as it is written or read. The blocks -contain quantized coefficients everywhere outside the DCT/IDCT subsystems. -(This latter decision may need to be revisited to support variable -quantization a la JPEG Part 3.) - -Notice that the allocation unit is now a row of 8x8 coefficient blocks, -corresponding to N rows of samples. Otherwise the structure is much the same -as for samples, and for the same reasons. - -On machines where malloc() can't handle a request bigger than 64Kb, this data -structure limits us to rows of less than 512 JBLOCKs, or a picture width of -4000+ pixels. This seems an acceptable restriction. - - -On 80x86 machines, the bottom-level pointer types (JSAMPROW and JBLOCKROW) -must be declared as "far" pointers, but the upper levels can be "near" -(implying that the pointer lists are allocated in the DS segment). -We use a #define symbol FAR, which expands to the "far" keyword when -compiling on 80x86 machines and to nothing elsewhere. - - -*** Suspendable processing *** - -In some applications it is desirable to use the JPEG library as an -incremental, memory-to-memory filter. In this situation the data source or -destination may be a limited-size buffer, and we can't rely on being able to -empty or refill the buffer at arbitrary times. Instead the application would -like to have control return from the library at buffer overflow/underrun, and -then resume compression or decompression at a later time. - -This scenario is supported for simple cases. (For anything more complex, we -recommend that the application "bite the bullet" and develop real multitasking -capability.) The libjpeg.txt file goes into more detail about the usage and -limitations of this capability; here we address the implications for library -structure. - -The essence of the problem is that the entropy codec (coder or decoder) must -be prepared to stop at arbitrary times. In turn, the controllers that call -the entropy codec must be able to stop before having produced or consumed all -the data that they normally would handle in one call. That part is reasonably -straightforward: we make the controller call interfaces include "progress -counters" which indicate the number of data chunks successfully processed, and -we require callers to test the counter rather than just assume all of the data -was processed. - -Rather than trying to restart at an arbitrary point, the current Huffman -codecs are designed to restart at the beginning of the current MCU after a -suspension due to buffer overflow/underrun. At the start of each call, the -codec's internal state is loaded from permanent storage (in the JPEG object -structures) into local variables. On successful completion of the MCU, the -permanent state is updated. (This copying is not very expensive, and may even -lead to *improved* performance if the local variables can be registerized.) -If a suspension occurs, the codec simply returns without updating the state, -thus effectively reverting to the start of the MCU. Note that this implies -leaving some data unprocessed in the source/destination buffer (ie, the -compressed partial MCU). The data source/destination module interfaces are -specified so as to make this possible. This also implies that the data buffer -must be large enough to hold a worst-case compressed MCU; a couple thousand -bytes should be enough. - -In a successive-approximation AC refinement scan, the progressive Huffman -decoder has to be able to undo assignments of newly nonzero coefficients if it -suspends before the MCU is complete, since decoding requires distinguishing -previously-zero and previously-nonzero coefficients. This is a bit tedious -but probably won't have much effect on performance. Other variants of Huffman -decoding need not worry about this, since they will just store the same values -again if forced to repeat the MCU. - -This approach would probably not work for an arithmetic codec, since its -modifiable state is quite large and couldn't be copied cheaply. Instead it -would have to suspend and resume exactly at the point of the buffer end. - -The JPEG marker reader is designed to cope with suspension at an arbitrary -point. It does so by backing up to the start of the marker parameter segment, -so the data buffer must be big enough to hold the largest marker of interest. -Again, a couple KB should be adequate. (A special "skip" convention is used -to bypass COM and APPn markers, so these can be larger than the buffer size -without causing problems; otherwise a 64K buffer would be needed in the worst -case.) - -The JPEG marker writer currently does *not* cope with suspension. -We feel that this is not necessary; it is much easier simply to require -the application to ensure there is enough buffer space before starting. (An -empty 2K buffer is more than sufficient for the header markers; and ensuring -there are a dozen or two bytes available before calling jpeg_finish_compress() -will suffice for the trailer.) This would not work for writing multi-scan -JPEG files, but we simply do not intend to support that capability with -suspension. - - -*** Memory manager services *** - -The JPEG library's memory manager controls allocation and deallocation of -memory, and it manages large "virtual" data arrays on machines where the -operating system does not provide virtual memory. Note that the same -memory manager serves both compression and decompression operations. - -In all cases, allocated objects are tied to a particular compression or -decompression master record, and they will be released when that master -record is destroyed. - -The memory manager does not provide explicit deallocation of objects. -Instead, objects are created in "pools" of free storage, and a whole pool -can be freed at once. This approach helps prevent storage-leak bugs, and -it speeds up operations whenever malloc/free are slow (as they often are). -The pools can be regarded as lifetime identifiers for objects. Two -pools/lifetimes are defined: - * JPOOL_PERMANENT lasts until master record is destroyed - * JPOOL_IMAGE lasts until done with image (JPEG datastream) -Permanent lifetime is used for parameters and tables that should be carried -across from one datastream to another; this includes all application-visible -parameters. Image lifetime is used for everything else. (A third lifetime, -JPOOL_PASS = one processing pass, was originally planned. However it was -dropped as not being worthwhile. The actual usage patterns are such that the -peak memory usage would be about the same anyway; and having per-pass storage -substantially complicates the virtual memory allocation rules --- see below.) - -The memory manager deals with three kinds of object: -1. "Small" objects. Typically these require no more than 10K-20K total. -2. "Large" objects. These may require tens to hundreds of K depending on - image size. Semantically they behave the same as small objects, but we - distinguish them for two reasons: - * On MS-DOS machines, large objects are referenced by FAR pointers, - small objects by NEAR pointers. - * Pool allocation heuristics may differ for large and small objects. - Note that individual "large" objects cannot exceed the size allowed by - type size_t, which may be 64K or less on some machines. -3. "Virtual" objects. These are large 2-D arrays of JSAMPLEs or JBLOCKs - (typically large enough for the entire image being processed). The - memory manager provides stripwise access to these arrays. On machines - without virtual memory, the rest of the array may be swapped out to a - temporary file. - -(Note: JSAMPARRAY and JBLOCKARRAY data structures are a combination of large -objects for the data proper and small objects for the row pointers. For -convenience and speed, the memory manager provides single routines to create -these structures. Similarly, virtual arrays include a small control block -and a JSAMPARRAY or JBLOCKARRAY working buffer, all created with one call.) - -In the present implementation, virtual arrays are only permitted to have image -lifespan. (Permanent lifespan would not be reasonable, and pass lifespan is -not very useful since a virtual array's raison d'etre is to store data for -multiple passes through the image.) We also expect that only "small" objects -will be given permanent lifespan, though this restriction is not required by -the memory manager. - -In a non-virtual-memory machine, some performance benefit can be gained by -making the in-memory buffers for virtual arrays be as large as possible. -(For small images, the buffers might fit entirely in memory, so blind -swapping would be very wasteful.) The memory manager will adjust the height -of the buffers to fit within a prespecified maximum memory usage. In order -to do this in a reasonably optimal fashion, the manager needs to allocate all -of the virtual arrays at once. Therefore, there isn't a one-step allocation -routine for virtual arrays; instead, there is a "request" routine that simply -allocates the control block, and a "realize" routine (called just once) that -determines space allocation and creates all of the actual buffers. The -realize routine must allow for space occupied by non-virtual large objects. -(We don't bother to factor in the space needed for small objects, on the -grounds that it isn't worth the trouble.) - -To support all this, we establish the following protocol for doing business -with the memory manager: - 1. Modules must request virtual arrays (which may have only image lifespan) - during the initial setup phase, i.e., in their jinit_xxx routines. - 2. All "large" objects (including JSAMPARRAYs and JBLOCKARRAYs) must also be - allocated during initial setup. - 3. realize_virt_arrays will be called at the completion of initial setup. - The above conventions ensure that sufficient information is available - for it to choose a good size for virtual array buffers. -Small objects of any lifespan may be allocated at any time. We expect that -the total space used for small objects will be small enough to be negligible -in the realize_virt_arrays computation. - -In a virtual-memory machine, we simply pretend that the available space is -infinite, thus causing realize_virt_arrays to decide that it can allocate all -the virtual arrays as full-size in-memory buffers. The overhead of the -virtual-array access protocol is very small when no swapping occurs. - -A virtual array can be specified to be "pre-zeroed"; when this flag is set, -never-yet-written sections of the array are set to zero before being made -available to the caller. If this flag is not set, never-written sections -of the array contain garbage. (This feature exists primarily because the -equivalent logic would otherwise be needed in jdcoefct.c for progressive -JPEG mode; we may as well make it available for possible other uses.) - -The first write pass on a virtual array is required to occur in top-to-bottom -order; read passes, as well as any write passes after the first one, may -access the array in any order. This restriction exists partly to simplify -the virtual array control logic, and partly because some file systems may not -support seeking beyond the current end-of-file in a temporary file. The main -implication of this restriction is that rearrangement of rows (such as -converting top-to-bottom data order to bottom-to-top) must be handled while -reading data out of the virtual array, not while putting it in. - - -*** Memory manager internal structure *** - -To isolate system dependencies as much as possible, we have broken the -memory manager into two parts. There is a reasonably system-independent -"front end" (jmemmgr.c) and a "back end" that contains only the code -likely to change across systems. All of the memory management methods -outlined above are implemented by the front end. The back end provides -the following routines for use by the front end (none of these routines -are known to the rest of the JPEG code): - -jpeg_mem_init, jpeg_mem_term system-dependent initialization/shutdown - -jpeg_get_small, jpeg_free_small interface to malloc and free library routines - (or their equivalents) - -jpeg_get_large, jpeg_free_large interface to FAR malloc/free in MSDOS machines; - else usually the same as - jpeg_get_small/jpeg_free_small - -jpeg_mem_available estimate available memory - -jpeg_open_backing_store create a backing-store object - -read_backing_store, manipulate a backing-store object -write_backing_store, -close_backing_store - -On some systems there will be more than one type of backing-store object -(specifically, in MS-DOS a backing store file might be an area of extended -memory as well as a disk file). jpeg_open_backing_store is responsible for -choosing how to implement a given object. The read/write/close routines -are method pointers in the structure that describes a given object; this -lets them be different for different object types. - -It may be necessary to ensure that backing store objects are explicitly -released upon abnormal program termination. For example, MS-DOS won't free -extended memory by itself. To support this, we will expect the main program -or surrounding application to arrange to call self_destruct (typically via -jpeg_destroy) upon abnormal termination. This may require a SIGINT signal -handler or equivalent. We don't want to have the back end module install its -own signal handler, because that would pre-empt the surrounding application's -ability to control signal handling. - -The IJG distribution includes several memory manager back end implementations. -Usually the same back end should be suitable for all applications on a given -system, but it is possible for an application to supply its own back end at -need. - - -*** Implications of DNL marker *** - -Some JPEG files may use a DNL marker to postpone definition of the image -height (this would be useful for a fax-like scanner's output, for instance). -In these files the SOF marker claims the image height is 0, and you only -find out the true image height at the end of the first scan. - -We could read these files as follows: -1. Upon seeing zero image height, replace it by 65535 (the maximum allowed). -2. When the DNL is found, update the image height in the global image - descriptor. -This implies that control modules must avoid making copies of the image -height, and must re-test for termination after each MCU row. This would -be easy enough to do. - -In cases where image-size data structures are allocated, this approach will -result in very inefficient use of virtual memory or much-larger-than-necessary -temporary files. This seems acceptable for something that probably won't be a -mainstream usage. People might have to forgo use of memory-hogging options -(such as two-pass color quantization or noninterleaved JPEG files) if they -want efficient conversion of such files. (One could improve efficiency by -demanding a user-supplied upper bound for the height, less than 65536; in most -cases it could be much less.) - -The standard also permits the SOF marker to overestimate the image height, -with a DNL to give the true, smaller height at the end of the first scan. -This would solve the space problems if the overestimate wasn't too great. -However, it implies that you don't even know whether DNL will be used. - -This leads to a couple of very serious objections: -1. Testing for a DNL marker must occur in the inner loop of the decompressor's - Huffman decoder; this implies a speed penalty whether the feature is used - or not. -2. There is no way to hide the last-minute change in image height from an - application using the decoder. Thus *every* application using the IJG - library would suffer a complexity penalty whether it cared about DNL or - not. -We currently do not support DNL because of these problems. - -A different approach is to insist that DNL-using files be preprocessed by a -separate program that reads ahead to the DNL, then goes back and fixes the SOF -marker. This is a much simpler solution and is probably far more efficient. -Even if one wants piped input, buffering the first scan of the JPEG file needs -a lot smaller temp file than is implied by the maximum-height method. For -this approach we'd simply treat DNL as a no-op in the decompressor (at most, -check that it matches the SOF image height). - -We will not worry about making the compressor capable of outputting DNL. -Something similar to the first scheme above could be applied if anyone ever -wants to make that work. |