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diff --git a/jpg/structure.txt b/jpg/structure.txt new file mode 100644 index 0000000..44e48ca --- /dev/null +++ b/jpg/structure.txt @@ -0,0 +1,941 @@ +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. |