1IJG JPEG LIBRARY: SYSTEM ARCHITECTURE 2 3Copyright (C) 1991-2009, Thomas G. Lane, Guido Vollbeding. 4This file is part of the Independent JPEG Group's software. 5For conditions of distribution and use, see the accompanying README file. 6 7 8This file provides an overview of the architecture of the IJG JPEG software; 9that is, the functions of the various modules in the system and the interfaces 10between modules. For more precise details about any data structure or calling 11convention, see the include files and comments in the source code. 12 13We assume that the reader is already somewhat familiar with the JPEG standard. 14The README file includes references for learning about JPEG. The file 15libjpeg.txt describes the library from the viewpoint of an application 16programmer using the library; it's best to read that file before this one. 17Also, the file coderules.txt describes the coding style conventions we use. 18 19In this document, JPEG-specific terminology follows the JPEG standard: 20 A "component" means a color channel, e.g., Red or Luminance. 21 A "sample" is a single component value (i.e., one number in the image data). 22 A "coefficient" is a frequency coefficient (a DCT transform output number). 23 A "block" is an 8x8 group of samples or coefficients. 24 An "MCU" (minimum coded unit) is an interleaved set of blocks of size 25 determined by the sampling factors, or a single block in a 26 noninterleaved scan. 27We do not use the terms "pixel" and "sample" interchangeably. When we say 28pixel, we mean an element of the full-size image, while a sample is an element 29of the downsampled image. Thus the number of samples may vary across 30components while the number of pixels does not. (This terminology is not used 31rigorously throughout the code, but it is used in places where confusion would 32otherwise result.) 33 34 35*** System features *** 36 37The IJG distribution contains two parts: 38 * A subroutine library for JPEG compression and decompression. 39 * cjpeg/djpeg, two sample applications that use the library to transform 40 JFIF JPEG files to and from several other image formats. 41cjpeg/djpeg are of no great intellectual complexity: they merely add a simple 42command-line user interface and I/O routines for several uncompressed image 43formats. This document concentrates on the library itself. 44 45We desire the library to be capable of supporting all JPEG baseline, extended 46sequential, and progressive DCT processes. Hierarchical processes are not 47supported. 48 49The library does not support the lossless (spatial) JPEG process. Lossless 50JPEG shares little or no code with lossy JPEG, and would normally be used 51without the extensive pre- and post-processing provided by this library. 52We feel that lossless JPEG is better handled by a separate library. 53 54Within these limits, any set of compression parameters allowed by the JPEG 55spec should be readable for decompression. (We can be more restrictive about 56what formats we can generate.) Although the system design allows for all 57parameter values, some uncommon settings are not yet implemented and may 58never be; nonintegral sampling ratios are the prime example. Furthermore, 59we treat 8-bit vs. 12-bit data precision as a compile-time switch, not a 60run-time option, because most machines can store 8-bit pixels much more 61compactly than 12-bit. 62 63By itself, the library handles only interchange JPEG datastreams --- in 64particular the widely used JFIF file format. The library can be used by 65surrounding code to process interchange or abbreviated JPEG datastreams that 66are embedded in more complex file formats. (For example, libtiff uses this 67library to implement JPEG compression within the TIFF file format.) 68 69The library includes a substantial amount of code that is not covered by the 70JPEG standard but is necessary for typical applications of JPEG. These 71functions preprocess the image before JPEG compression or postprocess it after 72decompression. They include colorspace conversion, downsampling/upsampling, 73and color quantization. This code can be omitted if not needed. 74 75A wide range of quality vs. speed tradeoffs are possible in JPEG processing, 76and even more so in decompression postprocessing. The decompression library 77provides multiple implementations that cover most of the useful tradeoffs, 78ranging from very-high-quality down to fast-preview operation. On the 79compression side we have generally not provided low-quality choices, since 80compression is normally less time-critical. It should be understood that the 81low-quality modes may not meet the JPEG standard's accuracy requirements; 82nonetheless, they are useful for viewers. 83 84 85*** Portability issues *** 86 87Portability is an essential requirement for the library. The key portability 88issues that show up at the level of system architecture are: 89 901. Memory usage. We want the code to be able to run on PC-class machines 91with limited memory. Images should therefore be processed sequentially (in 92strips), to avoid holding the whole image in memory at once. Where a 93full-image buffer is necessary, we should be able to use either virtual memory 94or temporary files. 95 962. Near/far pointer distinction. To run efficiently on 80x86 machines, the 97code should distinguish "small" objects (kept in near data space) from 98"large" ones (kept in far data space). This is an annoying restriction, but 99fortunately it does not impact code quality for less brain-damaged machines, 100and the source code clutter turns out to be minimal with sufficient use of 101pointer typedefs. 102 1033. Data precision. We assume that "char" is at least 8 bits, "short" and 104"int" at least 16, "long" at least 32. The code will work fine with larger 105data sizes, although memory may be used inefficiently in some cases. However, 106the JPEG compressed datastream must ultimately appear on external storage as a 107sequence of 8-bit bytes if it is to conform to the standard. This may pose a 108problem on machines where char is wider than 8 bits. The library represents 109compressed data as an array of values of typedef JOCTET. If no data type 110exactly 8 bits wide is available, custom data source and data destination 111modules must be written to unpack and pack the chosen JOCTET datatype into 1128-bit external representation. 113 114 115*** System overview *** 116 117The compressor and decompressor are each divided into two main sections: 118the JPEG compressor or decompressor proper, and the preprocessing or 119postprocessing functions. The interface between these two sections is the 120image data that the official JPEG spec regards as its input or output: this 121data is in the colorspace to be used for compression, and it is downsampled 122to the sampling factors to be used. The preprocessing and postprocessing 123steps are responsible for converting a normal image representation to or from 124this form. (Those few applications that want to deal with YCbCr downsampled 125data can skip the preprocessing or postprocessing step.) 126 127Looking more closely, the compressor library contains the following main 128elements: 129 130 Preprocessing: 131 * Color space conversion (e.g., RGB to YCbCr). 132 * Edge expansion and downsampling. Optionally, this step can do simple 133 smoothing --- this is often helpful for low-quality source data. 134 JPEG proper: 135 * MCU assembly, DCT, quantization. 136 * Entropy coding (sequential or progressive, Huffman or arithmetic). 137 138In addition to these modules we need overall control, marker generation, 139and support code (memory management & error handling). There is also a 140module responsible for physically writing the output data --- typically 141this is just an interface to fwrite(), but some applications may need to 142do something else with the data. 143 144The decompressor library contains the following main elements: 145 146 JPEG proper: 147 * Entropy decoding (sequential or progressive, Huffman or arithmetic). 148 * Dequantization, inverse DCT, MCU disassembly. 149 Postprocessing: 150 * Upsampling. Optionally, this step may be able to do more general 151 rescaling of the image. 152 * Color space conversion (e.g., YCbCr to RGB). This step may also 153 provide gamma adjustment [ currently it does not ]. 154 * Optional color quantization (e.g., reduction to 256 colors). 155 * Optional color precision reduction (e.g., 24-bit to 15-bit color). 156 [This feature is not currently implemented.] 157 158We also need overall control, marker parsing, and a data source module. 159The support code (memory management & error handling) can be shared with 160the compression half of the library. 161 162There may be several implementations of each of these elements, particularly 163in the decompressor, where a wide range of speed/quality tradeoffs is very 164useful. It must be understood that some of the best speedups involve 165merging adjacent steps in the pipeline. For example, upsampling, color space 166conversion, and color quantization might all be done at once when using a 167low-quality ordered-dither technique. The system architecture is designed to 168allow such merging where appropriate. 169 170 171Note: it is convenient to regard edge expansion (padding to block boundaries) 172as a preprocessing/postprocessing function, even though the JPEG spec includes 173it in compression/decompression. We do this because downsampling/upsampling 174can be simplified a little if they work on padded data: it's not necessary to 175have special cases at the right and bottom edges. Therefore the interface 176buffer is always an integral number of blocks wide and high, and we expect 177compression preprocessing to pad the source data properly. Padding will occur 178only to the next block (8-sample) boundary. In an interleaved-scan situation, 179additional dummy blocks may be used to fill out MCUs, but the MCU assembly and 180disassembly logic will create or discard these blocks internally. (This is 181advantageous for speed reasons, since we avoid DCTing the dummy blocks. 182It also permits a small reduction in file size, because the compressor can 183choose dummy block contents so as to minimize their size in compressed form. 184Finally, it makes the interface buffer specification independent of whether 185the file is actually interleaved or not.) Applications that wish to deal 186directly with the downsampled data must provide similar buffering and padding 187for odd-sized images. 188 189 190*** Poor man's object-oriented programming *** 191 192It should be clear by now that we have a lot of quasi-independent processing 193steps, many of which have several possible behaviors. To avoid cluttering the 194code with lots of switch statements, we use a simple form of object-style 195programming to separate out the different possibilities. 196 197For example, two different color quantization algorithms could be implemented 198as two separate modules that present the same external interface; at runtime, 199the calling code will access the proper module indirectly through an "object". 200 201We can get the limited features we need while staying within portable C. 202The basic tool is a function pointer. An "object" is just a struct 203containing one or more function pointer fields, each of which corresponds to 204a method name in real object-oriented languages. During initialization we 205fill in the function pointers with references to whichever module we have 206determined we need to use in this run. Then invocation of the module is done 207by indirecting through a function pointer; on most machines this is no more 208expensive than a switch statement, which would be the only other way of 209making the required run-time choice. The really significant benefit, of 210course, is keeping the source code clean and well structured. 211 212We can also arrange to have private storage that varies between different 213implementations of the same kind of object. We do this by making all the 214module-specific object structs be separately allocated entities, which will 215be accessed via pointers in the master compression or decompression struct. 216The "public" fields or methods for a given kind of object are specified by 217a commonly known struct. But a module's initialization code can allocate 218a larger struct that contains the common struct as its first member, plus 219additional private fields. With appropriate pointer casting, the module's 220internal functions can access these private fields. (For a simple example, 221see jdatadst.c, which implements the external interface specified by struct 222jpeg_destination_mgr, but adds extra fields.) 223 224(Of course this would all be a lot easier if we were using C++, but we are 225not yet prepared to assume that everyone has a C++ compiler.) 226 227An important benefit of this scheme is that it is easy to provide multiple 228versions of any method, each tuned to a particular case. While a lot of 229precalculation might be done to select an optimal implementation of a method, 230the cost per invocation is constant. For example, the upsampling step might 231have a "generic" method, plus one or more "hardwired" methods for the most 232popular sampling factors; the hardwired methods would be faster because they'd 233use straight-line code instead of for-loops. The cost to determine which 234method to use is paid only once, at startup, and the selection criteria are 235hidden from the callers of the method. 236 237This plan differs a little bit from usual object-oriented structures, in that 238only one instance of each object class will exist during execution. The 239reason for having the class structure is that on different runs we may create 240different instances (choose to execute different modules). You can think of 241the term "method" as denoting the common interface presented by a particular 242set of interchangeable functions, and "object" as denoting a group of related 243methods, or the total shared interface behavior of a group of modules. 244 245 246*** Overall control structure *** 247 248We previously mentioned the need for overall control logic in the compression 249and decompression libraries. In IJG implementations prior to v5, overall 250control was mostly provided by "pipeline control" modules, which proved to be 251large, unwieldy, and hard to understand. To improve the situation, the 252control logic has been subdivided into multiple modules. The control modules 253consist of: 254 2551. Master control for module selection and initialization. This has two 256responsibilities: 257 258 1A. Startup initialization at the beginning of image processing. 259 The individual processing modules to be used in this run are selected 260 and given initialization calls. 261 262 1B. Per-pass control. This determines how many passes will be performed 263 and calls each active processing module to configure itself 264 appropriately at the beginning of each pass. End-of-pass processing, 265 where necessary, is also invoked from the master control module. 266 267 Method selection is partially distributed, in that a particular processing 268 module may contain several possible implementations of a particular method, 269 which it will select among when given its initialization call. The master 270 control code need only be concerned with decisions that affect more than 271 one module. 272 2732. Data buffering control. A separate control module exists for each 274 inter-processing-step data buffer. This module is responsible for 275 invoking the processing steps that write or read that data buffer. 276 277Each buffer controller sees the world as follows: 278 279input data => processing step A => buffer => processing step B => output data 280 | | | 281 ------------------ controller ------------------ 282 283The controller knows the dataflow requirements of steps A and B: how much data 284they want to accept in one chunk and how much they output in one chunk. Its 285function is to manage its buffer and call A and B at the proper times. 286 287A data buffer control module may itself be viewed as a processing step by a 288higher-level control module; thus the control modules form a binary tree with 289elementary processing steps at the leaves of the tree. 290 291The control modules are objects. A considerable amount of flexibility can 292be had by replacing implementations of a control module. For example: 293* Merging of adjacent steps in the pipeline is done by replacing a control 294 module and its pair of processing-step modules with a single processing- 295 step module. (Hence the possible merges are determined by the tree of 296 control modules.) 297* In some processing modes, a given interstep buffer need only be a "strip" 298 buffer large enough to accommodate the desired data chunk sizes. In other 299 modes, a full-image buffer is needed and several passes are required. 300 The control module determines which kind of buffer is used and manipulates 301 virtual array buffers as needed. One or both processing steps may be 302 unaware of the multi-pass behavior. 303 304In theory, we might be able to make all of the data buffer controllers 305interchangeable and provide just one set of implementations for all. In 306practice, each one contains considerable special-case processing for its 307particular job. The buffer controller concept should be regarded as an 308overall system structuring principle, not as a complete description of the 309task performed by any one controller. 310 311 312*** Compression object structure *** 313 314Here is a sketch of the logical structure of the JPEG compression library: 315 316 |-- Colorspace conversion 317 |-- Preprocessing controller --| 318 | |-- Downsampling 319Main controller --| 320 | |-- Forward DCT, quantize 321 |-- Coefficient controller --| 322 |-- Entropy encoding 323 324This sketch also describes the flow of control (subroutine calls) during 325typical image data processing. Each of the components shown in the diagram is 326an "object" which may have several different implementations available. One 327or more source code files contain the actual implementation(s) of each object. 328 329The objects shown above are: 330 331* Main controller: buffer controller for the subsampled-data buffer, which 332 holds the preprocessed input data. This controller invokes preprocessing to 333 fill the subsampled-data buffer, and JPEG compression to empty it. There is 334 usually no need for a full-image buffer here; a strip buffer is adequate. 335 336* Preprocessing controller: buffer controller for the downsampling input data 337 buffer, which lies between colorspace conversion and downsampling. Note 338 that a unified conversion/downsampling module would probably replace this 339 controller entirely. 340 341* Colorspace conversion: converts application image data into the desired 342 JPEG color space; also changes the data from pixel-interleaved layout to 343 separate component planes. Processes one pixel row at a time. 344 345* Downsampling: performs reduction of chroma components as required. 346 Optionally may perform pixel-level smoothing as well. Processes a "row 347 group" at a time, where a row group is defined as Vmax pixel rows of each 348 component before downsampling, and Vk sample rows afterwards (remember Vk 349 differs across components). Some downsampling or smoothing algorithms may 350 require context rows above and below the current row group; the 351 preprocessing controller is responsible for supplying these rows via proper 352 buffering. The downsampler is responsible for edge expansion at the right 353 edge (i.e., extending each sample row to a multiple of 8 samples); but the 354 preprocessing controller is responsible for vertical edge expansion (i.e., 355 duplicating the bottom sample row as needed to make a multiple of 8 rows). 356 357* Coefficient controller: buffer controller for the DCT-coefficient data. 358 This controller handles MCU assembly, including insertion of dummy DCT 359 blocks when needed at the right or bottom edge. When performing 360 Huffman-code optimization or emitting a multiscan JPEG file, this 361 controller is responsible for buffering the full image. The equivalent of 362 one fully interleaved MCU row of subsampled data is processed per call, 363 even when the JPEG file is noninterleaved. 364 365* Forward DCT and quantization: Perform DCT, quantize, and emit coefficients. 366 Works on one or more DCT blocks at a time. (Note: the coefficients are now 367 emitted in normal array order, which the entropy encoder is expected to 368 convert to zigzag order as necessary. Prior versions of the IJG code did 369 the conversion to zigzag order within the quantization step.) 370 371* Entropy encoding: Perform Huffman or arithmetic entropy coding and emit the 372 coded data to the data destination module. Works on one MCU per call. 373 For progressive JPEG, the same DCT blocks are fed to the entropy coder 374 during each pass, and the coder must emit the appropriate subset of 375 coefficients. 376 377In addition to the above objects, the compression library includes these 378objects: 379 380* Master control: determines the number of passes required, controls overall 381 and per-pass initialization of the other modules. 382 383* Marker writing: generates JPEG markers (except for RSTn, which is emitted 384 by the entropy encoder when needed). 385 386* Data destination manager: writes the output JPEG datastream to its final 387 destination (e.g., a file). The destination manager supplied with the 388 library knows how to write to a stdio stream; for other behaviors, the 389 surrounding application may provide its own destination manager. 390 391* Memory manager: allocates and releases memory, controls virtual arrays 392 (with backing store management, where required). 393 394* Error handler: performs formatting and output of error and trace messages; 395 determines handling of nonfatal errors. The surrounding application may 396 override some or all of this object's methods to change error handling. 397 398* Progress monitor: supports output of "percent-done" progress reports. 399 This object represents an optional callback to the surrounding application: 400 if wanted, it must be supplied by the application. 401 402The error handler, destination manager, and progress monitor objects are 403defined as separate objects in order to simplify application-specific 404customization of the JPEG library. A surrounding application may override 405individual methods or supply its own all-new implementation of one of these 406objects. The object interfaces for these objects are therefore treated as 407part of the application interface of the library, whereas the other objects 408are internal to the library. 409 410The error handler and memory manager are shared by JPEG compression and 411decompression; the progress monitor, if used, may be shared as well. 412 413 414*** Decompression object structure *** 415 416Here is a sketch of the logical structure of the JPEG decompression library: 417 418 |-- Entropy decoding 419 |-- Coefficient controller --| 420 | |-- Dequantize, Inverse DCT 421Main controller --| 422 | |-- Upsampling 423 |-- Postprocessing controller --| |-- Colorspace conversion 424 |-- Color quantization 425 |-- Color precision reduction 426 427As before, this diagram also represents typical control flow. The objects 428shown are: 429 430* Main controller: buffer controller for the subsampled-data buffer, which 431 holds the output of JPEG decompression proper. This controller's primary 432 task is to feed the postprocessing procedure. Some upsampling algorithms 433 may require context rows above and below the current row group; when this 434 is true, the main controller is responsible for managing its buffer so as 435 to make context rows available. In the current design, the main buffer is 436 always a strip buffer; a full-image buffer is never required. 437 438* Coefficient controller: buffer controller for the DCT-coefficient data. 439 This controller handles MCU disassembly, including deletion of any dummy 440 DCT blocks at the right or bottom edge. When reading a multiscan JPEG 441 file, this controller is responsible for buffering the full image. 442 (Buffering DCT coefficients, rather than samples, is necessary to support 443 progressive JPEG.) The equivalent of one fully interleaved MCU row of 444 subsampled data is processed per call, even when the source JPEG file is 445 noninterleaved. 446 447* Entropy decoding: Read coded data from the data source module and perform 448 Huffman or arithmetic entropy decoding. Works on one MCU per call. 449 For progressive JPEG decoding, the coefficient controller supplies the prior 450 coefficients of each MCU (initially all zeroes), which the entropy decoder 451 modifies in each scan. 452 453* Dequantization and inverse DCT: like it says. Note that the coefficients 454 buffered by the coefficient controller have NOT been dequantized; we 455 merge dequantization and inverse DCT into a single step for speed reasons. 456 When scaled-down output is asked for, simplified DCT algorithms may be used 457 that need fewer coefficients and emit fewer samples per DCT block, not the 458 full 8x8. Works on one DCT block at a time. 459 460* Postprocessing controller: buffer controller for the color quantization 461 input buffer, when quantization is in use. (Without quantization, this 462 controller just calls the upsampler.) For two-pass quantization, this 463 controller is responsible for buffering the full-image data. 464 465* Upsampling: restores chroma components to full size. (May support more 466 general output rescaling, too. Note that if undersized DCT outputs have 467 been emitted by the DCT module, this module must adjust so that properly 468 sized outputs are created.) Works on one row group at a time. This module 469 also calls the color conversion module, so its top level is effectively a 470 buffer controller for the upsampling->color conversion buffer. However, in 471 all but the highest-quality operating modes, upsampling and color 472 conversion are likely to be merged into a single step. 473 474* Colorspace conversion: convert from JPEG color space to output color space, 475 and change data layout from separate component planes to pixel-interleaved. 476 Works on one pixel row at a time. 477 478* Color quantization: reduce the data to colormapped form, using either an 479 externally specified colormap or an internally generated one. This module 480 is not used for full-color output. Works on one pixel row at a time; may 481 require two passes to generate a color map. Note that the output will 482 always be a single component representing colormap indexes. In the current 483 design, the output values are JSAMPLEs, so an 8-bit compilation cannot 484 quantize to more than 256 colors. This is unlikely to be a problem in 485 practice. 486 487* Color reduction: this module handles color precision reduction, e.g., 488 generating 15-bit color (5 bits/primary) from JPEG's 24-bit output. 489 Not quite clear yet how this should be handled... should we merge it with 490 colorspace conversion??? 491 492Note that some high-speed operating modes might condense the entire 493postprocessing sequence to a single module (upsample, color convert, and 494quantize in one step). 495 496In addition to the above objects, the decompression library includes these 497objects: 498 499* Master control: determines the number of passes required, controls overall 500 and per-pass initialization of the other modules. This is subdivided into 501 input and output control: jdinput.c controls only input-side processing, 502 while jdmaster.c handles overall initialization and output-side control. 503 504* Marker reading: decodes JPEG markers (except for RSTn). 505 506* Data source manager: supplies the input JPEG datastream. The source 507 manager supplied with the library knows how to read from a stdio stream; 508 for other behaviors, the surrounding application may provide its own source 509 manager. 510 511* Memory manager: same as for compression library. 512 513* Error handler: same as for compression library. 514 515* Progress monitor: same as for compression library. 516 517As with compression, the data source manager, error handler, and progress 518monitor are candidates for replacement by a surrounding application. 519 520 521*** Decompression input and output separation *** 522 523To support efficient incremental display of progressive JPEG files, the 524decompressor is divided into two sections that can run independently: 525 5261. Data input includes marker parsing, entropy decoding, and input into the 527 coefficient controller's DCT coefficient buffer. Note that this 528 processing is relatively cheap and fast. 529 5302. Data output reads from the DCT coefficient buffer and performs the IDCT 531 and all postprocessing steps. 532 533For a progressive JPEG file, the data input processing is allowed to get 534arbitrarily far ahead of the data output processing. (This occurs only 535if the application calls jpeg_consume_input(); otherwise input and output 536run in lockstep, since the input section is called only when the output 537section needs more data.) In this way the application can avoid making 538extra display passes when data is arriving faster than the display pass 539can run. Furthermore, it is possible to abort an output pass without 540losing anything, since the coefficient buffer is read-only as far as the 541output section is concerned. See libjpeg.txt for more detail. 542 543A full-image coefficient array is only created if the JPEG file has multiple 544scans (or if the application specifies buffered-image mode anyway). When 545reading a single-scan file, the coefficient controller normally creates only 546a one-MCU buffer, so input and output processing must run in lockstep in this 547case. jpeg_consume_input() is effectively a no-op in this situation. 548 549The main impact of dividing the decompressor in this fashion is that we must 550be very careful with shared variables in the cinfo data structure. Each 551variable that can change during the course of decompression must be 552classified as belonging to data input or data output, and each section must 553look only at its own variables. For example, the data output section may not 554depend on any of the variables that describe the current scan in the JPEG 555file, because these may change as the data input section advances into a new 556scan. 557 558The progress monitor is (somewhat arbitrarily) defined to treat input of the 559file as one pass when buffered-image mode is not used, and to ignore data 560input work completely when buffered-image mode is used. Note that the 561library has no reliable way to predict the number of passes when dealing 562with a progressive JPEG file, nor can it predict the number of output passes 563in buffered-image mode. So the work estimate is inherently bogus anyway. 564 565No comparable division is currently made in the compression library, because 566there isn't any real need for it. 567 568 569*** Data formats *** 570 571Arrays of pixel sample values use the following data structure: 572 573 typedef something JSAMPLE; a pixel component value, 0..MAXJSAMPLE 574 typedef JSAMPLE *JSAMPROW; ptr to a row of samples 575 typedef JSAMPROW *JSAMPARRAY; ptr to a list of rows 576 typedef JSAMPARRAY *JSAMPIMAGE; ptr to a list of color-component arrays 577 578The basic element type JSAMPLE will typically be one of unsigned char, 579(signed) char, or short. Short will be used if samples wider than 8 bits are 580to be supported (this is a compile-time option). Otherwise, unsigned char is 581used if possible. If the compiler only supports signed chars, then it is 582necessary to mask off the value when reading. Thus, all reads of JSAMPLE 583values must be coded as "GETJSAMPLE(value)", where the macro will be defined 584as "((value) & 0xFF)" on signed-char machines and "((int) (value))" elsewhere. 585 586With these conventions, JSAMPLE values can be assumed to be >= 0. This helps 587simplify correct rounding during downsampling, etc. The JPEG standard's 588specification that sample values run from -128..127 is accommodated by 589subtracting 128 from the sample value in the DCT step. Similarly, during 590decompression the output of the IDCT step will be immediately shifted back to 5910..255. (NB: different values are required when 12-bit samples are in use. 592The code is written in terms of MAXJSAMPLE and CENTERJSAMPLE, which will be 593defined as 255 and 128 respectively in an 8-bit implementation, and as 4095 594and 2048 in a 12-bit implementation.) 595 596We use a pointer per row, rather than a two-dimensional JSAMPLE array. This 597choice costs only a small amount of memory and has several benefits: 598* Code using the data structure doesn't need to know the allocated width of 599 the rows. This simplifies edge expansion/compression, since we can work 600 in an array that's wider than the logical picture width. 601* Indexing doesn't require multiplication; this is a performance win on many 602 machines. 603* Arrays with more than 64K total elements can be supported even on machines 604 where malloc() cannot allocate chunks larger than 64K. 605* The rows forming a component array may be allocated at different times 606 without extra copying. This trick allows some speedups in smoothing steps 607 that need access to the previous and next rows. 608 609Note that each color component is stored in a separate array; we don't use the 610traditional layout in which the components of a pixel are stored together. 611This simplifies coding of modules that work on each component independently, 612because they don't need to know how many components there are. Furthermore, 613we can read or write each component to a temporary file independently, which 614is helpful when dealing with noninterleaved JPEG files. 615 616In general, a specific sample value is accessed by code such as 617 GETJSAMPLE(image[colorcomponent][row][col]) 618where col is measured from the image left edge, but row is measured from the 619first sample row currently in memory. Either of the first two indexings can 620be precomputed by copying the relevant pointer. 621 622 623Since most image-processing applications prefer to work on images in which 624the components of a pixel are stored together, the data passed to or from the 625surrounding application uses the traditional convention: a single pixel is 626represented by N consecutive JSAMPLE values, and an image row is an array of 627(# of color components)*(image width) JSAMPLEs. One or more rows of data can 628be represented by a pointer of type JSAMPARRAY in this scheme. This scheme is 629converted to component-wise storage inside the JPEG library. (Applications 630that want to skip JPEG preprocessing or postprocessing will have to contend 631with component-wise storage.) 632 633 634Arrays of DCT-coefficient values use the following data structure: 635 636 typedef short JCOEF; a 16-bit signed integer 637 typedef JCOEF JBLOCK[DCTSIZE2]; an 8x8 block of coefficients 638 typedef JBLOCK *JBLOCKROW; ptr to one horizontal row of 8x8 blocks 639 typedef JBLOCKROW *JBLOCKARRAY; ptr to a list of such rows 640 typedef JBLOCKARRAY *JBLOCKIMAGE; ptr to a list of color component arrays 641 642The underlying type is at least a 16-bit signed integer; while "short" is big 643enough on all machines of interest, on some machines it is preferable to use 644"int" for speed reasons, despite the storage cost. Coefficients are grouped 645into 8x8 blocks (but we always use #defines DCTSIZE and DCTSIZE2 rather than 646"8" and "64"). 647 648The contents of a coefficient block may be in either "natural" or zigzagged 649order, and may be true values or divided by the quantization coefficients, 650depending on where the block is in the processing pipeline. In the current 651library, coefficient blocks are kept in natural order everywhere; the entropy 652codecs zigzag or dezigzag the data as it is written or read. The blocks 653contain quantized coefficients everywhere outside the DCT/IDCT subsystems. 654(This latter decision may need to be revisited to support variable 655quantization a la JPEG Part 3.) 656 657Notice that the allocation unit is now a row of 8x8 blocks, corresponding to 658eight rows of samples. Otherwise the structure is much the same as for 659samples, and for the same reasons. 660 661On machines where malloc() can't handle a request bigger than 64Kb, this data 662structure limits us to rows of less than 512 JBLOCKs, or a picture width of 6634000+ pixels. This seems an acceptable restriction. 664 665 666On 80x86 machines, the bottom-level pointer types (JSAMPROW and JBLOCKROW) 667must be declared as "far" pointers, but the upper levels can be "near" 668(implying that the pointer lists are allocated in the DS segment). 669We use a #define symbol FAR, which expands to the "far" keyword when 670compiling on 80x86 machines and to nothing elsewhere. 671 672 673*** Suspendable processing *** 674 675In some applications it is desirable to use the JPEG library as an 676incremental, memory-to-memory filter. In this situation the data source or 677destination may be a limited-size buffer, and we can't rely on being able to 678empty or refill the buffer at arbitrary times. Instead the application would 679like to have control return from the library at buffer overflow/underrun, and 680then resume compression or decompression at a later time. 681 682This scenario is supported for simple cases. (For anything more complex, we 683recommend that the application "bite the bullet" and develop real multitasking 684capability.) The libjpeg.txt file goes into more detail about the usage and 685limitations of this capability; here we address the implications for library 686structure. 687 688The essence of the problem is that the entropy codec (coder or decoder) must 689be prepared to stop at arbitrary times. In turn, the controllers that call 690the entropy codec must be able to stop before having produced or consumed all 691the data that they normally would handle in one call. That part is reasonably 692straightforward: we make the controller call interfaces include "progress 693counters" which indicate the number of data chunks successfully processed, and 694we require callers to test the counter rather than just assume all of the data 695was processed. 696 697Rather than trying to restart at an arbitrary point, the current Huffman 698codecs are designed to restart at the beginning of the current MCU after a 699suspension due to buffer overflow/underrun. At the start of each call, the 700codec's internal state is loaded from permanent storage (in the JPEG object 701structures) into local variables. On successful completion of the MCU, the 702permanent state is updated. (This copying is not very expensive, and may even 703lead to *improved* performance if the local variables can be registerized.) 704If a suspension occurs, the codec simply returns without updating the state, 705thus effectively reverting to the start of the MCU. Note that this implies 706leaving some data unprocessed in the source/destination buffer (ie, the 707compressed partial MCU). The data source/destination module interfaces are 708specified so as to make this possible. This also implies that the data buffer 709must be large enough to hold a worst-case compressed MCU; a couple thousand 710bytes should be enough. 711 712In a successive-approximation AC refinement scan, the progressive Huffman 713decoder has to be able to undo assignments of newly nonzero coefficients if it 714suspends before the MCU is complete, since decoding requires distinguishing 715previously-zero and previously-nonzero coefficients. This is a bit tedious 716but probably won't have much effect on performance. Other variants of Huffman 717decoding need not worry about this, since they will just store the same values 718again if forced to repeat the MCU. 719 720This approach would probably not work for an arithmetic codec, since its 721modifiable state is quite large and couldn't be copied cheaply. Instead it 722would have to suspend and resume exactly at the point of the buffer end. 723 724The JPEG marker reader is designed to cope with suspension at an arbitrary 725point. It does so by backing up to the start of the marker parameter segment, 726so the data buffer must be big enough to hold the largest marker of interest. 727Again, a couple KB should be adequate. (A special "skip" convention is used 728to bypass COM and APPn markers, so these can be larger than the buffer size 729without causing problems; otherwise a 64K buffer would be needed in the worst 730case.) 731 732The JPEG marker writer currently does *not* cope with suspension. 733We feel that this is not necessary; it is much easier simply to require 734the application to ensure there is enough buffer space before starting. (An 735empty 2K buffer is more than sufficient for the header markers; and ensuring 736there are a dozen or two bytes available before calling jpeg_finish_compress() 737will suffice for the trailer.) This would not work for writing multi-scan 738JPEG files, but we simply do not intend to support that capability with 739suspension. 740 741 742*** Memory manager services *** 743 744The JPEG library's memory manager controls allocation and deallocation of 745memory, and it manages large "virtual" data arrays on machines where the 746operating system does not provide virtual memory. Note that the same 747memory manager serves both compression and decompression operations. 748 749In all cases, allocated objects are tied to a particular compression or 750decompression master record, and they will be released when that master 751record is destroyed. 752 753The memory manager does not provide explicit deallocation of objects. 754Instead, objects are created in "pools" of free storage, and a whole pool 755can be freed at once. This approach helps prevent storage-leak bugs, and 756it speeds up operations whenever malloc/free are slow (as they often are). 757The pools can be regarded as lifetime identifiers for objects. Two 758pools/lifetimes are defined: 759 * JPOOL_PERMANENT lasts until master record is destroyed 760 * JPOOL_IMAGE lasts until done with image (JPEG datastream) 761Permanent lifetime is used for parameters and tables that should be carried 762across from one datastream to another; this includes all application-visible 763parameters. Image lifetime is used for everything else. (A third lifetime, 764JPOOL_PASS = one processing pass, was originally planned. However it was 765dropped as not being worthwhile. The actual usage patterns are such that the 766peak memory usage would be about the same anyway; and having per-pass storage 767substantially complicates the virtual memory allocation rules --- see below.) 768 769The memory manager deals with three kinds of object: 7701. "Small" objects. Typically these require no more than 10K-20K total. 7712. "Large" objects. These may require tens to hundreds of K depending on 772 image size. Semantically they behave the same as small objects, but we 773 distinguish them for two reasons: 774 * On MS-DOS machines, large objects are referenced by FAR pointers, 775 small objects by NEAR pointers. 776 * Pool allocation heuristics may differ for large and small objects. 777 Note that individual "large" objects cannot exceed the size allowed by 778 type size_t, which may be 64K or less on some machines. 7793. "Virtual" objects. These are large 2-D arrays of JSAMPLEs or JBLOCKs 780 (typically large enough for the entire image being processed). The 781 memory manager provides stripwise access to these arrays. On machines 782 without virtual memory, the rest of the array may be swapped out to a 783 temporary file. 784 785(Note: JSAMPARRAY and JBLOCKARRAY data structures are a combination of large 786objects for the data proper and small objects for the row pointers. For 787convenience and speed, the memory manager provides single routines to create 788these structures. Similarly, virtual arrays include a small control block 789and a JSAMPARRAY or JBLOCKARRAY working buffer, all created with one call.) 790 791In the present implementation, virtual arrays are only permitted to have image 792lifespan. (Permanent lifespan would not be reasonable, and pass lifespan is 793not very useful since a virtual array's raison d'etre is to store data for 794multiple passes through the image.) We also expect that only "small" objects 795will be given permanent lifespan, though this restriction is not required by 796the memory manager. 797 798In a non-virtual-memory machine, some performance benefit can be gained by 799making the in-memory buffers for virtual arrays be as large as possible. 800(For small images, the buffers might fit entirely in memory, so blind 801swapping would be very wasteful.) The memory manager will adjust the height 802of the buffers to fit within a prespecified maximum memory usage. In order 803to do this in a reasonably optimal fashion, the manager needs to allocate all 804of the virtual arrays at once. Therefore, there isn't a one-step allocation 805routine for virtual arrays; instead, there is a "request" routine that simply 806allocates the control block, and a "realize" routine (called just once) that 807determines space allocation and creates all of the actual buffers. The 808realize routine must allow for space occupied by non-virtual large objects. 809(We don't bother to factor in the space needed for small objects, on the 810grounds that it isn't worth the trouble.) 811 812To support all this, we establish the following protocol for doing business 813with the memory manager: 814 1. Modules must request virtual arrays (which may have only image lifespan) 815 during the initial setup phase, i.e., in their jinit_xxx routines. 816 2. All "large" objects (including JSAMPARRAYs and JBLOCKARRAYs) must also be 817 allocated during initial setup. 818 3. realize_virt_arrays will be called at the completion of initial setup. 819 The above conventions ensure that sufficient information is available 820 for it to choose a good size for virtual array buffers. 821Small objects of any lifespan may be allocated at any time. We expect that 822the total space used for small objects will be small enough to be negligible 823in the realize_virt_arrays computation. 824 825In a virtual-memory machine, we simply pretend that the available space is 826infinite, thus causing realize_virt_arrays to decide that it can allocate all 827the virtual arrays as full-size in-memory buffers. The overhead of the 828virtual-array access protocol is very small when no swapping occurs. 829 830A virtual array can be specified to be "pre-zeroed"; when this flag is set, 831never-yet-written sections of the array are set to zero before being made 832available to the caller. If this flag is not set, never-written sections 833of the array contain garbage. (This feature exists primarily because the 834equivalent logic would otherwise be needed in jdcoefct.c for progressive 835JPEG mode; we may as well make it available for possible other uses.) 836 837The first write pass on a virtual array is required to occur in top-to-bottom 838order; read passes, as well as any write passes after the first one, may 839access the array in any order. This restriction exists partly to simplify 840the virtual array control logic, and partly because some file systems may not 841support seeking beyond the current end-of-file in a temporary file. The main 842implication of this restriction is that rearrangement of rows (such as 843converting top-to-bottom data order to bottom-to-top) must be handled while 844reading data out of the virtual array, not while putting it in. 845 846 847*** Memory manager internal structure *** 848 849To isolate system dependencies as much as possible, we have broken the 850memory manager into two parts. There is a reasonably system-independent 851"front end" (jmemmgr.c) and a "back end" that contains only the code 852likely to change across systems. All of the memory management methods 853outlined above are implemented by the front end. The back end provides 854the following routines for use by the front end (none of these routines 855are known to the rest of the JPEG code): 856 857jpeg_mem_init, jpeg_mem_term system-dependent initialization/shutdown 858 859jpeg_get_small, jpeg_free_small interface to malloc and free library routines 860 (or their equivalents) 861 862jpeg_get_large, jpeg_free_large interface to FAR malloc/free in MSDOS machines; 863 else usually the same as 864 jpeg_get_small/jpeg_free_small 865 866jpeg_mem_available estimate available memory 867 868jpeg_open_backing_store create a backing-store object 869 870read_backing_store, manipulate a backing-store object 871write_backing_store, 872close_backing_store 873 874On some systems there will be more than one type of backing-store object 875(specifically, in MS-DOS a backing store file might be an area of extended 876memory as well as a disk file). jpeg_open_backing_store is responsible for 877choosing how to implement a given object. The read/write/close routines 878are method pointers in the structure that describes a given object; this 879lets them be different for different object types. 880 881It may be necessary to ensure that backing store objects are explicitly 882released upon abnormal program termination. For example, MS-DOS won't free 883extended memory by itself. To support this, we will expect the main program 884or surrounding application to arrange to call self_destruct (typically via 885jpeg_destroy) upon abnormal termination. This may require a SIGINT signal 886handler or equivalent. We don't want to have the back end module install its 887own signal handler, because that would pre-empt the surrounding application's 888ability to control signal handling. 889 890The IJG distribution includes several memory manager back end implementations. 891Usually the same back end should be suitable for all applications on a given 892system, but it is possible for an application to supply its own back end at 893need. 894 895 896*** Implications of DNL marker *** 897 898Some JPEG files may use a DNL marker to postpone definition of the image 899height (this would be useful for a fax-like scanner's output, for instance). 900In these files the SOF marker claims the image height is 0, and you only 901find out the true image height at the end of the first scan. 902 903We could read these files as follows: 9041. Upon seeing zero image height, replace it by 65535 (the maximum allowed). 9052. When the DNL is found, update the image height in the global image 906 descriptor. 907This implies that control modules must avoid making copies of the image 908height, and must re-test for termination after each MCU row. This would 909be easy enough to do. 910 911In cases where image-size data structures are allocated, this approach will 912result in very inefficient use of virtual memory or much-larger-than-necessary 913temporary files. This seems acceptable for something that probably won't be a 914mainstream usage. People might have to forgo use of memory-hogging options 915(such as two-pass color quantization or noninterleaved JPEG files) if they 916want efficient conversion of such files. (One could improve efficiency by 917demanding a user-supplied upper bound for the height, less than 65536; in most 918cases it could be much less.) 919 920The standard also permits the SOF marker to overestimate the image height, 921with a DNL to give the true, smaller height at the end of the first scan. 922This would solve the space problems if the overestimate wasn't too great. 923However, it implies that you don't even know whether DNL will be used. 924 925This leads to a couple of very serious objections: 9261. Testing for a DNL marker must occur in the inner loop of the decompressor's 927 Huffman decoder; this implies a speed penalty whether the feature is used 928 or not. 9292. There is no way to hide the last-minute change in image height from an 930 application using the decoder. Thus *every* application using the IJG 931 library would suffer a complexity penalty whether it cared about DNL or 932 not. 933We currently do not support DNL because of these problems. 934 935A different approach is to insist that DNL-using files be preprocessed by a 936separate program that reads ahead to the DNL, then goes back and fixes the SOF 937marker. This is a much simpler solution and is probably far more efficient. 938Even if one wants piped input, buffering the first scan of the JPEG file needs 939a lot smaller temp file than is implied by the maximum-height method. For 940this approach we'd simply treat DNL as a no-op in the decompressor (at most, 941check that it matches the SOF image height). 942 943We will not worry about making the compressor capable of outputting DNL. 944Something similar to the first scheme above could be applied if anyone ever 945wants to make that work. 946