1 IJG JPEG LIBRARY: SYSTEM ARCHITECTURE
3 Copyright (C) 1991-1995, Thomas G. Lane.
4 This file is part of the Independent JPEG Group's software.
5 For conditions of distribution and use, see the accompanying README file.
8 This file provides an overview of the architecture of the IJG JPEG software;
9 that is, the functions of the various modules in the system and the interfaces
10 between modules. For more precise details about any data structure or calling
11 convention, see the include files and comments in the source code.
13 We assume that the reader is already somewhat familiar with the JPEG standard.
14 The README file includes references for learning about JPEG. The file
15 libjpeg.doc describes the library from the viewpoint of an application
16 programmer using the library; it's best to read that file before this one.
17 Also, the file coderules.doc describes the coding style conventions we use.
19 In 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
27 We do not use the terms "pixel" and "sample" interchangeably. When we say
28 pixel, we mean an element of the full-size image, while a sample is an element
29 of the downsampled image. Thus the number of samples may vary across
30 components while the number of pixels does not. (This terminology is not used
31 rigorously throughout the code, but it is used in places where confusion would
35 *** System features ***
37 The 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.
41 cjpeg/djpeg are of no great intellectual complexity: they merely add a simple
42 command-line user interface and I/O routines for several uncompressed image
43 formats. This document concentrates on the library itself.
45 We desire the library to be capable of supporting all JPEG baseline, extended
46 sequential, and progressive DCT processes. Hierarchical processes are not
49 The library does not support the lossless (spatial) JPEG process. Lossless
50 JPEG shares little or no code with lossy JPEG, and would normally be used
51 without the extensive pre- and post-processing provided by this library.
52 We feel that lossless JPEG is better handled by a separate library.
54 Within these limits, any set of compression parameters allowed by the JPEG
55 spec should be readable for decompression. (We can be more restrictive about
56 what formats we can generate.) Although the system design allows for all
57 parameter values, some uncommon settings are not yet implemented and may
58 never be; nonintegral sampling ratios are the prime example. Furthermore,
59 we treat 8-bit vs. 12-bit data precision as a compile-time switch, not a
60 run-time option, because most machines can store 8-bit pixels much more
61 compactly than 12-bit.
63 For legal reasons, JPEG arithmetic coding is not currently supported, but
64 extending the library to include it would be straightforward.
66 By itself, the library handles only interchange JPEG datastreams --- in
67 particular the widely used JFIF file format. The library can be used by
68 surrounding code to process interchange or abbreviated JPEG datastreams that
69 are embedded in more complex file formats. (For example, libtiff uses this
70 library to implement JPEG compression within the TIFF file format.)
72 The library includes a substantial amount of code that is not covered by the
73 JPEG standard but is necessary for typical applications of JPEG. These
74 functions preprocess the image before JPEG compression or postprocess it after
75 decompression. They include colorspace conversion, downsampling/upsampling,
76 and color quantization. This code can be omitted if not needed.
78 A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
79 and even more so in decompression postprocessing. The decompression library
80 provides multiple implementations that cover most of the useful tradeoffs,
81 ranging from very-high-quality down to fast-preview operation. On the
82 compression side we have generally not provided low-quality choices, since
83 compression is normally less time-critical. It should be understood that the
84 low-quality modes may not meet the JPEG standard's accuracy requirements;
85 nonetheless, they are useful for viewers.
88 *** Portability issues ***
90 Portability is an essential requirement for the library. The key portability
91 issues that show up at the level of system architecture are:
93 1. Memory usage. We want the code to be able to run on PC-class machines
94 with limited memory. Images should therefore be processed sequentially (in
95 strips), to avoid holding the whole image in memory at once. Where a
96 full-image buffer is necessary, we should be able to use either virtual memory
99 2. Near/far pointer distinction. To run efficiently on 80x86 machines, the
100 code should distinguish "small" objects (kept in near data space) from
101 "large" ones (kept in far data space). This is an annoying restriction, but
102 fortunately it does not impact code quality for less brain-damaged machines,
103 and the source code clutter turns out to be minimal with sufficient use of
106 3. Data precision. We assume that "char" is at least 8 bits, "short" and
107 "int" at least 16, "long" at least 32. The code will work fine with larger
108 data sizes, although memory may be used inefficiently in some cases. However,
109 the JPEG compressed datastream must ultimately appear on external storage as a
110 sequence of 8-bit bytes if it is to conform to the standard. This may pose a
111 problem on machines where char is wider than 8 bits. The library represents
112 compressed data as an array of values of typedef JOCTET. If no data type
113 exactly 8 bits wide is available, custom data source and data destination
114 modules must be written to unpack and pack the chosen JOCTET datatype into
115 8-bit external representation.
118 *** System overview ***
120 The compressor and decompressor are each divided into two main sections:
121 the JPEG compressor or decompressor proper, and the preprocessing or
122 postprocessing functions. The interface between these two sections is the
123 image data that the official JPEG spec regards as its input or output: this
124 data is in the colorspace to be used for compression, and it is downsampled
125 to the sampling factors to be used. The preprocessing and postprocessing
126 steps are responsible for converting a normal image representation to or from
127 this form. (Those few applications that want to deal with YCbCr downsampled
128 data can skip the preprocessing or postprocessing step.)
130 Looking more closely, the compressor library contains the following main
134 * Color space conversion (e.g., RGB to YCbCr).
135 * Edge expansion and downsampling. Optionally, this step can do simple
136 smoothing --- this is often helpful for low-quality source data.
138 * MCU assembly, DCT, quantization.
139 * Entropy coding (sequential or progressive, Huffman or arithmetic).
141 In addition to these modules we need overall control, marker generation,
142 and support code (memory management & error handling). There is also a
143 module responsible for physically writing the output data --- typically
144 this is just an interface to fwrite(), but some applications may need to
145 do something else with the data.
147 The decompressor library contains the following main elements:
150 * Entropy decoding (sequential or progressive, Huffman or arithmetic).
151 * Dequantization, inverse DCT, MCU disassembly.
153 * Upsampling. Optionally, this step may be able to do more general
154 rescaling of the image.
155 * Color space conversion (e.g., YCbCr to RGB). This step may also
156 provide gamma adjustment [ currently it does not ].
157 * Optional color quantization (e.g., reduction to 256 colors).
158 * Optional color precision reduction (e.g., 24-bit to 15-bit color).
159 [This feature is not currently implemented.]
161 We also need overall control, marker parsing, and a data source module.
162 The support code (memory management & error handling) can be shared with
163 the compression half of the library.
165 There may be several implementations of each of these elements, particularly
166 in the decompressor, where a wide range of speed/quality tradeoffs is very
167 useful. It must be understood that some of the best speedups involve
168 merging adjacent steps in the pipeline. For example, upsampling, color space
169 conversion, and color quantization might all be done at once when using a
170 low-quality ordered-dither technique. The system architecture is designed to
171 allow such merging where appropriate.
174 Note: it is convenient to regard edge expansion (padding to block boundaries)
175 as a preprocessing/postprocessing function, even though the JPEG spec includes
176 it in compression/decompression. We do this because downsampling/upsampling
177 can be simplified a little if they work on padded data: it's not necessary to
178 have special cases at the right and bottom edges. Therefore the interface
179 buffer is always an integral number of blocks wide and high, and we expect
180 compression preprocessing to pad the source data properly. Padding will occur
181 only to the next block (8-sample) boundary. In an interleaved-scan situation,
182 additional dummy blocks may be used to fill out MCUs, but the MCU assembly and
183 disassembly logic will create or discard these blocks internally. (This is
184 advantageous for speed reasons, since we avoid DCTing the dummy blocks.
185 It also permits a small reduction in file size, because the compressor can
186 choose dummy block contents so as to minimize their size in compressed form.
187 Finally, it makes the interface buffer specification independent of whether
188 the file is actually interleaved or not.) Applications that wish to deal
189 directly with the downsampled data must provide similar buffering and padding
190 for odd-sized images.
193 *** Poor man's object-oriented programming ***
195 It should be clear by now that we have a lot of quasi-independent processing
196 steps, many of which have several possible behaviors. To avoid cluttering the
197 code with lots of switch statements, we use a simple form of object-style
198 programming to separate out the different possibilities.
200 For example, two different color quantization algorithms could be implemented
201 as two separate modules that present the same external interface; at runtime,
202 the calling code will access the proper module indirectly through an "object".
204 We can get the limited features we need while staying within portable C.
205 The basic tool is a function pointer. An "object" is just a struct
206 containing one or more function pointer fields, each of which corresponds to
207 a method name in real object-oriented languages. During initialization we
208 fill in the function pointers with references to whichever module we have
209 determined we need to use in this run. Then invocation of the module is done
210 by indirecting through a function pointer; on most machines this is no more
211 expensive than a switch statement, which would be the only other way of
212 making the required run-time choice. The really significant benefit, of
213 course, is keeping the source code clean and well structured.
215 We can also arrange to have private storage that varies between different
216 implementations of the same kind of object. We do this by making all the
217 module-specific object structs be separately allocated entities, which will
218 be accessed via pointers in the master compression or decompression struct.
219 The "public" fields or methods for a given kind of object are specified by
220 a commonly known struct. But a module's initialization code can allocate
221 a larger struct that contains the common struct as its first member, plus
222 additional private fields. With appropriate pointer casting, the module's
223 internal functions can access these private fields. (For a simple example,
224 see jdatadst.c, which implements the external interface specified by struct
225 jpeg_destination_mgr, but adds extra fields.)
227 (Of course this would all be a lot easier if we were using C++, but we are
228 not yet prepared to assume that everyone has a C++ compiler.)
230 An important benefit of this scheme is that it is easy to provide multiple
231 versions of any method, each tuned to a particular case. While a lot of
232 precalculation might be done to select an optimal implementation of a method,
233 the cost per invocation is constant. For example, the upsampling step might
234 have a "generic" method, plus one or more "hardwired" methods for the most
235 popular sampling factors; the hardwired methods would be faster because they'd
236 use straight-line code instead of for-loops. The cost to determine which
237 method to use is paid only once, at startup, and the selection criteria are
238 hidden from the callers of the method.
240 This plan differs a little bit from usual object-oriented structures, in that
241 only one instance of each object class will exist during execution. The
242 reason for having the class structure is that on different runs we may create
243 different instances (choose to execute different modules). You can think of
244 the term "method" as denoting the common interface presented by a particular
245 set of interchangeable functions, and "object" as denoting a group of related
246 methods, or the total shared interface behavior of a group of modules.
249 *** Overall control structure ***
251 We previously mentioned the need for overall control logic in the compression
252 and decompression libraries. In IJG implementations prior to v5, overall
253 control was mostly provided by "pipeline control" modules, which proved to be
254 large, unwieldy, and hard to understand. To improve the situation, the
255 control logic has been subdivided into multiple modules. The control modules
258 1. Master control for module selection and initialization. This has two
261 1A. Startup initialization at the beginning of image processing.
262 The individual processing modules to be used in this run are selected
263 and given initialization calls.
265 1B. Per-pass control. This determines how many passes will be performed
266 and calls each active processing module to configure itself
267 appropriately at the beginning of each pass. End-of-pass processing,
268 where necessary, is also invoked from the master control module.
270 Method selection is partially distributed, in that a particular processing
271 module may contain several possible implementations of a particular method,
272 which it will select among when given its initialization call. The master
273 control code need only be concerned with decisions that affect more than
276 2. Data buffering control. A separate control module exists for each
277 inter-processing-step data buffer. This module is responsible for
278 invoking the processing steps that write or read that data buffer.
280 Each buffer controller sees the world as follows:
282 input data => processing step A => buffer => processing step B => output data
284 ------------------ controller ------------------
286 The controller knows the dataflow requirements of steps A and B: how much data
287 they want to accept in one chunk and how much they output in one chunk. Its
288 function is to manage its buffer and call A and B at the proper times.
290 A data buffer control module may itself be viewed as a processing step by a
291 higher-level control module; thus the control modules form a binary tree with
292 elementary processing steps at the leaves of the tree.
294 The control modules are objects. A considerable amount of flexibility can
295 be had by replacing implementations of a control module. For example:
296 * Merging of adjacent steps in the pipeline is done by replacing a control
297 module and its pair of processing-step modules with a single processing-
298 step module. (Hence the possible merges are determined by the tree of
300 * In some processing modes, a given interstep buffer need only be a "strip"
301 buffer large enough to accommodate the desired data chunk sizes. In other
302 modes, a full-image buffer is needed and several passes are required.
303 The control module determines which kind of buffer is used and manipulates
304 virtual array buffers as needed. One or both processing steps may be
305 unaware of the multi-pass behavior.
307 In theory, we might be able to make all of the data buffer controllers
308 interchangeable and provide just one set of implementations for all. In
309 practice, each one contains considerable special-case processing for its
310 particular job. The buffer controller concept should be regarded as an
311 overall system structuring principle, not as a complete description of the
312 task performed by any one controller.
315 *** Compression object structure ***
317 Here is a sketch of the logical structure of the JPEG compression library:
319 |-- Colorspace conversion
320 |-- Preprocessing controller --|
323 | |-- Forward DCT, quantize
324 |-- Coefficient controller --|
327 This sketch also describes the flow of control (subroutine calls) during
328 typical image data processing. Each of the components shown in the diagram is
329 an "object" which may have several different implementations available. One
330 or more source code files contain the actual implementation(s) of each object.
332 The objects shown above are:
334 * Main controller: buffer controller for the subsampled-data buffer, which
335 holds the preprocessed input data. This controller invokes preprocessing to
336 fill the subsampled-data buffer, and JPEG compression to empty it. There is
337 usually no need for a full-image buffer here; a strip buffer is adequate.
339 * Preprocessing controller: buffer controller for the downsampling input data
340 buffer, which lies between colorspace conversion and downsampling. Note
341 that a unified conversion/downsampling module would probably replace this
344 * Colorspace conversion: converts application image data into the desired
345 JPEG color space; also changes the data from pixel-interleaved layout to
346 separate component planes. Processes one pixel row at a time.
348 * Downsampling: performs reduction of chroma components as required.
349 Optionally may perform pixel-level smoothing as well. Processes a "row
350 group" at a time, where a row group is defined as Vmax pixel rows of each
351 component before downsampling, and Vk sample rows afterwards (remember Vk
352 differs across components). Some downsampling or smoothing algorithms may
353 require context rows above and below the current row group; the
354 preprocessing controller is responsible for supplying these rows via proper
355 buffering. The downsampler is responsible for edge expansion at the right
356 edge (i.e., extending each sample row to a multiple of 8 samples); but the
357 preprocessing controller is responsible for vertical edge expansion (i.e.,
358 duplicating the bottom sample row as needed to make a multiple of 8 rows).
360 * Coefficient controller: buffer controller for the DCT-coefficient data.
361 This controller handles MCU assembly, including insertion of dummy DCT
362 blocks when needed at the right or bottom edge. When performing
363 Huffman-code optimization or emitting a multiscan JPEG file, this
364 controller is responsible for buffering the full image. The equivalent of
365 one fully interleaved MCU row of subsampled data is processed per call,
366 even when the JPEG file is noninterleaved.
368 * Forward DCT and quantization: Perform DCT, quantize, and emit coefficients.
369 Works on one or more DCT blocks at a time. (Note: the coefficients are now
370 emitted in normal array order, which the entropy encoder is expected to
371 convert to zigzag order as necessary. Prior versions of the IJG code did
372 the conversion to zigzag order within the quantization step.)
374 * Entropy encoding: Perform Huffman or arithmetic entropy coding and emit the
375 coded data to the data destination module. Works on one MCU per call.
376 For progressive JPEG, the same DCT blocks are fed to the entropy coder
377 during each pass, and the coder must emit the appropriate subset of
380 In addition to the above objects, the compression library includes these
383 * Master control: determines the number of passes required, controls overall
384 and per-pass initialization of the other modules.
386 * Marker writing: generates JPEG markers (except for RSTn, which is emitted
387 by the entropy encoder when needed).
389 * Data destination manager: writes the output JPEG datastream to its final
390 destination (e.g., a file). The destination manager supplied with the
391 library knows how to write to a stdio stream; for other behaviors, the
392 surrounding application may provide its own destination manager.
394 * Memory manager: allocates and releases memory, controls virtual arrays
395 (with backing store management, where required).
397 * Error handler: performs formatting and output of error and trace messages;
398 determines handling of nonfatal errors. The surrounding application may
399 override some or all of this object's methods to change error handling.
401 * Progress monitor: supports output of "percent-done" progress reports.
402 This object represents an optional callback to the surrounding application:
403 if wanted, it must be supplied by the application.
405 The error handler, destination manager, and progress monitor objects are
406 defined as separate objects in order to simplify application-specific
407 customization of the JPEG library. A surrounding application may override
408 individual methods or supply its own all-new implementation of one of these
409 objects. The object interfaces for these objects are therefore treated as
410 part of the application interface of the library, whereas the other objects
411 are internal to the library.
413 The error handler and memory manager are shared by JPEG compression and
414 decompression; the progress monitor, if used, may be shared as well.
417 *** Decompression object structure ***
419 Here is a sketch of the logical structure of the JPEG decompression library:
422 |-- Coefficient controller --|
423 | |-- Dequantize, Inverse DCT
426 |-- Postprocessing controller --| |-- Colorspace conversion
427 |-- Color quantization
428 |-- Color precision reduction
430 As before, this diagram also represents typical control flow. The objects
433 * Main controller: buffer controller for the subsampled-data buffer, which
434 holds the output of JPEG decompression proper. This controller's primary
435 task is to feed the postprocessing procedure. Some upsampling algorithms
436 may require context rows above and below the current row group; when this
437 is true, the main controller is responsible for managing its buffer so as
438 to make context rows available. In the current design, the main buffer is
439 always a strip buffer; a full-image buffer is never required.
441 * Coefficient controller: buffer controller for the DCT-coefficient data.
442 This controller handles MCU disassembly, including deletion of any dummy
443 DCT blocks at the right or bottom edge. When reading a multiscan JPEG
444 file, this controller is responsible for buffering the full image.
445 (Buffering DCT coefficients, rather than samples, is necessary to support
446 progressive JPEG.) The equivalent of one fully interleaved MCU row of
447 subsampled data is processed per call, even when the source JPEG file is
450 * Entropy decoding: Read coded data from the data source module and perform
451 Huffman or arithmetic entropy decoding. Works on one MCU per call.
452 For progressive JPEG decoding, the coefficient controller supplies the prior
453 coefficients of each MCU (initially all zeroes), which the entropy decoder
454 modifies in each scan.
456 * Dequantization and inverse DCT: like it says. Note that the coefficients
457 buffered by the coefficient controller have NOT been dequantized; we
458 merge dequantization and inverse DCT into a single step for speed reasons.
459 When scaled-down output is asked for, simplified DCT algorithms may be used
460 that emit only 1x1, 2x2, or 4x4 samples per DCT block, not the full 8x8.
461 Works on one DCT block at a time.
463 * Postprocessing controller: buffer controller for the color quantization
464 input buffer, when quantization is in use. (Without quantization, this
465 controller just calls the upsampler.) For two-pass quantization, this
466 controller is responsible for buffering the full-image data.
468 * Upsampling: restores chroma components to full size. (May support more
469 general output rescaling, too. Note that if undersized DCT outputs have
470 been emitted by the DCT module, this module must adjust so that properly
471 sized outputs are created.) Works on one row group at a time. This module
472 also calls the color conversion module, so its top level is effectively a
473 buffer controller for the upsampling->color conversion buffer. However, in
474 all but the highest-quality operating modes, upsampling and color
475 conversion are likely to be merged into a single step.
477 * Colorspace conversion: convert from JPEG color space to output color space,
478 and change data layout from separate component planes to pixel-interleaved.
479 Works on one pixel row at a time.
481 * Color quantization: reduce the data to colormapped form, using either an
482 externally specified colormap or an internally generated one. This module
483 is not used for full-color output. Works on one pixel row at a time; may
484 require two passes to generate a color map. Note that the output will
485 always be a single component representing colormap indexes. In the current
486 design, the output values are JSAMPLEs, so an 8-bit compilation cannot
487 quantize to more than 256 colors. This is unlikely to be a problem in
490 * Color reduction: this module handles color precision reduction, e.g.,
491 generating 15-bit color (5 bits/primary) from JPEG's 24-bit output.
492 Not quite clear yet how this should be handled... should we merge it with
493 colorspace conversion???
495 Note that some high-speed operating modes might condense the entire
496 postprocessing sequence to a single module (upsample, color convert, and
497 quantize in one step).
499 In addition to the above objects, the decompression library includes these
502 * Master control: determines the number of passes required, controls overall
503 and per-pass initialization of the other modules. This is subdivided into
504 input and output control: jdinput.c controls only input-side processing,
505 while jdmaster.c handles overall initialization and output-side control.
507 * Marker reading: decodes JPEG markers (except for RSTn).
509 * Data source manager: supplies the input JPEG datastream. The source
510 manager supplied with the library knows how to read from a stdio stream;
511 for other behaviors, the surrounding application may provide its own source
514 * Memory manager: same as for compression library.
516 * Error handler: same as for compression library.
518 * Progress monitor: same as for compression library.
520 As with compression, the data source manager, error handler, and progress
521 monitor are candidates for replacement by a surrounding application.
524 *** Decompression input and output separation ***
526 To support efficient incremental display of progressive JPEG files, the
527 decompressor is divided into two sections that can run independently:
529 1. Data input includes marker parsing, entropy decoding, and input into the
530 coefficient controller's DCT coefficient buffer. Note that this
531 processing is relatively cheap and fast.
533 2. Data output reads from the DCT coefficient buffer and performs the IDCT
534 and all postprocessing steps.
536 For a progressive JPEG file, the data input processing is allowed to get
537 arbitrarily far ahead of the data output processing. (This occurs only
538 if the application calls jpeg_consume_input(); otherwise input and output
539 run in lockstep, since the input section is called only when the output
540 section needs more data.) In this way the application can avoid making
541 extra display passes when data is arriving faster than the display pass
542 can run. Furthermore, it is possible to abort an output pass without
543 losing anything, since the coefficient buffer is read-only as far as the
544 output section is concerned. See libjpeg.doc for more detail.
546 A full-image coefficient array is only created if the JPEG file has multiple
547 scans (or if the application specifies buffered-image mode anyway). When
548 reading a single-scan file, the coefficient controller normally creates only
549 a one-MCU buffer, so input and output processing must run in lockstep in this
550 case. jpeg_consume_input() is effectively a no-op in this situation.
552 The main impact of dividing the decompressor in this fashion is that we must
553 be very careful with shared variables in the cinfo data structure. Each
554 variable that can change during the course of decompression must be
555 classified as belonging to data input or data output, and each section must
556 look only at its own variables. For example, the data output section may not
557 depend on any of the variables that describe the current scan in the JPEG
558 file, because these may change as the data input section advances into a new
561 The progress monitor is (somewhat arbitrarily) defined to treat input of the
562 file as one pass when buffered-image mode is not used, and to ignore data
563 input work completely when buffered-image mode is used. Note that the
564 library has no reliable way to predict the number of passes when dealing
565 with a progressive JPEG file, nor can it predict the number of output passes
566 in buffered-image mode. So the work estimate is inherently bogus anyway.
568 No comparable division is currently made in the compression library, because
569 there isn't any real need for it.
574 Arrays of pixel sample values use the following data structure:
576 typedef something JSAMPLE; a pixel component value, 0..MAXJSAMPLE
577 typedef JSAMPLE *JSAMPROW; ptr to a row of samples
578 typedef JSAMPROW *JSAMPARRAY; ptr to a list of rows
579 typedef JSAMPARRAY *JSAMPIMAGE; ptr to a list of color-component arrays
581 The basic element type JSAMPLE will typically be one of unsigned char,
582 (signed) char, or short. Short will be used if samples wider than 8 bits are
583 to be supported (this is a compile-time option). Otherwise, unsigned char is
584 used if possible. If the compiler only supports signed chars, then it is
585 necessary to mask off the value when reading. Thus, all reads of JSAMPLE
586 values must be coded as "GETJSAMPLE(value)", where the macro will be defined
587 as "((value) & 0xFF)" on signed-char machines and "((int) (value))" elsewhere.
589 With these conventions, JSAMPLE values can be assumed to be >= 0. This helps
590 simplify correct rounding during downsampling, etc. The JPEG standard's
591 specification that sample values run from -128..127 is accommodated by
592 subtracting 128 just as the sample value is copied into the source array for
593 the DCT step (this will be an array of signed ints). Similarly, during
594 decompression the output of the IDCT step will be immediately shifted back to
595 0..255. (NB: different values are required when 12-bit samples are in use.
596 The code is written in terms of MAXJSAMPLE and CENTERJSAMPLE, which will be
597 defined as 255 and 128 respectively in an 8-bit implementation, and as 4095
598 and 2048 in a 12-bit implementation.)
600 We use a pointer per row, rather than a two-dimensional JSAMPLE array. This
601 choice costs only a small amount of memory and has several benefits:
602 * Code using the data structure doesn't need to know the allocated width of
603 the rows. This simplifies edge expansion/compression, since we can work
604 in an array that's wider than the logical picture width.
605 * Indexing doesn't require multiplication; this is a performance win on many
607 * Arrays with more than 64K total elements can be supported even on machines
608 where malloc() cannot allocate chunks larger than 64K.
609 * The rows forming a component array may be allocated at different times
610 without extra copying. This trick allows some speedups in smoothing steps
611 that need access to the previous and next rows.
613 Note that each color component is stored in a separate array; we don't use the
614 traditional layout in which the components of a pixel are stored together.
615 This simplifies coding of modules that work on each component independently,
616 because they don't need to know how many components there are. Furthermore,
617 we can read or write each component to a temporary file independently, which
618 is helpful when dealing with noninterleaved JPEG files.
620 In general, a specific sample value is accessed by code such as
621 GETJSAMPLE(image[colorcomponent][row][col])
622 where col is measured from the image left edge, but row is measured from the
623 first sample row currently in memory. Either of the first two indexings can
624 be precomputed by copying the relevant pointer.
627 Since most image-processing applications prefer to work on images in which
628 the components of a pixel are stored together, the data passed to or from the
629 surrounding application uses the traditional convention: a single pixel is
630 represented by N consecutive JSAMPLE values, and an image row is an array of
631 (# of color components)*(image width) JSAMPLEs. One or more rows of data can
632 be represented by a pointer of type JSAMPARRAY in this scheme. This scheme is
633 converted to component-wise storage inside the JPEG library. (Applications
634 that want to skip JPEG preprocessing or postprocessing will have to contend
635 with component-wise storage.)
638 Arrays of DCT-coefficient values use the following data structure:
640 typedef short JCOEF; a 16-bit signed integer
641 typedef JCOEF JBLOCK[DCTSIZE2]; an 8x8 block of coefficients
642 typedef JBLOCK *JBLOCKROW; ptr to one horizontal row of 8x8 blocks
643 typedef JBLOCKROW *JBLOCKARRAY; ptr to a list of such rows
644 typedef JBLOCKARRAY *JBLOCKIMAGE; ptr to a list of color component arrays
646 The underlying type is at least a 16-bit signed integer; while "short" is big
647 enough on all machines of interest, on some machines it is preferable to use
648 "int" for speed reasons, despite the storage cost. Coefficients are grouped
649 into 8x8 blocks (but we always use #defines DCTSIZE and DCTSIZE2 rather than
652 The contents of a coefficient block may be in either "natural" or zigzagged
653 order, and may be true values or divided by the quantization coefficients,
654 depending on where the block is in the processing pipeline. In the current
655 library, coefficient blocks are kept in natural order everywhere; the entropy
656 codecs zigzag or dezigzag the data as it is written or read. The blocks
657 contain quantized coefficients everywhere outside the DCT/IDCT subsystems.
658 (This latter decision may need to be revisited to support variable
659 quantization a la JPEG Part 3.)
661 Notice that the allocation unit is now a row of 8x8 blocks, corresponding to
662 eight rows of samples. Otherwise the structure is much the same as for
663 samples, and for the same reasons.
665 On machines where malloc() can't handle a request bigger than 64Kb, this data
666 structure limits us to rows of less than 512 JBLOCKs, or a picture width of
667 4000+ pixels. This seems an acceptable restriction.
670 On 80x86 machines, the bottom-level pointer types (JSAMPROW and JBLOCKROW)
671 must be declared as "far" pointers, but the upper levels can be "near"
672 (implying that the pointer lists are allocated in the DS segment).
673 We use a #define symbol FAR, which expands to the "far" keyword when
674 compiling on 80x86 machines and to nothing elsewhere.
677 *** Suspendable processing ***
679 In some applications it is desirable to use the JPEG library as an
680 incremental, memory-to-memory filter. In this situation the data source or
681 destination may be a limited-size buffer, and we can't rely on being able to
682 empty or refill the buffer at arbitrary times. Instead the application would
683 like to have control return from the library at buffer overflow/underrun, and
684 then resume compression or decompression at a later time.
686 This scenario is supported for simple cases. (For anything more complex, we
687 recommend that the application "bite the bullet" and develop real multitasking
688 capability.) The libjpeg.doc file goes into more detail about the usage and
689 limitations of this capability; here we address the implications for library
692 The essence of the problem is that the entropy codec (coder or decoder) must
693 be prepared to stop at arbitrary times. In turn, the controllers that call
694 the entropy codec must be able to stop before having produced or consumed all
695 the data that they normally would handle in one call. That part is reasonably
696 straightforward: we make the controller call interfaces include "progress
697 counters" which indicate the number of data chunks successfully processed, and
698 we require callers to test the counter rather than just assume all of the data
701 Rather than trying to restart at an arbitrary point, the current Huffman
702 codecs are designed to restart at the beginning of the current MCU after a
703 suspension due to buffer overflow/underrun. At the start of each call, the
704 codec's internal state is loaded from permanent storage (in the JPEG object
705 structures) into local variables. On successful completion of the MCU, the
706 permanent state is updated. (This copying is not very expensive, and may even
707 lead to *improved* performance if the local variables can be registerized.)
708 If a suspension occurs, the codec simply returns without updating the state,
709 thus effectively reverting to the start of the MCU. Note that this implies
710 leaving some data unprocessed in the source/destination buffer (ie, the
711 compressed partial MCU). The data source/destination module interfaces are
712 specified so as to make this possible. This also implies that the data buffer
713 must be large enough to hold a worst-case compressed MCU; a couple thousand
714 bytes should be enough.
716 In a successive-approximation AC refinement scan, the progressive Huffman
717 decoder has to be able to undo assignments of newly nonzero coefficients if it
718 suspends before the MCU is complete, since decoding requires distinguishing
719 previously-zero and previously-nonzero coefficients. This is a bit tedious
720 but probably won't have much effect on performance. Other variants of Huffman
721 decoding need not worry about this, since they will just store the same values
722 again if forced to repeat the MCU.
724 This approach would probably not work for an arithmetic codec, since its
725 modifiable state is quite large and couldn't be copied cheaply. Instead it
726 would have to suspend and resume exactly at the point of the buffer end.
728 The JPEG marker reader is designed to cope with suspension at an arbitrary
729 point. It does so by backing up to the start of the marker parameter segment,
730 so the data buffer must be big enough to hold the largest marker of interest.
731 Again, a couple KB should be adequate. (A special "skip" convention is used
732 to bypass COM and APPn markers, so these can be larger than the buffer size
733 without causing problems; otherwise a 64K buffer would be needed in the worst
736 The JPEG marker writer currently does *not* cope with suspension. I feel that
737 this is not necessary; it is much easier simply to require the application to
738 ensure there is enough buffer space before starting. (An empty 2K buffer is
739 more than sufficient for the header markers; and ensuring there are a dozen or
740 two bytes available before calling jpeg_finish_compress() will suffice for the
741 trailer.) This would not work for writing multi-scan JPEG files, but
742 we simply do not intend to support that capability with suspension.
745 *** Memory manager services ***
747 The JPEG library's memory manager controls allocation and deallocation of
748 memory, and it manages large "virtual" data arrays on machines where the
749 operating system does not provide virtual memory. Note that the same
750 memory manager serves both compression and decompression operations.
752 In all cases, allocated objects are tied to a particular compression or
753 decompression master record, and they will be released when that master
756 The memory manager does not provide explicit deallocation of objects.
757 Instead, objects are created in "pools" of free storage, and a whole pool
758 can be freed at once. This approach helps prevent storage-leak bugs, and
759 it speeds up operations whenever malloc/free are slow (as they often are).
760 The pools can be regarded as lifetime identifiers for objects. Two
761 pools/lifetimes are defined:
762 * JPOOL_PERMANENT lasts until master record is destroyed
763 * JPOOL_IMAGE lasts until done with image (JPEG datastream)
764 Permanent lifetime is used for parameters and tables that should be carried
765 across from one datastream to another; this includes all application-visible
766 parameters. Image lifetime is used for everything else. (A third lifetime,
767 JPOOL_PASS = one processing pass, was originally planned. However it was
768 dropped as not being worthwhile. The actual usage patterns are such that the
769 peak memory usage would be about the same anyway; and having per-pass storage
770 substantially complicates the virtual memory allocation rules --- see below.)
772 The memory manager deals with three kinds of object:
773 1. "Small" objects. Typically these require no more than 10K-20K total.
774 2. "Large" objects. These may require tens to hundreds of K depending on
775 image size. Semantically they behave the same as small objects, but we
776 distinguish them for two reasons:
777 * On MS-DOS machines, large objects are referenced by FAR pointers,
778 small objects by NEAR pointers.
779 * Pool allocation heuristics may differ for large and small objects.
780 Note that individual "large" objects cannot exceed the size allowed by
781 type size_t, which may be 64K or less on some machines.
782 3. "Virtual" objects. These are large 2-D arrays of JSAMPLEs or JBLOCKs
783 (typically large enough for the entire image being processed). The
784 memory manager provides stripwise access to these arrays. On machines
785 without virtual memory, the rest of the array may be swapped out to a
788 (Note: JSAMPARRAY and JBLOCKARRAY data structures are a combination of large
789 objects for the data proper and small objects for the row pointers. For
790 convenience and speed, the memory manager provides single routines to create
791 these structures. Similarly, virtual arrays include a small control block
792 and a JSAMPARRAY or JBLOCKARRAY working buffer, all created with one call.)
794 In the present implementation, virtual arrays are only permitted to have image
795 lifespan. (Permanent lifespan would not be reasonable, and pass lifespan is
796 not very useful since a virtual array's raison d'etre is to store data for
797 multiple passes through the image.) We also expect that only "small" objects
798 will be given permanent lifespan, though this restriction is not required by
801 In a non-virtual-memory machine, some performance benefit can be gained by
802 making the in-memory buffers for virtual arrays be as large as possible.
803 (For small images, the buffers might fit entirely in memory, so blind
804 swapping would be very wasteful.) The memory manager will adjust the height
805 of the buffers to fit within a prespecified maximum memory usage. In order
806 to do this in a reasonably optimal fashion, the manager needs to allocate all
807 of the virtual arrays at once. Therefore, there isn't a one-step allocation
808 routine for virtual arrays; instead, there is a "request" routine that simply
809 allocates the control block, and a "realize" routine (called just once) that
810 determines space allocation and creates all of the actual buffers. The
811 realize routine must allow for space occupied by non-virtual large objects.
812 (We don't bother to factor in the space needed for small objects, on the
813 grounds that it isn't worth the trouble.)
815 To support all this, we establish the following protocol for doing business
816 with the memory manager:
817 1. Modules must request virtual arrays (which may have only image lifespan)
818 during the initial setup phase, i.e., in their jinit_xxx routines.
819 2. All "large" objects (including JSAMPARRAYs and JBLOCKARRAYs) must also be
820 allocated during initial setup.
821 3. realize_virt_arrays will be called at the completion of initial setup.
822 The above conventions ensure that sufficient information is available
823 for it to choose a good size for virtual array buffers.
824 Small objects of any lifespan may be allocated at any time. We expect that
825 the total space used for small objects will be small enough to be negligible
826 in the realize_virt_arrays computation.
828 In a virtual-memory machine, we simply pretend that the available space is
829 infinite, thus causing realize_virt_arrays to decide that it can allocate all
830 the virtual arrays as full-size in-memory buffers. The overhead of the
831 virtual-array access protocol is very small when no swapping occurs.
833 A virtual array can be specified to be "pre-zeroed"; when this flag is set,
834 never-yet-written sections of the array are set to zero before being made
835 available to the caller. If this flag is not set, never-written sections
836 of the array contain garbage. (This feature exists primarily because the
837 equivalent logic would otherwise be needed in jdcoefct.c for progressive
838 JPEG mode; we may as well make it available for possible other uses.)
840 The first write pass on a virtual array is required to occur in top-to-bottom
841 order; read passes, as well as any write passes after the first one, may
842 access the array in any order. This restriction exists partly to simplify
843 the virtual array control logic, and partly because some file systems may not
844 support seeking beyond the current end-of-file in a temporary file. The main
845 implication of this restriction is that rearrangement of rows (such as
846 converting top-to-bottom data order to bottom-to-top) must be handled while
847 reading data out of the virtual array, not while putting it in.
850 *** Memory manager internal structure ***
852 To isolate system dependencies as much as possible, we have broken the
853 memory manager into two parts. There is a reasonably system-independent
854 "front end" (jmemmgr.c) and a "back end" that contains only the code
855 likely to change across systems. All of the memory management methods
856 outlined above are implemented by the front end. The back end provides
857 the following routines for use by the front end (none of these routines
858 are known to the rest of the JPEG code):
860 jpeg_mem_init, jpeg_mem_term system-dependent initialization/shutdown
862 jpeg_get_small, jpeg_free_small interface to malloc and free library routines
863 (or their equivalents)
865 jpeg_get_large, jpeg_free_large interface to FAR malloc/free in MSDOS machines;
866 else usually the same as
867 jpeg_get_small/jpeg_free_small
869 jpeg_mem_available estimate available memory
871 jpeg_open_backing_store create a backing-store object
873 read_backing_store, manipulate a backing-store object
877 On some systems there will be more than one type of backing-store object
878 (specifically, in MS-DOS a backing store file might be an area of extended
879 memory as well as a disk file). jpeg_open_backing_store is responsible for
880 choosing how to implement a given object. The read/write/close routines
881 are method pointers in the structure that describes a given object; this
882 lets them be different for different object types.
884 It may be necessary to ensure that backing store objects are explicitly
885 released upon abnormal program termination. For example, MS-DOS won't free
886 extended memory by itself. To support this, we will expect the main program
887 or surrounding application to arrange to call self_destruct (typically via
888 jpeg_destroy) upon abnormal termination. This may require a SIGINT signal
889 handler or equivalent. We don't want to have the back end module install its
890 own signal handler, because that would pre-empt the surrounding application's
891 ability to control signal handling.
893 The IJG distribution includes several memory manager back end implementations.
894 Usually the same back end should be suitable for all applications on a given
895 system, but it is possible for an application to supply its own back end at
899 *** Implications of DNL marker ***
901 Some JPEG files may use a DNL marker to postpone definition of the image
902 height (this would be useful for a fax-like scanner's output, for instance).
903 In these files the SOF marker claims the image height is 0, and you only
904 find out the true image height at the end of the first scan.
906 We could read these files as follows:
907 1. Upon seeing zero image height, replace it by 65535 (the maximum allowed).
908 2. When the DNL is found, update the image height in the global image
910 This implies that control modules must avoid making copies of the image
911 height, and must re-test for termination after each MCU row. This would
912 be easy enough to do.
914 In cases where image-size data structures are allocated, this approach will
915 result in very inefficient use of virtual memory or much-larger-than-necessary
916 temporary files. This seems acceptable for something that probably won't be a
917 mainstream usage. People might have to forgo use of memory-hogging options
918 (such as two-pass color quantization or noninterleaved JPEG files) if they
919 want efficient conversion of such files. (One could improve efficiency by
920 demanding a user-supplied upper bound for the height, less than 65536; in most
921 cases it could be much less.)
923 The standard also permits the SOF marker to overestimate the image height,
924 with a DNL to give the true, smaller height at the end of the first scan.
925 This would solve the space problems if the overestimate wasn't too great.
926 However, it implies that you don't even know whether DNL will be used.
928 This leads to a couple of very serious objections:
929 1. Testing for a DNL marker must occur in the inner loop of the decompressor's
930 Huffman decoder; this implies a speed penalty whether the feature is used
932 2. There is no way to hide the last-minute change in image height from an
933 application using the decoder. Thus *every* application using the IJG
934 library would suffer a complexity penalty whether it cared about DNL or
936 We currently do not support DNL because of these problems.
938 A different approach is to insist that DNL-using files be preprocessed by a
939 separate program that reads ahead to the DNL, then goes back and fixes the SOF
940 marker. This is a much simpler solution and is probably far more efficient.
941 Even if one wants piped input, buffering the first scan of the JPEG file needs
942 a lot smaller temp file than is implied by the maximum-height method. For
943 this approach we'd simply treat DNL as a no-op in the decompressor (at most,
944 check that it matches the SOF image height).
946 We will not worry about making the compressor capable of outputting DNL.
947 Something similar to the first scheme above could be applied if anyone ever
948 wants to make that work.