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IJG JPEG LIBRARY:  SYSTEM ARCHITECTURE

Copyright (C) 1991-2013, Thomas G. Lane, Guido Vollbeding.
This file is part of the Independent JPEG Group's software.
For conditions of distribution and use, see the accompanying README file.


This file provides an overview of the architecture of the IJG JPEG software;
that is, the functions of the various modules in the system and the interfaces
between modules.  For more precise details about any data structure or calling
convention, see the include files and comments in the source code.

We assume that the reader is already somewhat familiar with the JPEG standard.
The README file includes references for learning about JPEG.  The file
libjpeg.txt describes the library from the viewpoint of an application
programmer using the library; it's best to read that file before this one.
Also, the file coderules.txt describes the coding style conventions we use.

In this document, JPEG-specific terminology follows the JPEG standard:
  A "component" means a color channel, e.g., Red or Luminance.
  A "sample" is a single component value (i.e., one number in the image data).
  A "coefficient" is a frequency coefficient (a DCT transform output number).
  A "block" is an array of samples or coefficients.
  An "MCU" (minimum coded unit) is an interleaved set of blocks of size
	determined by the sampling factors, or a single block in a
	noninterleaved scan.
We do not use the terms "pixel" and "sample" interchangeably.  When we say
pixel, we mean an element of the full-size image, while a sample is an element
of the downsampled image.  Thus the number of samples may vary across
components while the number of pixels does not.  (This terminology is not used
rigorously throughout the code, but it is used in places where confusion would
otherwise result.)


*** System features ***

The IJG distribution contains two parts:
  * A subroutine library for JPEG compression and decompression.
  * cjpeg/djpeg, two sample applications that use the library to transform
    JFIF JPEG files to and from several other image formats.
cjpeg/djpeg are of no great intellectual complexity: they merely add a simple
command-line user interface and I/O routines for several uncompressed image
formats.  This document concentrates on the library itself.

We desire the library to be capable of supporting all JPEG baseline, extended
sequential, and progressive DCT processes.  The library does not support the
hierarchical or lossless processes defined in the standard.

Within these limits, any set of compression parameters allowed by the JPEG
spec should be readable for decompression.  (We can be more restrictive about
what formats we can generate.)  Although the system design allows for all
parameter values, some uncommon settings are not yet implemented and may
never be; nonintegral sampling ratios are the prime example.  Furthermore,
we treat 8-bit vs. 12-bit data precision as a compile-time switch, not a
run-time option, because most machines can store 8-bit pixels much more
compactly than 12-bit.

By itself, the library handles only interchange JPEG datastreams --- in
particular the widely used JFIF file format.  The library can be used by
surrounding code to process interchange or abbreviated JPEG datastreams that
are embedded in more complex file formats.  (For example, libtiff uses this
library to implement JPEG compression within the TIFF file format.)

The library includes a substantial amount of code that is not covered by the
JPEG standard but is necessary for typical applications of JPEG.  These
functions preprocess the image before JPEG compression or postprocess it after
decompression.  They include colorspace conversion, downsampling/upsampling,
and color quantization.  This code can be omitted if not needed.

A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
and even more so in decompression postprocessing.  The decompression library
provides multiple implementations that cover most of the useful tradeoffs,
ranging from very-high-quality down to fast-preview operation.  On the
compression side we have generally not provided low-quality choices, since
compression is normally less time-critical.  It should be understood that the
low-quality modes may not meet the JPEG standard's accuracy requirements;
nonetheless, they are useful for viewers.

src/Source/LibJPEG/structure.txt  view on Meta::CPAN

  JPEG proper:
    * MCU assembly, DCT, quantization.
    * Entropy coding (sequential or progressive, Huffman or arithmetic).

In addition to these modules we need overall control, marker generation,
and support code (memory management & error handling).  There is also a
module responsible for physically writing the output data --- typically
this is just an interface to fwrite(), but some applications may need to
do something else with the data.

The decompressor library contains the following main elements:

  JPEG proper:
    * Entropy decoding (sequential or progressive, Huffman or arithmetic).
    * Dequantization, inverse DCT, MCU disassembly.
  Postprocessing:
    * Upsampling.  Optionally, this step may be able to do more general
      rescaling of the image.
    * Color space conversion (e.g., YCbCr to RGB).  This step may also
      provide gamma adjustment [ currently it does not ].
    * Optional color quantization (e.g., reduction to 256 colors).
    * Optional color precision reduction (e.g., 24-bit to 15-bit color).
      [This feature is not currently implemented.]

We also need overall control, marker parsing, and a data source module.
The support code (memory management & error handling) can be shared with
the compression half of the library.

There may be several implementations of each of these elements, particularly
in the decompressor, where a wide range of speed/quality tradeoffs is very
useful.  It must be understood that some of the best speedups involve
merging adjacent steps in the pipeline.  For example, upsampling, color space
conversion, and color quantization might all be done at once when using a
low-quality ordered-dither technique.  The system architecture is designed to
allow such merging where appropriate.


Note: it is convenient to regard edge expansion (padding to block boundaries)
as a preprocessing/postprocessing function, even though the JPEG spec includes
it in compression/decompression.  We do this because downsampling/upsampling
can be simplified a little if they work on padded data: it's not necessary to
have special cases at the right and bottom edges.  Therefore the interface
buffer is always an integral number of blocks wide and high, and we expect
compression preprocessing to pad the source data properly.  Padding will occur
only to the next block (block_size-sample) boundary.  In an interleaved-scan
situation, additional dummy blocks may be used to fill out MCUs, but the MCU
assembly and disassembly logic will create or discard these blocks internally.
(This is advantageous for speed reasons, since we avoid DCTing the dummy
blocks.  It also permits a small reduction in file size, because the
compressor can choose dummy block contents so as to minimize their size
in compressed form.  Finally, it makes the interface buffer specification
independent of whether the file is actually interleaved or not.)
Applications that wish to deal directly with the downsampled data must
provide similar buffering and padding for odd-sized images.


*** Poor man's object-oriented programming ***

It should be clear by now that we have a lot of quasi-independent processing
steps, many of which have several possible behaviors.  To avoid cluttering the
code with lots of switch statements, we use a simple form of object-style
programming to separate out the different possibilities.

For example, two different color quantization algorithms could be implemented
as two separate modules that present the same external interface; at runtime,
the calling code will access the proper module indirectly through an "object".

We can get the limited features we need while staying within portable C.
The basic tool is a function pointer.  An "object" is just a struct
containing one or more function pointer fields, each of which corresponds to
a method name in real object-oriented languages.  During initialization we
fill in the function pointers with references to whichever module we have
determined we need to use in this run.  Then invocation of the module is done
by indirecting through a function pointer; on most machines this is no more
expensive than a switch statement, which would be the only other way of
making the required run-time choice.  The really significant benefit, of
course, is keeping the source code clean and well structured.

We can also arrange to have private storage that varies between different
implementations of the same kind of object.  We do this by making all the
module-specific object structs be separately allocated entities, which will
be accessed via pointers in the master compression or decompression struct.
The "public" fields or methods for a given kind of object are specified by
a commonly known struct.  But a module's initialization code can allocate
a larger struct that contains the common struct as its first member, plus
additional private fields.  With appropriate pointer casting, the module's
internal functions can access these private fields.  (For a simple example,
see jdatadst.c, which implements the external interface specified by struct
jpeg_destination_mgr, but adds extra fields.)

(Of course this would all be a lot easier if we were using C++, but we are
not yet prepared to assume that everyone has a C++ compiler.)

An important benefit of this scheme is that it is easy to provide multiple
versions of any method, each tuned to a particular case.  While a lot of
precalculation might be done to select an optimal implementation of a method,
the cost per invocation is constant.  For example, the upsampling step might
have a "generic" method, plus one or more "hardwired" methods for the most
popular sampling factors; the hardwired methods would be faster because they'd
use straight-line code instead of for-loops.  The cost to determine which
method to use is paid only once, at startup, and the selection criteria are
hidden from the callers of the method.

This plan differs a little bit from usual object-oriented structures, in that
only one instance of each object class will exist during execution.  The
reason for having the class structure is that on different runs we may create
different instances (choose to execute different modules).  You can think of
the term "method" as denoting the common interface presented by a particular
set of interchangeable functions, and "object" as denoting a group of related
methods, or the total shared interface behavior of a group of modules.


*** Overall control structure ***

We previously mentioned the need for overall control logic in the compression
and decompression libraries.  In IJG implementations prior to v5, overall
control was mostly provided by "pipeline control" modules, which proved to be
large, unwieldy, and hard to understand.  To improve the situation, the
control logic has been subdivided into multiple modules.  The control modules
consist of: