JPEGView/Independent JPEG Group/structure.doc

IJG JPEG LIBRARY:  SYSTEM ARCHITECTURE

Copyright (C) 1991-1994, Thomas G. Lane.
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.doc 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.doc 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 8x8 group 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; 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 simple 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 and
extended sequential DCT processes.  Progressive processes are also allowed
for in the system architecture, although they are not likely to be
implemented very soon.  Hierarchical processes are not supported.

The library does not support the lossless (spatial) JPEG process.  Lossless
JPEG shares little or no code with lossy JPEG, and would normally be used
without the extensive pre- and post-processing provided by this library.
We feel that lossless JPEG is better handled by a separate library.

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.

For legal reasons, JPEG arithmetic coding is not currently supported, but
extending the library to include it would be straightforward.

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, we anticipate that
Sam Leffler's TIFF library will use this code to support the revised TIFF
JPEG 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