Release Notes

Open Source Used In AsyncOS 8.8 for Cisco Web Security Appliances

You should read at least the overview and basic usage sections before trying

to program with the library. The sections on advanced features can be read

if and when you need them.

OVERVIEW

========

Functions provided by the library

---------------------------------

The IJG JPEG library provides C code to read and write JPEG-compressed image

files. The surrounding application program receives or supplies image data a

scanline at a time, using a straightforward uncompressed image format. All

details of color conversion and other preprocessing/postprocessing can be

handled by the library.

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. The application indirectly selects use of this code

by specifying the format in which it wishes to supply or receive image data.

For example, if colormapped output is requested, then the decompression

library automatically invokes color quantization.

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.

A word about functions *not* provided by the library. We handle a subset of

the ISO JPEG standard; most baseline, extended-sequential, and progressive

JPEG processes are supported. (Our subset includes all features now in common

use.) Unsupported ISO options include:

* Hierarchical storage

* Lossless JPEG

* DNL marker

* Nonintegral subsampling ratios

We support both 8- and 12-bit data precision, but this is a compile-time

choice rather than a run-time choice; hence it is difficult to use both

precisions in a single application.

By itself, the library handles only interchange JPEG datastreams --- in