Compressing Data

Throughout the articles included, you'll see references to methods of data compression or data encoding. Compression is the process used to reduce the physical size of a block of information. By compressing graphics data, we're able to fit more information in a physical storage space. Because graphics images usually require a very large amount of storage space, compression is an important consideration for graphics file formats. Almost every graphics file format uses some compression method.

There are several ways to look at compression. We can talk about differences between physical and logical compression, symmetric and asymmetric compression, and lossless and lossy compression. These terms are described in detail in Chapter 9, Data Compression. That chapter also describes the most common methods of, or algorithms for, compression, which we mention here briefly:

Pixel packing--Not a method of data compression per se, but an efficient way to store data in contiguous bytes of memory. This method is used by the Macintosh PICT format and other formats that are capable of storing multiple 1-, 2-, or 4-bit pixels per byte of memory or disk space.
Run-length encoding (RLE)--A very common compression algorithm used by such bitmap formats as BMP, TIFF, and PCX to reduce the amount of redundant graphics data.
Lempel-Ziv-Welch (LZW)--Used by GIF and TIFF, this algorithm is also a part of the v.42bis modem compression standard and of PostScript Level 2.
CCITT encoding--A form of data compression used for facsimile transmission and standardized by the CCITT. One particular standard is based on the keyed compression scheme introduced by David Huffman and known widely as Huffman encoding.
Joint Photographic Experts Group (JPEG)--A toolkit of compression methods used particularly for continuous-tone image data and multimedia. The baseline JPEG implementation uses an encoding scheme based on the Discrete Cosine Transform (DCT) algorithm.
Joint Bi-level Image Experts Group (JBIG)--A method of compressing bi-level (two-color) image data, which is intended to replace the MR (Modified READ) and MMR (Modified Modified READ) compression algorithms used by CCITT Group 3 and Group 4.
ART--A proprietary compression algorithm developed by Johnson-Grace that can be adapted to support audio, animation, and full-motion video in the future.
Fractal--A mathematical process used to encode bitmaps containing a real-world image as a set of mathematical data that describes the fractal (similar, repeating patterns) properties of the image.

Each of the articles in Part Two, Graphics File Formats, lists the compression algorithms used for the particular graphics file format described.