Data Compression: The Complete Reference

4Image CompressionThe first part of this chapter discusses the basic features and types of digital images andthe main approaches to image compression. This is followed by a description of about30 different compression methods. The author would like to start with the followingobservations:1. Why were these particular methods included in the book, while others were ignored?The simple answer is: Because of the documentation available to the author. Imagecompression methods that are well documented were included. Methods that are keptsecret, or whose documentation was not clear to the author, were left out.2. The treatment of the various methods is uneven. This, again, reflects the documentationavailable to the author. Some methods have been documented by their developersin great detail, and this is reflected in this chapter. Where no detailed documentationwas available for a compression algorithm, only its basic principles are outlined here.3. There is no attempt to compare the various methods described here. This is becausemost image compression methods have been designed for a particular type of image, andalso because of the practical difficulties of getting all the software and adapting it to runon the same platform.4. The compression methods described in this chapter are not arranged in any particularorder. After much thought and many trials, the author gave up any hope of sorting thecompression methods in any reasonable way. Readers looking for any particular methodcan use the table of contents and the detailed index to easily locate it.A digital image is a rectangular array of dots, or picture elements, arranged in mrows and n columns. The expression m × n is called the resolution of the image, and thedots are called pixels (except in the cases of fax images and video compression, wherethey are referred to as pels). The term “resolution” is sometimes also used to indicatethe number of pixels per unit length of the image. Thus, dpi stands for dots per inch.For the purpose of image compression it is useful to distinguish the following types ofimages: