Chapter 1

CHAPTER 2

Chapter 3

 It is possible to treat a set of images in 2 or 3 different ways: as a stream of bits, as a group of still images or if possible as a video sequence; each way of representing the image may lead itself to a variety of compression techniques. Before going into detail on these three different ways, we consider the fact that, in general, there are two different kind of compression or coding: lossless or lossy compression. In the first case the bits are reduced without changing the characteristics of the data, actually it is possible to encode and decode the data obtaining an identical copy of the starting data. In the second case the bits are reduced eliminating some non interesting, or less interesting, parts of the data, in order to compress the remaining part more or less. In this case compressing and decompressing the image, an identical copy it can not be obtained, but only a similar copy; obviously the bigger the lost part is, the bigger the difference between the two data is.

It is possible to see the group of images as a stream of bits or better the images are seen such as if they are simple normal files, if it is not known that they are images. In this way it is possible to use some different kind of lossless coding, like Huffman, Run-length, Lempel-Ziv, Variable Length Coding or LZW, to compress and reduce the size of these files. There are a lot of different programs for UNIX and Windows operating systems that allow to do this compression, like WinZip, Gzip, PKzip, Arj and Rare; in a later chapter more accurate statistics about this kind of compression are seen. We have to specify only few things about it:

• The compression ratio  is usually between 1.3 :1 and 2 :1, for the image files, and it depends on the kind of image and on the information that it contains,
• Some parts of this coding are already part of the standards for still images and for video like JPEG, JPEG 2000, MPEG-2 and MPEG-4,
• This coding could be really useful after algorithms or standards that do not use it.

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2.3. VIDEO STANDARDS

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