A black-and-white television picture is generated by exciting the phosphor on the television screen using an electron beam whose intensity is modulated to generate the image we see. The path that the modulated electron beam traces is shown in Figure 18.4. The line created by the horizontal traversal of the electron beam is called a … Read more
Video compression can be viewed as the compression of a sequence of images; in other words, image compression with a temporal component. Motion video may mask coding artifacts that would be visible in still images. On the other hand, artifacts that may not be visible in reconstructed still images can be very annoying in reconstructed … Read more
The ITU-T Recommendation V.42 bis is a compression standard devised for use over a telephone network along with error-correcting procedures described in CCITT Recommen- dation V.42. This algorithm is used in modems connecting computers to remote users. The algorithm described in this recommendation operates in two modes, a transparent mode and a compressed mode. In … Read more
The PNG standard is one of the first standards to be collaboratively developed over the Internet. The impetus for it was an announcement in December 1994 by Unisys (which had acquired the patent for LZW from Sperry) and CompuServe that they would start charging royalties to authors of software that included support for GIF. The … Read more
The Graphics Interchange Format (GIF) was developed by Compuserve Information Service to encode graphical images. It is another implementation of the LZW algorithm and is very similar to the compress command. The compressed image is stored with the first byte being the minimum number of bits b per pixel in the original image. For the … Read more
The UNIX compress command is one of the earlier applications of LZW. The size of the dictionary is adaptive. We start with a dictionary of size 512. This means that the transmitted codewords are 9 bits long. Once the dictionary has filled up, the size of the dictionary is doubled to 1024 entries. The codewords … Read more
Most adaptive-dictionary-based techniques have their roots in two landmark papers by Jacob Ziv and Abraham Lempel in 1977 and 1978 . These papers provide two different approaches to adaptively building dictionaries, and each approach has given rise to a number of variations. The approaches based on the 1977 paper are said to belong to the … Read more
One of the more common forms of static dictionary coding is diagram coding. In this form of coding, the dictionary consists of all letters of the source alphabet followed by as many pairs of letters, called diagrams, as can be accommodated by the dictionary. For example, suppose we were to construct a dictionary of size … Read more
Choosing a static dictionary technique is most appropriate when considerable prior knowledge about the source is available. This technique is especially suitable for use in specific applications. For example, if the task were to compress the student records at a university, a static dictionary approach may be the best. This is because we know ahead … Read more
Arithmetic code for two-symbol sequences The average length per message is 4.5 bits. Therefore, using two symbols at a time we get a rate of 2.25 bits/symbol (certainly better than 2.75 bits/symbol, but still not as good as the best rate of 1.75 bits/symbol). However, we see that as we increase the number of symbols … Read more