 a table of 7.4.5.2 Block transform coding - the source symbol probabilities, and then constructs code- discrete cosine transform words whose lengths grow according to the decreasing probability of a symbol's occurrence. Modified versions Block transform coding is widely used in image and video of the traditional algorithm are used in the current gen- compression systems. The transforms used in video eration of image and video encoders. The H.261 encoder encoders are unitary, which means that the transform uses two sets of static Huffman codewords (one each for operation has an inverse operation that uniquely AC and DC DCT coefficients). A set of 32 codewords is reconstructs the original input. The discrete cosine used for encoding the AC coefficients. The zigzag transform (DCT), successively operates on 8 X 8 image scanned coefficients are classified according to the zero blocks, and is used in the H.261, H.263, and MPEG coefficient run-length and first nonzero coefficient value. standards. Block transforms make use of the high degree A simple table lookup is all that is then required to assign of correlation between adjacent image pixels to provide the codeword for each classified pair. energy compaction or coding gain in the transformed Markov and random field source models are well domain. The block transform coding gain GTC is defined suited to describing the source characteristics of natural as the logarithmic ratio of the arithmetic and geometric images. A Markov source has memory of some number of means of the transformed block variances, i.e., preceding or adjacent events. In a natural image block, the value of the current pixel is dependent on the values GTC = log10 (7.4.6) of some of the surrounding pixels because they are part of the same object, texture, contour, etc. This can be modeled as an m-th order Markov source, in which the probability of source symbol ei depends on the last m where the transformed image block is divided into N source symbols. This dependence is expressed as the subbands, and