So far we have described several image processing methods that enable us to modify a given image so that a specific image characteristic is altered. This chapter describes another image processing method used in compressing images. Thus, image compression is a digital process by which the amount of data (in bits) in a given image is reduced to as low as desired. The need for image compression stems from the fact that more and more image and video data are used for transmission and storage in this Internet age. As more and more TV channels are introduced, transmission bandwidth becomes very precious. For example, the data rates for SDTV and HDTV are shown in Table 8-1. As shown in Table 8-1, raw video data rates range from about 20 to 120MB/s. However, the transmission channel bandwidths are around 4 and 8MHz, respectively, for SDTV and HDTV, hence the need for data compression. In fact the required compression ratios for the two TV systems are about 20 :1 and 60 :1. The term compression ratio used earlier refers to the ratio of the number of bits in the original digital source data to that in compressed digital data.