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Image and Videl Comoression P7

In this chapter, the JPEG standard is introduced. This standard allows for lossy and lossless encoding of still images and four distinct modes of operation are supported: sequential DCT-based mode, progressive DCT-based mode, lossless mode and hierarchical mode. 7.1 INTRODUCTION Still image coding is an important application of data compression. When an analog image or picture is digitized, each pixel is represented by a fixed number of bits, which correspond to a certain number of gray levels. In this uncompressed format, the digitized image requires a large number of bits to be stored or transmitted. As a result, compression become necessary due. | Section II Still Image Compression 2000 by CRC Press LLC 7 Still Image Coding Standard JPEG In this chapter the JPEG standard is introduced. This standard allows for lossy and lossless encoding of still images and four distinct modes of operation are supported sequential DCT-based mode progressive DCT-based mode lossless mode and hierarchical mode. 7.1 INTRODUCTION Still image coding is an important application of data compression. When an analog image or picture is digitized each pixel is represented by a fixed number of bits which correspond to a certain number of gray levels. In this uncompressed format the digitized image requires a large number of bits to be stored or transmitted. As a result compression become necessary due to the limited communication bandwidth or storage size. Since the mid-1980s the ITU and ISO have been working together to develop a joint international standard for the compression of still images. Officially JPEG jpeg is the ISO IEC international standard 10918-1 digital compression and coding of continuous-tone still images or the ITU-T Recommendation T.81. JPEG became an international standard in 1992. The JPEG standard allows for both lossy and lossless encoding of still images. The algorithm for lossy coding is a DCT-based coding scheme. This is the baseline of JPEG and is sufficient for many applications. However to meet the needs of applications that cannot tolerate loss e.g. compression of medical images a lossless coding scheme is also provided and is based on a predictive coding scheme. From the algorithmic point of view JPEG includes four distinct modes of operation namely sequential DCT-based mode progressive DCT-based mode lossless mode and hierarchical mode. In the following sections an overview of these modes is provided. Further technical details can be found in the books by Pennelbaker and Mitchell 1992 and Symes 1998 . In the sequential DCT-based mode an image is first partitioned into blocks of 8 x 8 pixels. The blocks .