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4/28/20081Image compressionImage compression methods and standardsSpring 2008ELEN 4304/5365 DIP1by Gleb V. Tcheslavski: [email protected]JPEGJPEG – while being one of the most popular continuous tone image compression standards – defines three basic coding schemes:1)A lossy baseline coding system based on DCT;2)An extended coding system for greater compression higherAn extended coding system for greater compression, higher precision, or progressive reconstruction applications;3)A lossless independent coding system for reversible compression.In a baseline format, the image is subdivided into 8x8 pixel blocks, which are processed left to right, top to bottom. For each block, its 64 pixels are level-shifted by subtracting 2k-1, where 2kis the maximum Spring 2008ELEN 4304/5365 DIP2number of intensity levels. Next, a 2D DCT of the block is computed, quantized, and reordered using the zigzag pattern to form a 1D sequence of quantized coefficients. Next, the nonzero AC coefficients are coded using a variable-length code. The DC coefficient is difference coded relative to the DC coefficient of the previous block.
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4/28/20082JPEGThe JPEG recommended luminance quantization array can be scaled to provide a variety of compression levels (select the quality of JPEG compression)compression).Consider compression and reconstruction of the following 8x8 subimage:525561667061647363596690 109856972625968 113 144 1046673635871 122 154 1067069Scale by 27= 128767367625867645565696238194359566669601516246255657057626225859−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Spring 2008ELEN 4304/5365 DIP3676168 104 126886870796560707763587585716459556165838779696865767894The original 256 = 28levels image616760242406058496368585165705343576469736763454149596063525034−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−JPEGDCT of the scaled imageQuantized transformed arrayNext, the zigzag ordering pattern will lead to[-26 -3 1 -3 -2 -6 2 -4 1 -4 1 1 5 0 2 0 0 EOB]Spring 2008ELEN 4304/5365 DIP4Where EOB is a special end-of-block symbol.Next, the difference between the current block’s DC symbol and the DC symbol from the previous block is computed and coded. The nonzero AC coefficients are coded according to another code table.