08-3 - Image compression

08-3 - Image compression - 4/28/2008 Image compression...

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4/28/2008 1 Image compression Image compression methods and standards Spring 2008 ELEN 4304/5365 DIP 1 by Gleb V. Tcheslavski: gleb@ee.lamar.edu http://ee.lamar.edu/gleb/dip/index.htm JPEG JPEG – 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 higher 2) An 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 2 k-1 , where 2 k is the maximum Spring 2008 ELEN 4304/5365 DIP 2 number 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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4/28/2008 2 JPEG The 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: 52 55 61 66 70 61 64 73 63 59 66 90 109 85 69 72 62 59 68 113 144 104 66 73 63 58 71 122 154 106 70 69 Scale by 2 7 = 128 76 73 67 62 58 67 64 55 65 69 62 38 19 43 59 56 66 69 60 15 16 24 62 55 65 70 57 6 26 22 58 59 −−−−−−− −−− −− − Spring 2008 ELEN 4304/5365 DIP 3 67 61 68 104 126 88 68 70 79 65 60 70 77 63 58 75 85 71 64 59 55 61 65 83 87 79 69 68 65 76 78 94 The original 256 = 2 8 levels image 61 67 60 24 2 40 60 58 49 63 68 58 51 65 70 53 43 57 64 69 73 67 63 45 41 49 59 60 63 52 50 34 −−− −−−− JPEG DCT of the scaled image Quantized transformed array Next, 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 2008 ELEN 4304/5365 DIP 4 Where 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.
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4/28/2008 3 JPEG JPEG decompression begins from decoding DC and AC coefficients and recovering an array of quantized coefficients from a 1D zigzag. De-normalized DCT coefficients Spring 2008 ELEN 4304/5365 DIP 5 IDCT array Upscaled IDCT – reconstructed image JPEG The error between the original and reconstructed images is due to the lossy nature of the JPEG compression. The rms error is approximately 5.8 intensity levels. Spring 2008 ELEN 4304/5365 DIP 6
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4/28/2008 4 JPEG JPEG approximation with compression 25:1, rms error 5.4 JPEG approximation with compression 52:1, Spring 2008 ELEN 4304/5365 DIP 7 rms error 10.7 Predictive coding Predictive coding is based on eliminating the redundancies of closely spaced pixels – in space and/or in time – by extracting and coding only the new information in each pixel. The new information is defined as
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This note was uploaded on 12/08/2010 for the course ELEN 4304 taught by Professor Staff during the Spring '08 term at Lamar University.

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08-3 - Image compression - 4/28/2008 Image compression...

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