compression2.slides.printing

compression2.slides.printing - Compression (contd)...

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Unformatted text preview: Compression (contd) Compression (contd) CS 450: Introduction to Digital Signal and Image Processing Bryan Morse BYU Computer Science Compression (contd) Interpixel Redundancy Interpixel Redundancy The basis of interpixel redundancy is I repetition I prediction Compression (contd) Interpixel Redundancy Repetition Run-Length Encoding I Encode sequences of identical symbols as (symbol,count) pairs. I Can use fixed-size counts or special prefixes to indicate the number of bits for the count I Fixed: can reduce compression if either too large or too small I Variable: overhead for the prefixes I Can extend to multiple dimensions I Encode difference from the previous line (hopefully long runs of 0s) I Encode using lengths or markers from previous line I Useful for binary signals and black-and-white images. 2-D RLE is used in the CCITT fax standard. Compression (contd) Interpixel Redundancy Repetition Lempel-Ziv-Welch I Basic idea: encode longest possible previously-seen sequence I Coding stream is mixture of symbols and back-pointers I Better yet: I Keep a codebook of previously-seen sequences I Store codebook index instead of backwards pointers. I Used in most common text compression algorithms, zip, and the GIF image standard. Compression (contd) Interpixel Redundancy Repetition Lempel-Ziv-Welch codebook = all single symbols sequence = empty while (get(symbol)) if sequence + symbol is in codebook sequence += symbol else output(code for sequence) add sequence + symbol to codebook sequence = symbol Compression (contd) Interpixel Redundancy Repetition Example: Lempel-Ziv Mary had a little lamb, little lamb, little lamb, Mary had a little lamb, its fleece was white as snow. Compression (contd) Interpixel Redundancy Prediction Predictive Coding I Use one set of symbols/pixels to predict another.Use one set of symbols/pixels to predict another....
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compression2.slides.printing - Compression (contd)...

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