compression2.slides.printing

Predictors next pixel is like the last one next

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Predictors: Next pixel is like the last one Next scanline is like the last one Next frame was like the last one Next pixel is the average of the already-known neighbors The error from the prediction ( residual ) hopefully has smaller entropy than the original signal. The information used to make the prediction is the context .
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Compression (cont’d) Interpixel Redundancy Prediction Predictive Coding Key: Sender and receiver use the same predictive model. Sender Receiver Make prediction (no peeking!) Make prediction Send the residual (difference) Add the residual to get the correct value Lossless: entropy code the residual Lossy: quantize the residual
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Compression (cont’d) Interpixel Redundancy Prediction Simple Predictive Audio Compression: Delta Modulation Basic Algorithm: Prediction: next signal value is the same as the last. Residual is the difference (delta) from the previous one. Residual is encoded in a smaller number of bits than the original. Often used in audio systems (phones). Problem: limited-range delta can cause under/overshoot.
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Compression (cont’d) Interpixel Redundancy Prediction Delta Modulation
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Compression (cont’d) Interpixel Redundancy Prediction Predictive Image Coding Predict next pixel based on neighbors that have been already seen ? Simplest predictor: average of the four neighbors Can use larger context Can quantize (lossy) or entropy code (lossless) the residual Used in DPCM and the lossless part of the JPEG standard. Newer algorithms (CALOC, LOCO-I) use multiple contexts and smarter predictors.
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Compression (cont’d) Interpixel Redundancy Prediction Predictive Image Coding
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Compression (cont’d) Interpixel Redundancy Prediction Residual: Entropy Coding / Nonuniform Quantization The residual values should be very small, so Use fewer bits for smaller values (entropy coding), or Use finer quantization (less loss) for smaller values
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Compression (cont’d)
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  • Winter '08
  • Morse,B
  • (Cont’d), Codebook, interpixel redundancy, entropy coding

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