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The original image is 786,486 bytes (with or without delta coding). The followingtable shows the compressed sizes when compressed with an order 0 indirect contextmodel (ICM-0), with each of the 3 colors compressed in a separate stream.
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The images below show the effects of 3 passes of delta coding horizontally and verticallyof the image (a widely used benchmark image).The original image is in BMP format, which consists of a 54 byte header and a 512by 512 array of pixels, scanned in rows starting at the bottom left. Each pixelis 3 bytes with the numbers 0..255 representing the brightness of the blue, green,and red components. The image is delta coded by subtracting the pixel value to theleft of the same color, and again on the result by subtracting the pixel value below.(The order of the two encodings does not matter). To show the effects better, 128is added to all pixel values (which does not affect compression). Thus, a pixelequal to its neighbors appears medium gray.
A predictive filter is a transform which can be used to compress numeric data suchas audio, images, or video. The idea is to predict the next sample, and then encodethe difference (the error) with an order 0 model. The decompresser makes the samesequence of predictions and adds them to the decoded prediction errors. Better predictionslead to smaller errors, which generally compress better.
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Typically the best compressors use dynamic models and arithmetic coding. The compressoruses past input to estimate a probability distribution (prediction) for the nextsymbol without looking at it. Then it passes the prediction and symbol to the arithmeticcoder, and finally updates the model with the symbol it just coded. The decompressermakes an identical prediction using the data it has already decoded, decodes thesymbol, then updates its model with the decoded output symbol. The model is unawareof whether it is compressing or decompressing. This is the technique we will usein the rest of this chapter.
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PAQ7 (Dec. 2005) was a complete rewrite. It uses logistic mixing rather than linearmixing, as described in section 4.3.2. It has models for color BMP, TIFF, and JPEGimages. The BMP and TIFF models use adjacent pixels as context. JPEG is alreadycompressed. The model partially undoes the compression back to the DCT (discretecosine transform) coefficients and uses these as context to predict the Huffmancodes.
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The World Wide Web has standardized on the use of asthe recommended (and still reasonably simple) default color space, and shouldthus be used for all images that do not contain any colorspace profileinformation.
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PNG is compressed by predictive filtering (section 5.6) followed by deflate (section5.2.2). There are 5 filters which can be selected for each scan line. The imageis scanned left to right starting at the top. Let A, B, and C be the previouslycoded neighboring pixels of the predicted pixel x:
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Aug image compression thesis 18, 2016 · This page has information about general copyright restrictions that apply to the material that is available through the PubMed Central (PMC) site.