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S2TC - a subset of a wellknown texture compression scheme (actually Color Cell Compression) https://github.com/divVerent/s2tc/wiki
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S2TC Environment Variables ========================== Color Distance Function ----------------------- The following color distance functions can be selected by setting the environment variable `S2TC_COLORDIST_MODE`: * `RGB`: weighted YCbCr difference assuming linear input * `YUV`: weighted Y'Cb'Cr' difference assuming sRGB input * `SRGB`: weighted YCbCr difference assuming sRGB input * `SRGB_MIXED`: weighted "Y'(Y)Cb'Cr'" difference assuming sRGB input * `AVG`: standard average of component difference (generic) * `WAVG`: weighted RGB average of component difference * `W0AVG`: superfast weighted RGB average of component difference * `NORMALMAP`: vector distance after normalization The default is `WAVG`, which is a good compromise between speed and quality for RGB and sRGB data. For optimum quality on sRGB input, try `SRGB_MIXED`. The color distance function defines how "closeness" of pixel values is judged when the pixel color values are evaluated, colors are selected, or during refinement. Color Selection --------------- The environment variable `S2TC_RANDOM_COLORS` can be set the following way: * `-1`: quick selection (darkest and brightest color are chosen, which is similar to the method in the Color Cell Compression paper) * `0`: all 16 input colors of a block are considered * greater than `0`: additionally, that many random color values in the range of the input color values are considered The default is `-1`, which is fast but poor quality, however ideally suited for online compression. For optimum quality, try `64`. A bad color selection can later be compensated for by color refinement. Color Refinement ---------------- The environment variable `S2TC_REFINE_COLORS` can be set to the following values: * `NEVER`: never run color refinement * `ALWAYS`: unconditionally perform color refinement * `LOOP`: perform color refinement, evaluate its output and discard it if it didn't improve quality, re-evaluate the pixel color values, and repeat until no improvement could be made The default is `ALWAYS`, which is fast and decent quality, and usually doesn't make things worse. For optimum quality, try `LOOP`. Color refinement recalculates the color palette of a block after the pixel value decision by averaging the color values of those encoded as c0 or c1, and is a technique that helps a lot of the initial color selection was poor (e.g. if `S2TC_RANDOM_COLORS` was not set, or set to `-1`).