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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
* `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`).