1 S2TC Environment Variables
2 ==========================
4 Color Distance Function
5 -----------------------
6 The following color distance functions can be selected by setting the
7 environment variable `S2TC_COLORDIST_MODE`:
9 * `RGB`: weighted YCbCr difference assuming linear input
10 * `YUV`: weighted Y'Cb'Cr' difference assuming sRGB input
11 * `SRGB`: weighted YCbCr difference assuming sRGB input
12 * `SRGB_MIXED`: weighted "Y'(Y)Cb'Cr'" difference assuming sRGB input
13 * `AVG`: standard average of component difference (generic)
14 * `WAVG`: weighted RGB average of component difference
15 * `W0AVG`: superfast weighted RGB average of component difference
16 * `NORMALMAP`: vector distance after normalization
18 The default is `WAVG`, which is a good compromise between speed and quality for
19 RGB and sRGB data. For optimum quality on sRGB input, try `SRGB_MIXED`.
21 The color distance function defines how "closeness" of pixel values is judged
22 when the pixel color values are evaluated, colors are selected, or during
27 The environment variable `S2TC_RANDOM_COLORS` can be set the following way:
29 * `-1`: quick selection (darkest and brightest color are chosen, which is
30 similar to the method in the Color Cell Compression paper)
31 * `0`: all 16 input colors of a block are considered
32 * greater than `0`: additionally, that many random color values in the range
33 of the input color values are considered
35 The default is `-1`, which is fast but poor quality, however ideally suited for
36 online compression. For optimum quality, try `64`.
38 A bad color selection can later be compensated for by color refinement.
42 The environment variable `S2TC_REFINE_COLORS` can be set to the following values:
44 * `NEVER`: never run color refinement
45 * `ALWAYS`: unconditionally perform color refinement
46 * `LOOP`: perform color refinement, evaluate its output and discard it if it
47 didn't improve quality, re-evaluate the pixel color values, and repeat
48 until no improvement could be made
50 The default is `ALWAYS`, which is fast and decent quality, and usually doesn't
51 make things worse. For optimum quality, try `LOOP`.
53 Color refinement recalculates the color palette of a block after the pixel
54 value decision by averaging the color values of those encoded as c0 or c1, and
55 is a technique that helps a lot of the initial color selection was poor (e.g.
56 if `S2TC_RANDOM_COLORS` was not set, or set to `-1`).