-S2TC - Texture Compression using Color Cell Compression
-
-
-What is S2TC?
-
-S2TC is a texture compression scheme that encodes 4x4 blocks with a 2 color
-palette each. The storage format is chosen so that most current graphics cards
-can decode it without any special updates or shaders; however, the format is
-simpler and thus can allow for quicker online compression, or simpler decoding.
-
-S2TC textures are stored in a DDS (DirectDraw Surface) container. After the 128
-header bytes, alpha blocks and color blocks may follow. Blocks are encoded top
-to bottom, left to right.
-
-Notation:
-
-When nothing else is specified, pixels are ordered from top to bottom, then
-left to right. Bits are read from the least to the most significant bit. Larger
-than a byte values are stored little-endian.
-
-Subformats:
-
-DXT1:
-
-No alpha blocks are stored. Each color block starts with two 16-bit words c0,
-c1. The most significant 5 bits encode red, the least significant bits encode
-blue, the remaining 6 bits encode green. It must always be that c1 >= c0.
-
-The remaining 32 bits are a little endian longword, of which each two adjacent
-bits, encode one of the 16 pixels of the block. The two bits for a pixel can
-encode the following values:
-
-00 - color c0, alpha 255 (opaque)
-01 - color c1, alpha 255 (opaque)
-10 - reserved
-11 - no color, alpha 0 (transparent)
-
-DXT3:
-
-Before each color block, an alpha block is stored. The alpha block is a 64bit
-qword, of which each four adjacent bits encode one pixel value of alpha in
-standard 4bpp encoding.
-
-Each color block starts with two 16-bit words c0, c1. The most significant 5
-bits encode red, the least significant bits encode blue, the remaining 6 bits
-encode green. It must always be that c0 >= c1.
-
-The remaining 32 bits are a little endian dword, of which each two adjacent
-bits encode one of the 16 pixels of the block. The two bits for a pixel can
-encode the following values:
-
-00 - color c0
-01 - color c1
-10 - reserved
-11 - reserved
-
-DXT5:
-
-Before each color block, an alpha block is stored. The alpha block starts with
-two bytes for alpha values a0 and a1. It must always be that a1 >= a0. Then a
-48-bit integer is encoded, of which each three adjacent bits encode one of the
-16 alpha values of the block.
-
-000 - alpha a0
-001 - alpha a1
-010 - reserved
-011 - reserved
-100 - reserved
-101 - reserved
-110 - alpha 0 (transparent)
-111 - alpha 255 (opaque)
-
-Each color block starts with two 16-bit words c0, c1. The most significant 5
-bits encode red, the least significant bits encode blue, the remaining 6 bits
-encode green. It must always be that c0 >= c1.
-
-The remaining 32 bits are a little endian longword, of which each two adjacent
-bits encode one of the 16 pixels of the block. The two bits for a pixel can
-encode the following values:
-
-00 - color c0
-01 - color c1
-10 - reserved
-11 - reserved
-
-
-Environment variables:
-
-Color distance function: S2TC_COLORDIST_MODE (default: WAVG)
-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
-LAB: L*a*b* difference
-AVG: standard average of component difference (generic)
-WAVG: weighted RGB average of component difference
-NORMALMAP: vector distance after normalization
-
-Color selection: S2TC_RANDOM_COLORS (default: -1)
--1: quick selection (just darkest and brightest color of each block)
-0: normal selection (all 16 colors of each block are candidates)
->0: randomized selection (S2TC_RANDOM_COLORS random colors are added to the
- >candidates for each block)
-
-Color refinement: S2TC_REFINE_COLORS (default: ALWAYS)
-NEVER: never run color refinement
-ALWAYS: always run color refinement (i.e. replace the colors by possibly better
- colors by averaging the original colors for the encoded pixels)
-CHECK: always run color refinement, but only use its result if it is actually
- a closer match for the block
-LOOP: always run color refinement, but only use its result if it is actually
- a closer match for the block; after refining, reassign colors and refine
- till no change for the better happened
+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`).
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