GAMECUBE Compressed Texture Format

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    As usual texture, CMPR-texture is divided on tiles, each per 32-bytes,
    to fit texture cache line. Every tile is sub-tiled on four parts, in
    zigzag order :

        0-1
         /
        2-3

    Format of sub-tile is pretty simple, and looks like DXT1. First goes two
    base colors in RGB565, followed by 16 sub-tile texels. Every texel is
    2-bit wide, to lookup from four colors : 00, 01, 10 and 11. First two
    are given already, and last two are interpolated from first ones, by 
    following rule :

        COLOR0 and COLOR1 are base colors.

        RGB0 <- unpack RGB565 COLOR0
        A0 = 255

        RGB1 <- unpack RGB565 COLOR1
        A1 = 255

        if COLOR0 > COLOR1
            RGB2 = (2 * RGB0 + RGB1) / 3
            A2 = 255

            RGB3 = (2 * RGB1 + RGB0) / 3
            A3 = 255
        else
            RGB2 = (RGB0 + RGB1) / 2
            A2 = 255

            RGB3 = (2 * RGB1 + RGB0) / 3
            A3 = 0

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    This document is maintaining by Dolwin team.
    Last edited by org, 21 Sep 2004