See Also
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It is important to note that any single texture must specify that its data is stored as 64 or 128 bits per group of 16 texels. If 64-bit blocks — that is, format DXT1 (see D3DMFORMAT) — are used for the texture, it is possible to mix the opaque and one-bit alpha formats on a per-block basis within the same texture. In other words, the comparison of the unsigned integer magnitude of color_0 and color_1 is performed uniquely for each block of 16 texels.
Once the 128-bit blocks are used, the alpha channel must be specified in either explicit (format DXT2 and DXT3) or interpolated mode (format DXT4 and DXT5) for the entire texture. As with color, once interpolated mode (format DXT4 and DXT5) is selected, then either eight interpolated alphas or six interpolated alphas mode can be used on a block-by-block basis. Again the magnitude comparison of alpha_0 and alpha_1 is done uniquely on a block-by-block basis.