Editing S3 Texture Compression (section)

== DXT4 and DXT5 ==
DXT4 and DXT5 (collectively also known as Block Compression 3 or BC3) converts 16 input pixels into 128 bits of output, consisting of 64 bits of alpha channel data (two 8-bit alpha values and a 4×4 3-bit lookup table) followed by 64 bits of color data (encoded the same way as DXT1).

If <math>\alpha_0 > \alpha_1</math>, then six other alpha values are calculated, such that <math display="inline">\alpha_2 = {{6\alpha_0 + 1\alpha_1} \over 7}</math>, <math display="inline">\alpha_3 = {{5\alpha_0 + 2\alpha_1} \over 7}</math>, <math display="inline">\alpha_4 = {{4\alpha_0 + 3\alpha_1} \over 7}</math>, <math display="inline">\alpha_5 = {{3\alpha_0 + 4\alpha_1} \over 7}</math>, <math display="inline">\alpha_6 = {{2\alpha_0 + 5\alpha_1} \over 7}</math>, and <math display="inline">\alpha_7 = {{1\alpha_0 + 6\alpha_1} \over 7}</math>.

Otherwise, if <math display="inline">\alpha_0 \le \alpha_1</math>, four other alpha values are calculated such that <math display="inline">\alpha_2 = {{4\alpha_0 + 1\alpha_1} \over 5}</math>, <math display="inline">\alpha_3 = {{3\alpha_0 + 2\alpha_1} \over 5}</math>, <math display="inline">\alpha_4 = {{2\alpha_0 + 3\alpha_1} \over 5}</math>, and <math display="inline">\alpha_5 = {{1\alpha_0 + 4\alpha_1} \over 5}</math> with <math>\alpha_6 = 0</math> and <math>\alpha_7 = 255</math>.

The lookup table is then consulted to determine the alpha value for each pixel, with a value of 0 corresponding to <math>\alpha_0</math> and a value of 7 corresponding to <math>\alpha_7</math>. DXT4's color data is premultiplied by alpha, whereas DXT5's is not. Because DXT4/5 use an interpolated alpha scheme, they generally produce superior results for alpha (transparency) gradients than DXT2/3.