Editing S3 Texture Compression (section)

== DXT1 ==
DXT1 (also known as Block Compression 1 or BC1) is the smallest variation of S3TC, storing 16 input pixels in 64 bits of output, consisting of two 16-bit RGB 5:6:5 color values <math>c_0</math> and <math>c_1</math>, and a 4×4 two-bit lookup table.

If <math>c_0 > c_1</math>(compare these colors by interpreting them as two 16-bit unsigned numbers), then two other colors are calculated, such that for each component, <math display="inline">c_2 = {2 \over 3} c_0 + {1 \over 3} c_1</math> and <math display="inline">c_3 = {1 \over 3} c_0 + {2 \over 3} c_1</math>.
This mode operates similarly to mode 0xC0 of the [[Apple Video|original Apple Video codec]].<ref>Togni, Roberto, et al. "[http://wiki.multimedia.cx/index.php?title=Apple_RPZA Apple RPZA] {{Webarchive|url=https://web.archive.org/web/20170704235213/https://wiki.multimedia.cx/index.php?title=Apple_RPZA |date=2017-07-04 }}". MultimediaWiki.</ref>

Otherwise, if <math>c_0 \le c_1</math>, then <math display="inline">c_2 = {1 \over 2} c_0 + {1 \over 2} c_1</math> and <math>c_3</math> is transparent black corresponding to a [[Alpha compositing|premultiplied alpha format]]. This color sometimes causes a black border surrounding the transparent area when linear texture filtering and alpha test is used, due to colors being interpolated between the color of opaque texel and neighbouring black transparent texel.

The lookup table is then consulted to determine the color value for each pixel, with a value of 0 corresponding to <math>c_0</math> and a value of 3 corresponding to <math>c_3</math>.