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 , then six other alpha values are calculated, such that , , , , , and . Otherwise, if , four other alpha values are calculated such that , , , and with and . The lookup table is then consulted to determine the alpha value for each pixel, with a value of 0 corresponding to and a value of 7 corresponding to . 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. Further variants BC4 and BC5 BC4 and BC5 (Block Compression 4 and 5) are added in Direct3D 10. They reuse the alpha channel encoding found in DXT4/5 (BC3). BC4 stores 16 input single-channel (e.g. greyscale) pixels into 64 bits of output, encoded in the same way as BC3 alphas. The expanded palette provides higher quality. BC5 stores 16 input double-channel (e.g. tangent space normal map) pixels into 128 bits of output, consisting of two halves each encoded in the same way as BC3 alphas. BC6H and BC7 BC6H (sometimes BC6) and BC7 (Block Compression 6H and 7) are added in Direct3D 11. BC6H encodes 16 input RGB HDR (float16) pixels into 128 bits of output. BC7 encodes 16 input RGB8/RGBA8 pixels into 128 bits of output. BC6H and BC7 have a much more complex algorithm with a selection of encoding modes. The quality is much better as a result. S3TC format comparison See also S2TC, patentless workaround FXT1 DirectDraw Surface PVRTC Adaptive Scalable Texture Compression (ASTC) Ericsson Texture Compression (ETC1 & ETC2) Color Cell Compression References External links US Patent & Trademark Office, Patent Full Text and Image Database, result for US 5956431 A USPTO Patent Assignment Search for US 5956431 A Microsoft Developer Network article on Block Compression in Direct3D 10 squish, an MIT-licensed S3TC compressor. The site also contains an article giving an introduction to compression algorithms. Comparison between S3TC and FXT1 texture compression The Truth about S3TC Note: This article used an early S3TC compression engine, not nVidia's or ATI's updated codecs. (Wayback Machine copy) Texture compression survey A fast, SSE2-enabled DXT1/5 compressor by Intel Betsy, a GPU-accelerated compressor. Modern open source BC1-7 encoders