DXT3

DXT3 is a block-based texture compression format in the S3 Texture Compression (S3TC) family, commonly referred to as BC2 in DirectX nomenclature. It compresses textures in 4x4 texel blocks, with a fixed 16-bytes per block layout: 8 bytes for per-texel alpha data and 8 bytes for color data, allowing efficient storage and fast decompression on many GPUs. DXT3 is designed to preserve sharp per-pixel transparency by storing explicit alpha values and applying them to the decompressed color information.

The alpha part of a DXT3 block consists of 16 four-bit alpha values, one for each texel in the 4x4 block, totaling 64 bits. These alpha values range from 0 to 15 and are typically scaled to 0–255 when used by applications. The color part mirrors the DXT1 encoding: two 16-bit color endpoints encoded in 5-6-5 RGB format, plus a 32-bit lookup table that provides a 2-bit index for each texel to select among four interpolated colors derived from the endpoints.

Decoding a DXT3 block involves reconstructing the per-texel color from the color endpoints and their indices, then applying the per-texel alpha value from the corresponding 4-bit field. This yields a final RGBA texel with a fixed 4-bit alpha resolution. The format is widely supported and was common in older DirectX/OpenGL pipelines. DXT3 is often contrasted with DXT5 (BC3), which combines the same color block layout with interpolated alpha instead of explicit per-texel alpha, offering improved alpha quality in many cases.