DXT5

DXT5 is a lossy texture compression format in the S3 Texture Compression (S3TC) family, commonly referred to as BC3 in DirectX terminology. It compresses a 4x4 texel block into 128 bits, providing roughly 4:1 compression for standard 32-bit RGBA textures. DXT5 achieves this by encoding color and alpha data separately: a 64-bit color block and a 64-bit alpha block.

The color block stores two 16-bit endpoints in RGB565 format and a 2-bit index for each texel, selecting one of four colors derived by interpolating between the endpoints. This allows a compact representation of the block’s color information. The alpha block stores two 8-bit endpoints (A0 and A1) and 16 3-bit indices (one per texel) that map to eight possible alpha values derived from A0 and A1 according to the standard DXT5 rule. The combination yields a full RGBA representation for the 4x4 texel block, with the two blocks occupying 8 bytes each.

DXT5 is widely supported by graphics hardware and software APIs, and remains a common choice for textures that require transparency. Its per-block alpha compression preserves smooth alpha gradients better than a simple binary alpha and is compatible with many existing pipelines and formats.

Limitations include artifacts at high-contrast edges or with very sharp alpha transitions, a fixed block-based structure that can struggle with highly detailed or rapidly changing alpha, and the general caveats of lossy compression. Modern alternatives such as BC7 or ASTC offer higher quality for some use cases, but DXT5 remains a staple for compatibility and performance.