DXT1

DXT1 is a lossy texture compression format that is part of the S3 Texture Compression (S3TC) family. It was developed by S3 Graphics and is widely supported across graphics hardware and APIs, including OpenGL, OpenGL ES, and DirectX. In DirectX terminology, DXT1 is often referred to as BC1. The format compresses each 4x4 texel block into 8 bytes, reducing memory usage and bandwidth for textures in real-time rendering.

Each 4x4 block stores two 16-bit color endpoints in RGB 5:6:5 format (c0 and c1). These endpoints define a palette of four colors derived from c0 and c1. If c0 > c1, the two interpolated colors c2 and c3 are computed by fixed-weight interpolation (c2 = 2/3 c0 + 1/3 c1; c3 = 1/3 c0 + 2/3 c1). If c0 <= c1, the palette uses three colors (c0, c1, c2) and a transparent color for the fourth entry, enabling binary per-texel alpha. The 16 texels in the block are then assigned colors via a 32-bit 2-bit-per-texel index map that references the palette. The result is an 8-byte block per 4x4 region.

Alpha handling in DXT1 is binary per texel via the c0 <= c1 case, where some texels can be fully transparent. DXT1 does not store a full alpha channel; per-texel transparency is limited to the presence or absence of a block-level transparent color.

Applications and limitations: DXT1 provides a favorable balance of compression ratio and image quality for many natural textures and is common in older and some modern pipelines. It can produce visible blockiness at low bitrates and is less suited for sharp text or highly detailed imagery. It remains a foundational format in the BC1 family and a predecessor to more advanced DXT variants.