.basis file format overview

.basis File Format Overview

Internally, .basis files are composed of a non-uniform texture array of one or more 2D ETC1S or UASTC texture "slices". ETC1S is a simple subset of the ETC1 texture format popular on Android. ETC1S has no block flips, no 4x2 or 2x4 subblocks, and each block only uses 555 base colors. ETC1S is still 100% standard ETC1, so transcoding to ETC1 or the color block of ETC2 is a no-op. We chose ETC1S because it has the very valuable property that it can be quickly transcoded to almost any other GPU texture format at very high quality using only simple per-block operations with small 1D lookup tables. Transcoding ETC1S to BC1 usually only introduces around .3 dB Y PSNR quality loss, with less loss for ETC1S->BC7. Transcoding to PVRTC1 involves only simple block level operations to compute the endpoints, and simple per-pixel scalar operations to compute the modulation values.

.basis files have a single set of compressed global endpoint/selector codebooks in ETC1S format, which all slices utilize. The ETC1S texture data is compressed using vector quantization (VQ) separately on the endpoints and selectors, followed by DPCM+RLE+psuedo-MTF+canonical Huffman coding. The ETC1S compressor's backend supports a simple form of Rate Distortion Optimization (RDO). Each ETC1S texture slice may be a different resolution. Mipmaps (if any) are always stored in order from largest to smallest level. The endpoint and selector codebooks are compressed using DPCM+Huffman coding.

Once the codebook and Huffman tables are decompressed, the slices are randomly accessible in any order. Opaque ETC1S files always have one slice per image mipmap level, and files with alpha channels always have two slices per image mipmap level (even if some images in the file don't have alpha channels, i.e. alpha is all or nothing at the file level). The transcoder abstracts these details away into a simple "image" API, which is what most callers will use. An image is either one or more RGB slices (one per mipmap level), or one or more pairs of RGB/A slices (two per mipmap level). Internally, alpha slices are also stored in ETC1S format, like the color data, so selector correlations across color/alpha can be exploited. This also allows both RGB and alpha slices to be transcoded to opaque-only texture formats like ETC1, BC1, or PVRTC1 with no transparency.

We currently only support CPU transcoding, but GPU assisted transcoding/format conversion is also possible by uploading the decompressed codebooks as textures and using compute shaders to convert the ETC1S codebook block indices to the desired output texture or pixel format.