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Rich Geldreich
2019-04-29 20:42:58 -07:00
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@@ -51,7 +51,7 @@ transcode_image_level() and transcode_slice() are thread safe, i.e. you can deco
### Quick Basis file details
Internally, Basis files are composed of a non-uniform texture array of one or more 2D ETC1S 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 ETC2 is a no-op. We choose ETC1S because it has the valuable property that it can be quickly transcoded (converted) to any other GPU texture format at high quality using only simple per-block operations with small 1D lookup tables.
Internally, Basis files are composed of a non-uniform texture array of one or more 2D ETC1S 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 choose ETC1S because it has the valuable property that it can be quickly transcoded (converted) to almost any other GPU texture format at high quality using only simple per-block operations with small 1D lookup tables. Transcoding to PVRTC1 involves only block level operations to compute the endpoints, and simple scalar-level operations to compute the modulation values.
Basis files have a single set of compressed global endpoint/selector codebooks, which all slices refer to. The ETC1S texture data is compressed using vector quantization (VQ) seperately on the endpoints and selectors, followed by DPCM/RLE/psuedo-MTF/canonical Huffman coding. Each ETC1S texture slice may be a different resolution. Mipmaps (if any) are always stored in order from largest to smallest. The file format supports either storing the selector codebook directly (using DPCM+Huffman), or storing the selector codebook using a hierarchical virtual codebook scheme.