| commit | 16c9e35c452e615de23589cbf6ad3ce31639f9d9 | [log] [tgz] |
|---|---|---|
| author | Rich Geldreich <richgel99@gmail.com> | Tue Apr 30 13:47:11 2019 -0700 |
| committer | Rich Geldreich <richgel99@gmail.com> | Tue Apr 30 13:50:01 2019 -0700 |
| tree | cc74c2ba96ba3b102011bbf42f469fc6d1770450 | |
| parent | c35e288c28aff71bb123635f84fd0b38625c41e3 [diff] |
Removing detex dependency Adding new GPU texture format decoders for BC1/4/5, ETC2 EAC alpha, and BC7 mode 6 Adding GPU texture unpack failure checks and warnings Adding support for 3 color blocks to transcoder, and making sure that BC3 blocks will never use 3 color (punchthrough alpha) blocks.
Basis Universal GPU Texture Compression Codec
Basis Universal is a GPU texture compression system that outputs a highly compressed intermediate file format (.basis) that can be quickly transcoded to a wide variety of GPU texture compression formats: PVRTC1 4bpp RGB, BC7 mode 6 RGB, BC1-5, ETC1, and ETC2. We will be adding ASTC RGB or RGBA, BC7 mode 4/5 RGBA, and PVRTC1 4bpp RGBA next. Basis files support non-uniform texture arrays, so cubemaps, volume textures, texture arrays, mipmap levels, video sequences, or arbitrary texture “tiles” can be stored in a single file. The compressor is able to exploit color and pattern correlations across the entire file.
So far, we‘ve compiled the code using MSVS 2019 and under Ubuntu x64 using cmake with either clang 3.8 or gcc 5.4. The compressor uses OpenMP for multithreading, but if you don’t have OpenMP it'll still work (just much more slowly).
The transcoder (in the “transcoder” directory) has no 3rd party code dependencies.
The encoder uses lodepng for loading and saving PNG images, which is Copyright (c) 2005-2019 Lode Vandevenne. It uses the zlib license.
The command line tool is named “basisu”. Run basisu without any parameters for help.
To compress an sRGB image to .basis:
basisu -srgb x.png
Note that basisu defaults to linear colorspace metrics, not sRGB. If the input is a photograph, or a diffuse/albedo/specular/etc. texture, you will definitely want to use sRGB metrics for less artifacts and better rate distortion performance.
To unpack a .basis file to .png/.ktx files:
basisu x.basis
The mipmapped .KTX files will be in a variety of GPU formats (PVRTC1 4bpp, ETC1-2, BC1-5, BC7), and to my knowledge there is no single .KTX viewer tool that supports every GPU texture format. BC1-5 and BC7 files are viewable using AMD‘s Compressonator, ETC1/2 using Mali’s Texture Compression Tool, and PVRTC1 using Imagination Tech's PVRTexTool. Links:
Mali Texture Compression Tool]
For the maximum possible achievable quality with the current former and encoder, use (remove -srgb for non-photographic images):
basisu x.png -srgb -slower -max_endpoint_clusters 8192 -max_selector_clusters 7936 -no_selector_rdo -no_auto_global_sel_pal
To use .basis files in an application, you only need the files in the “transcoder” directory. The entire transcoder lives in a single .cpp file: transcoder/basisu_transcoder.cpp. If compiling with gcc/clang, be sure strict aliasing is disabled when compiling this file, as I have not tested either the encoder or transcoder with strict aliasing enabled: -fno-strict-aliasing (The Linux kernel is also compiled with this option.)
To use the transcoder, #include “transcoder/basisu_transcoder.h” and “transcoder/basisu_global_selector_palette.h”. Call basist::basisu_transcoder_init() a single time (probably at startup). Also, ideally once at startup, you need to create a single instance of the basist::etc1_global_selector_codebook class, like this:
basist::etc1_global_selector_codebook sel_codebook(basist::g_global_selector_cb_size, basist::g_global_selector_cb);
Now you can use the transcoder, which is implemented in the “basisu_transcoder” class in transcoder/basisu_transcoder.h. The key methods are start_decoding(), get_total_images(), get_image_info(), get_image_level_info(), and transcode_image_level().
I will be simplifying the transcoder so the caller doesn‘t need to deal with etc1_global_selector_codebook’s next.
transcode_image_level() and transcode_slice() are thread safe, i.e. you can decompress multiple images/slices from multiple threads. start_decoding() is not thread safe.
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 per-pixel scalar 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.
The slices are randomly accessible. Opaque files always have one slice per image, and files with alpha channels always have two slices per image (even if some images in the file don't have alpha channels, i.e. alpha is all or nothing). The transcoder abstracts these details away into a simple “image” API, which is what most callers will use. An image is either a single RGB slice, or two (RGB+A) slices. Internally, alpha slices are also in ETC1S format.
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 data to the desired output format.