Basis Universal Supercompressed GPU Texture Codec

(This software is available under a free and permissive license (Apache 2.0), but like many other large open source projects it needs financial support to sustain its continued security, bug fixes and improvements. In addition to maintenance and stability there are many desirable features and new interchange formats we would like to add. If your company is using Basis Universal in a product, please consider reaching out.)

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Basis Universal is a “supercompressed” GPU texture data interchange system that supports two highly compressed intermediate file formats (.basis or the .KTX2 open standard from the Khronos Group) that can be quickly transcoded to a very wide variety of GPU compressed and uncompressed pixel formats: ASTC 4x4 L/LA/RGB/RGBA, PVRTC1 4bpp RGB/RGBA, PVRTC2 RGB/RGBA, BC7 mode 6 RGB, BC7 mode 5 RGB/RGBA, BC1-5 RGB/RGBA/X/XY, ETC1 RGB, ETC2 RGBA, ATC RGB/RGBA, ETC2 EAC R11 and RG11, FXT1 RGB, and uncompressed raster image formats 8888/565/4444.

The system now supports two modes: a high quality mode which is internally based off the UASTC compressed texture format, and the original lower quality mode which is based off a subset of ETC1 called “ETC1S”. UASTC is for extremely high quality (similar to BC7 quality) textures, and ETC1S is for very small files. The ETC1S system includes built-in data compression, while the UASTC system includes an optional Rate Distortion Optimization (RDO) post-process stage that conditions the encoded UASTC texture data in the .basis file so it can be more effectively LZ compressed by the end user. More technical details about UASTC integration are here.

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 multiple images with mipmaps can be stored very efficiently in a single file.

The system's bitrate depends on the quality setting and image content, but common usable ETC1S bitrates are .3-1.25 bits/texel. ETC1S .basis files are typically 10-25% smaller than using RDO texture compression of the internal texture data stored in the .basis file followed by LZMA. For UASTC files, the bitrate is fixed at 8bpp, but with RDO post-processing and user-provided LZ compression on the .basis file the effective bitrate can be as low as 2bpp for video or for individual textures approximately 4-6bpp.

The .basis and .KTX2 transcoders have been fuzz tested using zzuf.

So far, we've compiled the code using MSVC 2019, under Ubuntu 18.04 and 20 x64 using cmake with either clang 3.8 or gcc 5.4, and emscripten 1.35 to asm.js. (Be sure to use this version or later of emcc, as earlier versions fail with internal errors/exceptions during compilation.)

Basis Universal supports “skip blocks” in ETC1S compressed texture arrays, which makes it useful for basic compressed texture video applications. Note that Basis Universal is still at heart a GPU texture compression system, not a dedicated video codec, so bitrates will be larger than even MPEG1. 1/10/21 release notes:

For v1.13, we‘ve added numerous ETC1S encoder optimizations designed to greatly speed up single threaded encoding time, as well as greatly reducing overall CPU utilization when multithreading is enabled. For benchmarking, we’re using “-q 128 -no_multithreading -mip_fast”. The encoder now uses approximately 1/3rd as much total CPU time for the same PSNR. The encoder can now optionally utilize SSE 4.1 - see the “-no_sse” command line option.

Release Notes

The first texture (and possibly image/video) compression codec developed without “Lena”

We retired Lena years ago. No testing is done with this image:

Quick Introduction

Probably the most important concept to understand about Basis Universal before using it: The system supports two very different universal texture modes: The original “ETC1S” mode is low/medium quality, but the resulting file sizes are very small because the system has built-in compression for ETC1S texture format files. This is the command line encoding tool‘s default mode. ETC1S textures work best on images, photos, map data, or albedo/specular/etc. textures, but don’t work as well on normal maps.

There's the second “UASTC” mode, which is significantly higher quality (comparable to BC7 and highest quality LDR ASTC 4x4), and is usable on all texture types including complex normal maps. UASTC mode purposely does not have built-in file compression like ETC1S mode does, so the resulting files are quite large (8-bits/texel - same as BC7) compared to ETC1S mode. The UASTC encoder has an optional Rate Distortion Optimization (RDO) encoding mode (implemented as a post-process over the encoded UASTC texture data), which conditions the output texture data in a way that results in better lossless compression when UASTC .basis files are compressed with Deflate/Zstd, etc. In UASTC mode, you must losslessly compress .basis files yourself. .KTX2 files have built-in lossless compression support using Zstandard, which is used by default on UASTC textures.

Basis Universal is not an image compression codec, but a GPU texture compression codec. It can be used just like an image compression codec, but that‘s not the only use case. Here’s a good intro to GPU texture compression. If you're looking to primarily use the system as an image compression codec on sRGB photographic content, use the default ETC1S mode, because it has built-in compression.

The “-q X” option controls the output quality in ETC1S mode. The default is quality level 128. “-q 255” will increase quality quite a bit. If you want even higher quality, try “-max_selectors 16128 -max_endpoints 16128” instead of -q. -q internally tries to set the codebook sizes (or the # of quantization intervals for endpoints/selectors) for you. You need to experiment with the quality level on your content.

For tangent space normal maps, you should separate X into RGB and Y into Alpha, and provide the compressor with 32-bit/pixel input images. Or use the “-separate_rg_to_color_alpha” command line option which does this for you. The internal texture format that Basis Universal uses (ETC1S) doesn't handle tangent space normal maps encoded into RGB well. You need to separate the channels and recover Z in the pixel shader using z=sqrt(1-x^2-y^2).

License and 3rd party code dependencies

Detailed legal, license, and IP information is here. Basis Universal itself uses the Apache 2.0 licenses, but it also utilizes some optional BSD code (Zstandard). The supported texture formats are open Khronos Group standards.

All C/C++ code dependencies are present inside the Basis Universal repo itself to simplify building.

The encoder optionally uses Zstandard‘s single source file compressor (in zstd/zstd.c) to support compressing supercompressed KTX2 files. The stand-alone transcoder (in the “transcoder” directory) is a single .cpp source file library which has no 3rd party code dependencies apart from zstd/zstddeclib.c, which is also technically optional. It’s only used for decompressing UASTC KTX2 files that use Zstandard.

Command Line Compression Tool

The command line tool used to create, validate, and transcode/unpack .basis/.KTX2 files is named “basisu”. Run basisu without any parameters for help.

The library and command line tool have no other 3rd party dependencies (that are not already in the repo), so it's pretty easy to build.

To build basisu (without SSE 4.1 support - the default):

cmake CMakeLists.txt

To build with SSE 4.1 support on x86/x64 systems (encoding is roughly 15-30% faster):

cmake -D SSE=TRUE CMakeLists.txt

For Visual Studio 2019, you can now either use the CMakeLists.txt file or the included basisu.sln file. Earlier versions of Visual Studio (particularly 2017) should work but aren't actively tested. We develop with the most up to date version of 2019.

To compress a sRGB PNG/BMP/TGA/JPEG image to an ETC1S .KTX2 file:

basisu -ktx2 x.png

To compress a sRGB PNG/BMP/TGA/JPEG image to an UASTC .KTX2 file:

basisu -ktx2 -uastc x.png

To compress a sRGB PNG/BMP/TGA/JPEG image to an RDO UASTC .KTX2 file with mipmaps:

basisu -ktx2 -uastc -uastc_rdo_l 1.0 -mipmap x.png

To compress a sRGB PNG/BMP/TGA/JPEG image to an ETC1S .basis file:

basisu x.png

To compress a image to a higher quality UASTC .basis file:

basisu -uastc -uastc_level 2 x.png

To compress a image to a higher quality UASTC .basis file with RDO post processing, so the .basis file is more compressible:

basisu -uastc -uastc_level 2 -uastc_rdo_l .75 x.png

-uastc_level X ranges from 0-4 and controls the UASTC encoder's performance vs. quality tradeoff. Level 0 is very fast, but low quality, level 2 is the default quality, while level 3 is the highest practical quality. Level 4 is impractically slow, but highest quality.

-uastc_rdo_l X controls the rate distortion stage‘s quality setting. The lower this value, the higher the quality, but the larger the compressed file size. Good values to try are between .2-3.0. The default is 1.0. RDO post-processing is currently pretty slow, but we’ll be optimizing it over time.

UASTC texture video is supported and has been tested. In RDO mode with 7zip LZMA, we've seen average bitrates between 1-2 bpp. ETC1S mode is recommended for texture video, which gets bitrates around .25-.3 bpp.

Note that basisu defaults to sRGB colorspace metrics. If the input is a normal map, or some other type of non-sRGB (non-photographic) texture content, be sure to use -linear to avoid extra unnecessary artifacts. (Note: Currently, UASTC mode always uses linear colorspace metrics. sRGB and angulate metrics are comming soon.)

To add automatically generated mipmaps to the .basis file, at a higher than default quality level (which ranges from [1,255]):

basisu -mipmap -q 190 x.png

There are several mipmap options that allow you to change the filter kernel, the filter colorspace for the RGB channels (linear vs. sRGB), the smallest mipmap dimension, etc. The tool also supports generating cubemap files, 2D/cubemap texture arrays, etc.

To create a slightly higher quality ETC1S .basis file (one with better codebooks) at the default quality level (128) - note this is much slower to encode:

basisu -comp_level 2 x.png

On some rare images (ones with blue sky gradients come to bind), you may need to increase the ETC1S -comp_level setting. This controls the amount of overall effort the encoder uses to optimize the ETC1S codebooks (palettes) and compressed data stream. Higher comp_level‘s are significantly slower, and shouldn’t be used unless necessary:

basisu -ktx2 x.png -comp_level 5 -q 255

Or try: basisu -ktx2 x.png -comp_level 5 -max_endpoints 16128 -max_selectors 16128

Note -comp_level's 3-4 are almost as good as 5 and are a lot faster.

The compressor is multithreaded by default, but this can be disabled using the -no_multithreading command line option. The transcoder is currently single threaded although it supports multithreading decompression of multiple texture slices in parallel.

Unpacking .basis/.KTX2 files to .PNG/.KTX files

You can either use the command line tool or call the transcoder directly from JavaScript or C/C++ code to decompress .basis/.KTX2 files to GPU texture data or uncompressed images.

To use the command line tool to unpack a .basis or .KTX2 file to multiple .png/.ktx files:

basisu x.basis

Use the -no_ktx and -etc1_only options to unpack to less files. -info and -validate will just display file information and not output any files. The output .KTX1 files are currently in the KTX1 file format, not KTX2.

The mipmapped or cubemap .KTX files will be in a wide variety of compressed GPU texture formats (PVRTC1 4bpp, ETC1-2, BC1-5, BC7, etc.), and to my knowledge there is no single .KTX viewer tool that correctly and reliably supports every GPU texture format that we support. 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



After compression, the compressor transcodes all slices in the output .basis file to validate that the file decompresses correctly. It also validates all header, compressed data, and slice data CRC16's.

For best quality, you must supply basisu with original uncompressed source images. Any other type of lossy compression applied before basisu (including ETC1/BC1-5, BC7, JPEG, etc.) will cause multi-generational artifacts to appear in the final output textures.

For the maximum possible achievable ETC1S mode quality with the current format and encoder (completely ignoring encoding speed!), use:

basisu x.png -comp_level 5 -max_endpoints 16128 -max_selectors 16128 -no_selector_rdo -no_endpoint_rdo

Level 5 is extremely slow, so unless you have a very powerful machine, levels 1-4 are recommended.

Note that “-no_selector_rdo -no_endpoint_rdo” are optional. Using them hurts rate distortion performance, but increases quality. An alternative is to use -selector_rdo_thresh X and -endpoint_rdo_thresh, with X ranging from [1,2] (higher=lower quality/better compression - see the tool's help text).

To compress small video sequences, say using tools like ffmpeg and VirtualDub:

basisu -comp_level 2 -tex_type video -stats -debug -multifile_printf "pic%04u.png" -multifile_num 200 -multifile_first 1 -max_selectors 16128 -max_endpoints 16128 -endpoint_rdo_thresh 1.05 -selector_rdo_thresh 1.05

For video, the more cores your machine has, the better. Basis is intended for smaller videos of a few dozen seconds or so. If you are very patient and have a Threadripper or Xeon workstation, you should be able to encode up to a few thousand 720P frames. The “webgl_videotest” directory contains a very simple video viewer. For texture video, use -comp_level 2 or 3. The default is 1, which isn‘t quite good enough for texture video. Higher comp_level’s result in reduced ETC1S artifacts.

The .basis file will contain multiple images (all using the same global codebooks), which you can retrieve using the transcoder's image API. The system now supports conditional replenisment (CR, or “skip blocks”). CR can reduce the bitrate of some videos (highly dependent on how dynamic the content is) by over 50%. For videos using CR, the images must be requested from the transcoder in sequence from first to last, and random access is only allowed to I-Frames.

If you are doing rate distortion comparisons vs. other similar systems, be sure to experiment with increasing the endpoint RDO threshold (-endpoint_rdo_thresh X). This setting controls how aggressively the compressor's backend will combine together nearby blocks so they use the same block endpoint codebook vectors, for better coding efficiency. X defaults to a modest 1.5, which means the backend is allowed to increase the overall color distance by 1.5x while searching for merge candidates. The higher this setting, the better the compression, with the tradeoff of more block artifacts. Settings up to ~2.25 can work well, and make the codec more competitive. “-endpoint_rdo_thresh 1.75” is a good setting on many textures.

For video, level 1 should result in decent results on most clips. For less banding, level 2 can make a big difference. This is still an active area of development, and quality/encoding perf. will improve over time.

To control the ETC1S encoder's quality vs. encoding speed tradeoff, see ETC1S Compression Effort Levels.

More Examples

basisu x.png
Compress sRGB image x.png to a ETC1S format x.basis file using default settings (multiple filenames OK). ETC1S format files are typically very small on disk (around .5-1.5 bits/texel).

basisu -uastc x.png
Compress image x.png to a UASTC format x.basis file using default settings (multiple filenames OK). UASTC files are the same size as BC7 on disk (8-bpp). Be sure to compress UASTC .basis files yourself using Deflate, zstd, etc. To increase .basis file compressibility (trading off quality for smaller compressed files) use the “-uastc_rdo_q X” command line parameter.

basisu -q 255 x.png
Compress sRGB image x.png to x.basis at max quality level achievable without manually setting the codebook sizes (multiple filenames OK)

basisu x.basis
Unpack x.basis to PNG/KTX files (multiple filenames OK)

basisu -validate -file x.basis
Validate x.basis (check header, check file CRC's, attempt to transcode all slices)

basisu -unpack -file x.basis
Validates, transcodes and unpacks x.basis to mipmapped .KTX and RGB/A .PNG files (transcodes to all supported GPU texture formats)

basisu -q 255 -file x.png -mipmap -debug -stats
Compress sRGB x.png to x.basis at quality level 255 with compressor debug output/statistics

basisu -linear -max_endpoints 16128 -max_selectors 16128 -file x.png
Compress non-sRGB x.png to x.basis using the largest supported manually specified codebook sizes

basisu -linear -global_sel_pal -no_hybrid_sel_cb -file x.png
Compress a non-sRGB image, use virtual selector codebooks for improved compression (but slower encoding)

basisu -linear -global_sel_pal -file x.png
Compress a non-sRGB image, use hybrid selector codebooks for slightly improved compression (but slower encoding)

basisu -tex_type video -comp_level 2 -framerate 20 -multifile_printf "x%02u.png" -multifile_first 1 -multifile_count 20 -selector_rdo_thresh 1.05 -endpoint_rdo_thresh 1.05
Compress a 20 sRGB source image video sequence (x01.png, x02.png, x03.png, etc.) to x01.basis

basisu -comp_level 2 -q 255 -file x.png -mipmap -y_flip
Compress a mipmapped x.basis file from an sRGB image named x.png, Y flip each source image, set encoder to level 2 for slightly higher quality (but slower encoding).

WebGL test

The “WebGL” directory contains three simple WebGL demos that use the transcoder and compressor compiled to wasm with emscripten. See more details here.

Screenshot of texture example running in a browser.