Basis Universal is an open source supercompressed LDR/HDR GPU texture interchange system from Binomial LLC that supports two intermediate file formats: the .KTX2 open standard from the Khronos Group, and our own “.basis” file format. These file formats support rapid transcoding to virtually any GPU texture format released in the past ~25 years. Our overall goal is to simplify the encoding and efficient distribution of LDR and HDR GPU texture, image, and texture video content in a way that works well on any GPU.
The current system supports three modes: ETC1S, UASTC LDR, and UASTC HDR. The C/C++ encoder and transcoder libaries can be compiled to native code or WebAssembly. All encoder/transcoder features can be accessed from Javascript.
ETC1S and UASTC LDR files can be transcoded to:
UASTC HDR files can be transcoded to:
ETC1S: A roughly .3-3bpp low to medium quality supercompressed mode based off a subset of ETC1 called “ETC1S”. This mode supports variable quality vs. file size levels (like JPEG), alpha channels, built-in compression, and texture arrays optionally compressed as a video sequence using skip blocks (Conditional Replenishment). This mode can be rapidly transcoded to all of the supported LDR texture formats.
UASTC LDR: An 8 bits/pixel LDR high quality mode. UASTC LDR is a 19 mode subset of the standard ASTC LDR 4x4 (8bpp) texture format, but with a custom block format containing transcoding hints. Transcoding UASTC LDR to ASTC LDR and BC7 are particularly fast and simple, because UASTC LDR is a common subset of both BC7 and ASTC. The transcoders for the other texture formats are accelerated by several format-specific hint bits present in each UASTC LDR block.
This mode supports an optional Rate-Distortion Optimizated (RDO) post-process stage that conditions the encoded UASTC LDR texture data in the .KTX2/.basis file so it can be more effectively LZ compressed. More details here.
Here is the UASTC LDR specification document.
Here is the UASTC HDR specification document, and some compressed example images.
Both .basis and .KTX2 files support mipmap levels, texture arrays, cubemaps, cubemap arrays, and texture video, in all three modes. Additionally, .basis files support non-uniform texture arrays, where each image in the file can have a different resolution or number of mipmap levels.
In ETC1S mode, the compressor is able to exploit color and pattern correlations across all the images in the entire file using global endpoint/selector codebooks, so multiple images with mipmaps can be stored efficiently in a single file. The ETC1S mode also supports short video sequences, with skip blocks (Conditional Replenishment) used to not send blocks which haven't changed relative to the previous frame.
The LDR image formats supported for reading are .PNG, .DDS with mipmaps, .TGA, .QOI, and .JPG. The HDR image formats supported for reading are .EXR, .HDR, and .DDS with mipmaps. It can write .basis, .KTX2, .DDS, .KTX (v1), .ASTC, .OUT, .EXR, and .PNG files.
The system now supports loading basic 2D .DDS files with optional mipmaps, but the .DDS file must be in one of the supported uncompressed formats: 24bpp RGB, 32bpp RGBA/BGRA, half-float RGBA, or float RGBA. Using .DDS files allows the user to control exactly how the mipmaps are generated before compression.
The encoding library and command line tool have no required 3rd party dependencies that are not already in the repo itself. The transcoder is a single .cpp source file (in transcoder/basisu_transcoder.cpp
) which has no 3rd party dependencies.
We build and test under:
Under Windows with Visual Studio you can use the included basisu.sln
file. Alternatively, you can use cmake to create new VS solution/project files.
To build, first install cmake, then:
cd build cmake .. make
To build with SSE 4.1 support on x86/x64 systems (encoding is roughly 15-30% faster), add -DSSE=TRUE
to the cmake command line. Add -DOPENCL=TRUE
to build with (optional) OpenCL support. Use -DCMAKE_BUILD_TYPE=Debug
to build in debug. To build 32-bit executables, add -DBUILD_X64=FALSE
.
After building, the native command line tool used to create, validate, and transcode/unpack .basis/.KTX2 files is bin/basisu
.
The command line tool includes some automated LDR/HDR encoding/transcoding tests:
cd ../bin basisu -test basisu -test_hdr
To test the codec in OpenCL mode (must have OpenCL libs/headers/drivers installed and have compiled OpenCL support in by running cmake with -DOPENCL=TRUE
):
basisu -test -opencl
basisu -q 255 x.png
basisu -linear x.png
basisu -uastc x.png
basisu x.exr
Note the .EXR reader we‘re using is TinyEXR's, which doesn’t support all possible .EXR compression modes. Tools like ImageMagick can be used to create .EXR files that TinyEXR can read.
Alternatively, LDR images (such as .PNG) can be compressed to UASTC HDR by specifying -hdr
. By default, LDR images, when compressed to UASTC HDR, are first converted from sRGB to linear light before compression. This conversion step can be disabled by specifying -hdr_ldr_no_srgb_to_linear
.
Importantly, for best quality, you should 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.
-fastest
(which is equivalent to -uastc_level 0
) puts the UASTC LDR/HDR encoders in their fastest (but lower quality) modes.
-slower
puts the UASTC LDR/HDR encoders in higher quality but slower modes (equivalent to -uastc_level 3
). The default level is 1, and the highest is 4 (which is quite slow).
-q X
, where X ranges from [1,255], controls the ETC1S mode's quality vs. file size tradeoff level. 255 is the highest quality, and the default is 128.
-debug
causes the encoder to print internal and developer-oriented verbose debug information.
-stats
to see various quality (PSNR) statistics.
-linear
: ETC1S defaults to sRGB colorspace metrics, UASTC LDR currently always uses linear metrics, and UASTC HDR defaults to weighted RGB metrics (with 2,3,1 weights). 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. (Angular normal map metrics for UASTC LDR/HDR are definitely doable and on our TODO list.)
Specifying -opencl
enables OpenCL mode, which currently only accelerates ETC1S encoding.
The compressor is multithreaded by default, which can be disabled using the -no_multithreading
command line option. The transcoder is currently single threaded, although it is thread safe (i.e. it supports decompressing multiple texture slices in parallel).
basisu -uastc -uastc_rdo_l 1.0 -mipmap x.png
-uastc_rdo_l X
controls the RDO (Rate-Distortion Optimization) 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.
basisu -mipmap -q 200 x.png
There are several mipmap options 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 bypass the automatic mipmap generator, you can create LDR or HDR uncompressed .DDS texture files and feed them to the compressor.
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, which ranges from 1,6. This controls the amount of overall effort the encoder uses to optimize the ETC1S codebooks and the compressed data stream. Higher comp_level's are significantly slower.
basisu x.png -comp_level 2 -max_endpoints 16128 -max_selectors 16128
basisu -tonemap x.exr
basisu -compare a.png b.png
basisu -compare_hdr a.exr b.exr
See the help text for a complete listing of the tool's command line options. The command line tool is just a thin wrapper on top of the encoder library.
You can either use the command line tool or call the transcoder directly from JavaScript or C/C++ code to decompress .KTX2/.basis files to GPU texture data or uncompressed image data. To unpack a .KTX2 or.basis file to multiple .png/.exr/.ktx/.dds files:
basisu x.ktx2
Use the -no_ktx
and -etc1_only
/-format_only
options to unpack to less files.
-info
and -validate
will just display file information and not output any files.
The written mipmapped, cubemap, or texture array .KTX/.DDS files will be in a wide variety of compressed GPU texture formats (PVRTC1 4bpp, ETC1-2, BC1-5, BC7, etc.), and to our knowledge there is unfortunately (as of 2024) still no single .KTX or .DDS 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. RenderDoc has a useful texture file viewer for many formats. The Mac OSX Finder supports previewing .EXR and .KTX files in various GPU formats. The Windows 11 Explorer can preview .DDS files. The online OpenHDR Viewer is useful for viewing .EXR/.HDR image files.
The ‘WebGL’ directory contains four simple WebGL demos that use the transcoder and compressor compiled to WASM with emscripten. These demos are online here. See more details in the readme file here.