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# Texture Compressor
This directory contains a compressor for GPU texture formats. It is primarily
meant for compressing thumbnails to save memory. Currently, only the ETC1 format
is supported, and this is only used on Android.
## Goals
* Fast: Compressing a 4K screenshot takes less than 100ms (roughly 100
megapixels/s). Currently, it can achieve 220 megapixels/s on x64 desktop.
* Safe: Free of memory safety issues and suitable for running in the
privileged browser process.
* Portable: SIMD code shared across x86 (SSE) and ARM (NEON). Easy to modify
and validate for correctness.
## Algorithm
Vectorization is done by assigning each 4x4 SIMD lane to a different block. This
minimizes cross-lane shuffle operations (which are typically architecture
dependent) and makes the code significantly more portable.
Compression of ETC1 is done by first searching through the flip / no-flip and
individual / differential space to decide on the quantization of average, then
searching through selector tables and values for each subblock.
Detailed comments about the algorithm can be found in the source.
## Similar projects
* [etcpak][etcpak] heavily inspired this project. Most of the algorithm
(except vectorization scheme) and math tricks are borrowed from etcpak.
* [ISPC Texture Compressor][ispctexcomp] influenced the vectorization scheme.
Unlike this project, the compressor is written in ISPC and requires a
dedicated compiler.
* [AOSP ETC1 library][aospetc1] also uses a similar search algorithm and is
easier to read, thanks to the scalar implementation.
While reading the code, the [ETC1 specification][etc1spec] should also be
useful.
## (Currently) Missing Pieces
This project is functional, but still a work-in-progress. The following pieces
are missing:
* Testing coverage
* Dynamic dispatch (multiversioning) and AVX2 support
* Running on stable toolchain (using the `wide` crate instead of
`#[feature(portable_simd)]`)
[etcpak]: https://github.com/wolfpld/etcpak
[ispctexcomp]: https://github.com/GameTechDev/ISPCTextureCompressor
[aospetc1]: https://cs.android.com/android/platform/superproject/main/+/main:frameworks/native/opengl/libs/ETC1/
[etc1spec]: https://registry.khronos.org/DataFormat/specs/1.1/dataformat.1.1.html#ETC1
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