Release: ktx-rs 4.4.2, KTX-Software ported to Rust

ktx-rs 4.4.2 is out on crates.io. It’s an unofficial Rust port of Khronos KTX-Software - the libktx library and the ktx command line tool, together with the Basis Universal encoders and transcoders. It is not affiliated with or endorsed by The Khronos Group.

Why?

KTX2 with Basis Universal is the go-to container for GPU textures, but using it from Rust meant building the C++ library and going through FFI. Bindings would have been the easy route; a port means one cargo build with no C++ toolchain, no unsafe sprinkled around FFI, and a library that never panics on bad input.

The bar was simple: if the Rust version doesn’t produce exactly the same bytes as the C++ tools, it’s not a port.

What’s included

• KTX1 / KTX2 reading, writing and validation, with zstd and zlib supercompression.
• Basis Universal: ETC1S (Basis-LZ), UASTC, UASTC HDR 4x4 and 6x6i encoders, and transcoding to every target libktx supports.
• ASTC encoding and decoding (a port of astcenc), ETC unpack, PNG / JPEG / EXR input.
• GPU upload through a Vulkan loader (feature vulkan, default) and an OpenGL / OpenGL ES loader (feature opengl, opt in).
• The ktx CLI: create, convert, encode, deflate, transcode, extract, info, validate, compare and help, with man pages.

Output is byte identical to the upstream tools and every case of the upstream CLI conformance test suite (CTS) passes. The upstream unit, texture and transcode tests are ported as well.

Not ported: the legacy tools (toktx, ktx2check, …), the language bindings, OpenCL encoding and ASTC HDR encoding (broken upstream, so deferred).

Using it

The package is ktx-rs, the library is imported as ktx. Functions keep their libktx names in snake case (ktxTexture2_TranscodeBasis becomes Texture2::transcode_basis) and return KtxResult<T>:

use ktx::texture::KtxTexture;
use ktx::{KtxResult, Texture2, TranscodeFmt};

fn transcode_to_bc7(src: &str, dst: &str) -> KtxResult<()> {
    let mut texture =
        Texture2::create_from_named_file(src, ktx::KTX_TEXTURE_CREATE_LOAD_IMAGE_DATA_BIT)?;
    if texture.needs_transcoding() {
        texture.transcode_basis(TranscodeFmt::BC7_RGBA, 0)?;
    }
    texture.write_to_named_file(dst)
}

Or from the shell, same commands as upstream:

ktx create --format R8G8B8A8_SRGB --generate-mipmap albedo.png albedo.ktx2
ktx encode --codec basis-lz albedo.ktx2 albedo_blz.ktx2
ktx info albedo_blz.ktx2

Versioning

The crate version follows upstream KTX-Software, so 4.4.2 is the port of KTX-Software 4.4.2. Port-only fixes get a separate patch number while LIBKTX_VERSION keeps reporting the upstream version.

Performance

Being byte exact is not enough if it’s 20x slower. The encoders use multiple threads (--threads, all cores by default) with the same job splits as the C++ code, so the output never depends on the thread count. The astcenc compressor is compiled three times - SSE2, SSE4.1 and AVX2 - and the right copy is picked at runtime, so no target-cpu flags are needed.

Rust time vs C++ time on 24 threads (lower is better, 1.0 is parity):

ASTC started at 21.6x slower before the SIMD kernels and runtime dispatch went in, so it’s in a good place now, with some optional work left.

Design notes

• Pure Rust except the allowed C bindings: Vulkan (ash) and, behind opengl, OpenGL.
• unsafe is denied outside the GPU loaders and the astcenc SIMD code; unwrap, expect and panic! are denied everywhere.
• Each translated file keeps the license of the code it was ported from, so the crate is Apache-2.0 AND BSD-3-Clause AND MIT AND Zlib (details in NOTICE.md and REUSE.toml).

Source, roadmap and per-component implementation notes are on GitHub.