Add jxl-rs decoder

[iamcarbon]

Summary

This PR adds an experimental PhotoSauce.NativeCodecs.JxlRs decoder plugin backed by the pure-Rust jxl-rs decoder.

The change is additive and does not remove the existing libjxl implementation or encoder. Applications can opt in with:

CodecManager.Configure(codecs => {
    codecs.UseJxlRs();
});

Implementation

• Supports JPEG XL codestreams and container files.
• Supports still images, animation, orientation, ICC profiles, alpha, and uncompressed Exif boxes.
• Outputs BGR/BGRA directly, avoiding an RGB-to-BGR swizzle.
• Uses jxl-rs SIMD support through its default features.
• Builds a small Rust cdylib through Cargo and copies it alongside the managed assembly.
• Targets net8.0 and net472.

Input and decoding

• Contiguous MemoryStream and UnmanagedMemoryStream inputs are consumed in place.
• Other seekable streams use bounded callbacks and jxl-rs's NeedsMoreInput state machine.
• Still-image inspection stops after image/ICC metadata and retains the primed decoder for the first pixel request.
• A first full-frame request decodes directly into MagicScaler's caller-provided destination pointer.
• Partial or repeated requests use one pooled frame buffer.
• Animation frames are decoded on demand using jxl-rs visible-frame seek targets rather than storing all frames.

EXIF

jxl-rs does not currently expose arbitrary container boxes, so this adds a small allocation-free JPEG XL box parser.

The parser:

• Reads only 8- or 16-byte box headers using stack memory.
• Seeks over unrelated payloads.
• Stores only the EXIF stream offset and length.
• Copies EXIF directly into the caller-provided IExifSource destination when requested.
• Preserves the source stream position.
• Validates extended sizes, offsets, truncation, and integer overflow.

Brotli-compressed brob metadata boxes are not supports on .NETFramework (is it time to consider dropping .NETFramework ? :)) .

4K decode benchmark

Windows x64, .NET 10.0.10, 64 logical processors. The input was a deterministic 3840×2160 mixed-detail BGR image encoded by libjxl at distance 1.0, effort 5 (Hare), producing a 6.90 MiB payload.

The benchmark performs a complete MagicScaler open-and-full-frame-copy operation, with three warmups and twelve measured iterations in alternating order.

Decoder	Median	Mean	Min	Max	Throughput
libjxl 0.11.1	407.49 ms	406.79 ms	397.42 ms	419.38 ms	20.35 MP/s
jxl-rs 0.5.1	461.54 ms	461.69 ms	449.36 ms	476.26 ms	17.97 MP/s

On this workload, jxl-rs had 1.13× libjxl's median latency (+13.3%) and 0.88× its throughput.

The decoded outputs differed only by rounding:

• Mean absolute channel error: 0.244508
• Maximum channel delta: 1

These are single-machine directional results rather than a comprehensive cross-platform benchmark.

Validation

• cargo test --locked: 3 passed, 0 failed.
• Managed decoder project compiled for net8.0 and net472.
• EXIF parser checks cover:
  • Standard boxes
  • Extended-length boxes
  • Boxes extending to EOF
  • Non-zero stream origins and TIFF offsets
  • Truncated and malformed input
  • Unsupported brob boxes
  • Stream-position preservation
• 10,000 EXIF scan-and-copy iterations reported zero managed allocations.

Current limitations

• Decoder only; encoding remains with the existing libjxl plugin.
• Input streams must be seekable.
• brob-compressed EXIF is not exposed.
• The current source build produces the Rust native library for the host platform and architecture; RID-specific packaging would require additional build infrastructure.

Built with the assistance of GPT-5.6 Sol.