Prerequisites

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Description

The second part of ASTC (Adaptive scalable texture compression) decoding. ASTC is used with KTX containers to compress texture data.
This extends the ASTC decoder to add HDR support. It also adds tests that compare output with the ARM C++ reference decoder.

Prerequisites

• Test infrastructure improvements and fixes
• Base decoder and LDR support

To follow

• Benchmarks
• Fuzz testing

Limitations

• No support for 3D ASTC block types (4x4x4 etc)
• No encoding

Intentionally omitted

• Float output for HDR (doesn't appear to be useful for ImageSharp.Textures, but would be trivial to implement)
• Swizzle remapping (also not useful for ImageSharp.Textures, can be handled downstream)
• sRGB containers decode to raw UNORM8 (conversion is handled downstream)

Test data

Everything used is either created by myself, or sourced from
https://github.com/KhronosGroup/KTX-Software (Apache 2.0 license)
https://github.com/KhronosGroup/KTX-Software-CTS (Apache 2.0 license)
https://github.com/ARM-software/astc-encoder (Apache-2.0 license)

Performance

This implementation compares very favourably to the ARM C++ implementation. Single shot benchmarks show 3-4 faster on this small test set:

| Method         | Categories | Mean      | Error     | StdDev    | Ratio | RatioSD | Allocated | Alloc Ratio |
|--------------- |----------- |----------:|----------:|----------:|------:|--------:|----------:|------------:|
| Reference_Hdr  | HDR        |  7.118 ms | 0.1379 ms | 0.1588 ms |  1.00 |    0.03 |     460 B |        1.00 |
| ImageSharp_Hdr | HDR        |  1.932 ms | 0.0113 ms | 0.0100 ms |  0.27 |    0.01 |     288 B |        0.63 |

...however this includes the allocation overhead of the ARM decoder. Re-using the same decoder context in the ARM decoder shows a probably more realistic comparison:

| Method         | Categories | Mean       | Error    | StdDev   | Ratio | RatioSD | Allocated | Alloc Ratio |
|--------------- |----------- |-----------:|---------:|---------:|------:|--------:|----------:|------------:|
| Reference_Hdr  | HDR        |   943.3 us | 11.18 us | 10.46 us |  1.00 |    0.02 |         - |          NA |
| ImageSharp_Hdr | HDR        | 1,927.2 us | 10.61 us |  9.92 us |  2.04 |    0.02 |     288 B |          NA |