Bad endpoint encoding

[solidpixel]

Reproducer:

• Block size: 4x4
• Partition count: 1
• Plane count 1
• Weight grid: 4x4 QUANT_20
• PCA patch needed

Single block image:

For this block mode the compressor is computing sensible ideal endpoint value ranges during refinement, but the quantized value is coming out far too dark (looks to have half the value it should).

[solidpixel]

Bad case (PCA patch)

\\ Ideal endpoints:
v4_f32(214.3493 127.6201 1.0082 255.0000)
v4_f32(230.6912 109.4289 4.0289 255.0000)

\\ Packed endpoints:
v4_f32(116.9788 54.9719 3.0000 255.0000)
v4_f32(107.9517 63.9990 1.0000 255.0000)

\\ Using RGB+D

Good case (No PCA patch)

\\ Ideal endpoints
v4_f32(213.2434 125.5799 1.3944 255.0000)
v4_f32(231.7984 110.0338 3.9018 255.0000)

\\ Packed endpoints:
v4_f32(214.9072 125.9436 0.0000 255.0000)
v4_f32(230.9692 109.9438 4.9999 255.0000)

\\ Using RGB+RGB

[solidpixel]

Forcing color endpoints to use RGB+RGB endpoints solves the problem, so looks like the problem is in the implementation of the RGB+Delta encoding.

This doesn't look to be a new problem introduced by the optimization work:

• Decompressing the .astc with the original 1.x codec products the same dark output.
• The original 1.x delta encode patched into the new codec seems to give the same compressed output.

[solidpixel]

For some endpoint values the try_quantize_rgb_delta() function is producing encodings that trigger blue contraction, which is unexpected.