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Add HDR perceptual channel weighting to improve IQ - #759
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Apply brightness-dependent RGB channel weights to HDR blocks when perceptual mode is enabled, in addition to existing global color-based weighting. The search operates in ASTC's approximately logarithmic LNS space, where errors in dim channels can have excessive influence relative to their visible contribution. Estimate each channel's geometric mean brightness by converting its mean LNS value back to linear light. Normalize these estimates by the brightest RGB channel, retain a minimum weight of 1%, and multiply by the configured channel weights. Keep the alpha multiplier at one. This approximates display-space error and gives brighter channels more influence during encoding while retaining a minimum contribution from dim channels. Apply the same weighting to every block size in both HDR profiles. The weights are computed once per block and are independent of the exposure range used for CLI quality reporting.
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Only update records in reference CSVs when PSNR changes.
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More work on #364 to improve our HDR image quality. Currently this includes two changes that can be merged independently:
Image quality differences are expected with these changes, including PSNR regressions because perceptual error != PSNR.
This is not a full fix for all the HDR quality issues, and we still get a lot of block artifacts, but it's a good start and a lot better than 5.7!