Effect of ETC1 selector quantization on Luma SSIM/PSNR

This is like the previous post, except this time only the selectors are quantized while the endpoints are left alone. kodak test images, perceptual colorspace metrics:





Stats for non-RDO ETC1 compression:

best_luma_psnr: Avg: 40.009226, Std Dev: 2.154732, Min: 35.193684, Max: 42.750275, Mean: 41.113007
best_luma_ssim: Avg: 0.983419, Std Dev: 0.002109, Min: 0.980131, Max: 0.989254, Mean: 0.983190
best_bits_per_texel: Avg: 2.851078, Std Dev: 0.341672, Min: 2.184774, Max: 3.482361, Mean: 2.822876

RDO selectors 8192:

rdo_luma_psnr: Avg: 38.225255, Std Dev: 2.628415, Min: 31.853958, Max: 41.955276, Mean: 39.500504
rdo_luma_ssim: Avg: 0.966271, Std Dev: 0.007768, Min: 0.944449, Max: 0.981821, Mean: 0.966354
rdo_bits_per_texel: Avg: 2.366380, Std Dev: 0.231610, Min: 1.902201, Max: 2.793721, Mean: 2.337708

RDO selectors 4096:

rdo_luma_psnr: Avg: 36.581700, Std Dev: 2.874786, Min: 29.814810, Max: 40.718441, Mean: 37.796730
rdo_luma_ssim: Avg: 0.953993, Std Dev: 0.010954, Min: 0.922887, Max: 0.973516, Mean: 0.953305
rdo_bits_per_texel: Avg: 2.132147, Std Dev: 0.220503, Min: 1.668640, Max: 2.535848, Mean: 2.094666

RDO selectors: 2048:

rdo_luma_psnr: Avg: 35.129581, Std Dev: 2.967410, Min: 28.291447, Max: 39.650620, Mean: 36.413860
rdo_luma_ssim: Avg: 0.942579, Std Dev: 0.013760, Min: 0.903846, Max: 0.967203, Mean: 0.941114
rdo_bits_per_texel: Avg: 1.969779, Std Dev: 0.216071, Min: 1.506246, Max: 2.368530, Mean: 1.930033

RDO selectors 1024:

rdo_luma_psnr: Avg: 33.915408, Std Dev: 2.963184, Min: 27.143675, Max: 38.416290, Mean: 35.294361
rdo_luma_ssim: Avg: 0.931751, Std Dev: 0.016440, Min: 0.886028, Max: 0.960691, Mean: 0.929749
rdo_bits_per_texel: Avg: 1.848387, Std Dev: 0.216314, Min: 1.378805, Max: 2.245748, Mean: 1.809530

RDO selectors 512:

rdo_luma_psnr: Avg: 32.898390, Std Dev: 2.920482, Min: 26.292456, Max: 37.282799, Mean: 34.293579
rdo_luma_ssim: Avg: 0.920788, Std Dev: 0.019035, Min: 0.868281, Max: 0.953666, Mean: 0.918912
rdo_bits_per_texel: Avg: 1.753840, Std Dev: 0.215968, Min: 1.278585, Max: 2.150350, Mean: 1.717773

RDO selectors 256:

rdo_luma_psnr: Avg: 32.036631, Std Dev: 2.866251, Min: 25.595591, Max: 36.275482, Mean: 33.285240
rdo_luma_ssim: Avg: 0.909641, Std Dev: 0.021761, Min: 0.849128, Max: 0.946493, Mean: 0.907937
rdo_bits_per_texel: Avg: 1.673566, Std Dev: 0.215763, Min: 1.187663, Max: 2.065999, Mean: 1.631165

RDO selectors 128:

rdo_luma_psnr: Avg: 31.255766, Std Dev: 2.800476, Min: 24.977221, Max: 35.173336, Mean: 32.437733
rdo_luma_ssim: Avg: 0.896458, Std Dev: 0.024306, Min: 0.827130, Max: 0.934879, Mean: 0.895064
rdo_bits_per_texel: Avg: 1.600956, Std Dev: 0.215559, Min: 1.127218, Max: 1.991862, Mean: 1.550741