Block blurring (prefiltering)

Modern GPU texture compressors have a secret (but dangerous) superpower: prefiltering (blurring). Sometimes an encoder way overfits edges, causing overall perceptual quality to collapse. One way to overcome this is to blur the input and encode the block again. This is what we do in HDR on the very worst blocks (as measured by SSIM). It's paradoxical: blurring can boost perceptual quality.