XUASTC's next step: Intra-prediction of weight grids

Binomial has shown that image compression and GPU texture compression aren't separate fields. They're the same field, and the tools from one transfer directly to the other.

XUASTC is currently using JPEG-style DCT (from 1992) on ASTC weight grids:

https://github.com/BinomialLLC/basis_universal/wiki/XUASTC-LDR-Weight-Grid-DCT

We ported JPEG-style coding into ASTC, even preserving how libjpeg-style [1-100] Q factors are used to calculate quantization tables. (Our quantization table is the standard luminance JPEG table, with simple adaptive quantization added on top.)

This works, but it means the DCT has to carry the entire weight signal (just like JPEG). At the very lowest quality factors (Q levels 1-25 or so), the lowest spatial frequencies suffer (again, just like JPEG).

The next step is to port WebP-style intra-prediction into the weight grid domain. We can easily predict weight grids from nearby blocks, then code the weight residuals using DCT. It's the logical next step, and it'll push our bitrates even lower. While seemingly everyone is distracted by neural techniques, we're targeting billions of already shipped, hyper-efficient hardware decoders.