I posted this thread on X last night - mirroring it here:
On GPU textures the block lattice is a function of UV + known block size. This makes GPU texture seam artifacts entirely predictable, so filtering them is easy. This applies to all GPU texture formats, but is the most valuable on ASTC which supports up to 12x12 pixel block sizes.
No extra metadata is required. The geometry of GPU texture block artifacts is known before you even look at the compressed bits.
It's also easy to apply the seam filter while preserving mipmapping, trilinear filtering, etc.:
For normal maps: you can apply the seam filter (which only kicks in near/on block edges) then renormalize the results.
Encoders can optimize the GPU texture block parameters taking into account the reconstruction operator (the seam filter) during compression (in-loop). We use a form of SCD: Stochastic Coordinate Descent.
