A GPU that cannot sample ASTC gets the textures decoded to RGBA8, four times the video memory of the ASTC blocks. With astc_bc = 1 (default 0, GPU decoder only) a second compute pass, bc3_encode.comp, encodes the decoded texels to BC3 and the image is BC3_UNORM or BC3_SRGB: 8 bits per texel instead of 32. The encoder is the usual bounding-box method with the box diagonal turned to the sign of the colour covariance, and the eight-value alpha mode.
A region of a WriteTexels that does not start on a 4x4 block corner is left as it was and logged. The setting is in totk.ini and the Advanced tab. The SPIR-V header was built with glslang 16.6 (build_shaders.py uses glslc) with the same source hash.
On a GTX 1650 (4 GiB) in the Fire Temple the GPU time per frame went from 10-23 ms to 3-12 ms and the frame rate held at 59-60 fps.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
