The BC3 encoder's colour half is a four-colour BC1 block (c0 above c1, or every index 0 when they are equal), so a BC1 image of an opaque texture decodes to exactly the texels the BC3 one did. The encoder writes that half alone with its specialization constant BC1 set; its push constants are as before, so the BC3 pipeline is unchanged.
Every ASTC texture the CPU provides is made BC3, and its first upload checks its blocks while it copies them into staging: when all decode with alpha 255 (tex_astc.c astc_blocks_opaque, from the block header for all but the endpoint modes with alpha, which are decoded), the entry gets a BC1 image in place of the BC3 one before the decode. Later uploads into a BC1 image check again and go back to BC3 for content with alpha. A large texture's copy and check run on the ASTC decode workers, so its first upload copies faster than before. A check of its own where the texture is made cost 1.7 s of the render thread over a seven-minute teleport tour.
On that tour (Kakariko Village, the Graveyard, Death Mountain Trail, Kokiri Forest, Hyrule Field, RX 7900 XT), against BC3 alone in the same batch: the ASTC textures in Hyrule Field at the end 2,568 instead of 3,104 MiB, video memory at its peak 11,814 instead of 12,374 MiB, process commit at its peak 25,539 instead of 26,139 MiB; the first 7,200 frames in 93.7 instead of 101.3 s and the run's slowest 1 % at 24.1 instead of 21.1 fps. --probe-astc checks the header test against a decode of every generated block, the loop, its copy and the worker pool against it, and each BC1 block against the colour half of its BC3 block. NX2NAVI_ASTC_BC1=0 (astc_bc1 = 0) keeps every ASTC texture BC3; the sampling probe NX2NAVI_ASTC_BC_PROBE it replaces is gone.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Comments say what the code does and why, docs/maxwell.md keeps the Maxwell facts, and fetch_refs.sh clones only the checkout the build reads. Code is unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
