RT VRAM: build BLAS from a lower LOD for distant objects #578

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opened 2026-10-09 01:49:13 +00:00 by sky_line · 0 comments

sky_line commented

2026-10-09 01:49:13 +00:00

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Measured in a heavy region (2026-10-09): RT acceleration structures (rt-as) are about 1.7 GB, as much as all the geometry, and the opacity micromaps (rt-mask) are 354 MB. Follow-up to #573.

Idea

Static BLAS use a coarser LOD for objects beyond a distance or screen-size threshold. Objects near the camera keep their full-detail BLAS.

Trade-offs

Distant RT shadows, reflections and probe captures use coarser geometry, so silhouettes get slightly blockier at range.
Main risk: self-shadowing mismatch. The raster surface can poke outside its own coarser BLAS, causing shadow acne or light leaks. Mitigate with a conservative threshold, a per-instance ray-origin offset scaled by the LOD error, and hysteresis so a BLAS doesn't swap back and forth.
BLAS memory and build time scale with LOD residency, so tie this into the VRAM ladder.

Done when

rt-as drops substantially in a heavy region, with no visible acne or leaks at the threshold, and no popping when moving.

Measured in a heavy region (2026-10-09): RT acceleration structures (`rt-as`) are about 1.7 GB, as much as all the geometry, and the opacity micromaps (`rt-mask`) are 354 MB. Follow-up to #573. ## Idea Static BLAS use a coarser LOD for objects beyond a distance or screen-size threshold. Objects near the camera keep their full-detail BLAS. ## Trade-offs - Distant RT shadows, reflections and probe captures use coarser geometry, so silhouettes get slightly blockier at range. - Main risk: self-shadowing mismatch. The raster surface can poke outside its own coarser BLAS, causing shadow acne or light leaks. Mitigate with a conservative threshold, a per-instance ray-origin offset scaled by the LOD error, and hysteresis so a BLAS doesn't swap back and forth. - BLAS memory and build time scale with LOD residency, so tie this into the VRAM ladder. ## Done when `rt-as` drops substantially in a heavy region, with no visible acne or leaks at the threshold, and no popping when moving.

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