Probe captures: skip blended alpha for far scheduled probes (distance policy) #834

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opened 2026-10-17 22:16:24 +00:00 by sky_line · 1 comment

sky_line commented

2026-10-17 22:16:24 +00:00

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Probe captures draw alpha-blended geometry (glass, translucent prims, particles, alpha hair) for every face. Measured pr_rt_alpha ~0.44 ms at high on a GTX 1080 at 4K, plus its alpha sort.

Blended alpha matters little in a probe that covers few screen pixels. Alpha-mask geometry (most foliage / fences) is drawn in the opaque passes and is unaffected. Mirrors always keep alpha (viewed directly).

Plan

A policy setting (e.g. RenderVKProbeAlpha: 0 off, 1 near only (default candidate), 2 all):

keep alpha for the realtime probe and for probes within a distance of the camera;
skip it for scheduled probes beyond it.

Also:

skip the capture alpha pass and its sort entirely when the face's cull found no alpha draws (check whether this already happens);
optionally, a low-detail tier for far probes (no local lights, lower resolution), like LL's low-detail probe mode.

Visible risk: a large blended surface dominating a near-ish probe's view (a probe inside a glass dome, behind a big window). The distance rule should keep these.

Measure

GTX 1080 at 4K, high: pr_rt_alpha / pr_alpha + sort before vs after (at most ~0.5 ms), plus a probe image A/B near a glass build.

Probe captures draw alpha-blended geometry (glass, translucent prims, particles, alpha hair) for every face. Measured `pr_rt_alpha` ~0.44 ms at high on a GTX 1080 at 4K, plus its alpha sort. Blended alpha matters little in a probe that covers few screen pixels. Alpha-mask geometry (most foliage / fences) is drawn in the opaque passes and is unaffected. Mirrors always keep alpha (viewed directly). ### Plan A policy setting (e.g. `RenderVKProbeAlpha`: 0 off, 1 near only (default candidate), 2 all): - keep alpha for the realtime probe and for probes within a distance of the camera; - skip it for scheduled probes beyond it. Also: - skip the capture alpha pass and its sort entirely when the face's cull found no alpha draws (check whether this already happens); - optionally, a low-detail tier for far probes (no local lights, lower resolution), like LL's low-detail probe mode. **Visible risk:** a large blended surface dominating a near-ish probe's view (a probe inside a glass dome, behind a big window). The distance rule should keep these. ### Measure GTX 1080 at 4K, high: `pr_rt_alpha` / `pr_alpha` + sort before vs after (at most ~0.5 ms), plus a probe image A/B near a glass build.

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2026-10-17 22:16:24 +00:00

sky_line added this to the Slipstream project

2026-10-17 22:16:24 +00:00

sky_line referenced this issue from a commit

2026-10-22 19:32:55 +00:00

VK probes #834: blended-alpha distance policy for captures (RenderVKProbeAlpha / RenderVKProbeAlphaDist)