Weather: glass roofs let rain and snow through (alpha-blend casters below 0.5 count as open sky) #1159

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opened 2026-10-27 18:28:53 +00:00 by TheSlowly · 1 comment

TheSlowly commented

2026-10-27 18:28:53 +00:00

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A closed glass dome (PBR alpha 0.26) over an observatory's upper floor lets rain fall inside, and through a floor opening the floor below gets a wet patch too.

Cause: the weather occlusion map (#783, recordWeatherOcclusion, llcleanrenderervk.cpp) draws alpha-blend casters with the projector/shadow alpha test, which uses a 0.5 cutoff for blend materials (clean_shadow_mask.frag, clean_alpha_rule.glsl). Any glass below 0.5 is open sky for rain streaks, surface wetness and snow cover. The code comment there already names the intended fix as "M2: a weather-specific low-cutoff test"; it was never done.

Why the cutoff exists: fully transparent prims (phantom barriers, invisible platforms, security prims) must not become roofs.

Proposed: a weather-only alpha cutoff for blend casters in the occlusion map (e.g. a setting, default around 0.05-0.1), so tinted or clear glass (0.1-0.5) roofs while alpha-0 / near-invisible prims stay open. Physics (collision/phantom) is not visible to the renderer and plays no part.

Notes:

A retractable roof is excluded while it moves (mover tracker) and taken back in when it settles. That is correct.
Related: #1145 (rain under a sloped roof; same map), #1037 (indoor snow; same map).

Test: under a closed glass roof in rain or snow, nothing falls inside and the floor stays dry. An alpha-0 prim overhead still lets the rain through.

A closed glass dome (PBR alpha 0.26) over an observatory's upper floor lets rain fall inside, and through a floor opening the floor below gets a wet patch too. **Cause:** the weather occlusion map (#783, `recordWeatherOcclusion`, llcleanrenderervk.cpp) draws alpha-blend casters with the projector/shadow alpha test, which uses a 0.5 cutoff for blend materials (`clean_shadow_mask.frag`, clean_alpha_rule.glsl). Any glass below 0.5 is open sky for rain streaks, surface wetness and snow cover. The code comment there already names the intended fix as "M2: a weather-specific low-cutoff test"; it was never done. **Why the cutoff exists:** fully transparent prims (phantom barriers, invisible platforms, security prims) must not become roofs. **Proposed:** a weather-only alpha cutoff for blend casters in the occlusion map (e.g. a setting, default around 0.05-0.1), so tinted or clear glass (0.1-0.5) roofs while alpha-0 / near-invisible prims stay open. Physics (collision/phantom) is not visible to the renderer and plays no part. **Notes:** - A retractable roof is excluded while it moves (mover tracker) and taken back in when it settles. That is correct. - Related: #1145 (rain under a sloped roof; same map), #1037 (indoor snow; same map). **Test:** under a closed glass roof in rain or snow, nothing falls inside and the floor stays dry. An alpha-0 prim overhead still lets the rain through.

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