Translucent coloured shadows: alpha-blend casters tint and attenuate light (stained glass) #212

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opened 2026-09-30 21:16:57 +00:00 by sky_line · 0 comments

sky_line commented

2026-09-30 21:16:57 +00:00

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Goal

Light through a translucent caster is attenuated by its opacity and tinted by its colour:

More alpha: darker shadow. Less alpha: lighter shadow.
Coloured glass casts coloured light, so stained-glass windows throw colourful patterns.

Shadow-map path (sun cascades and projector atlas)

Translucency map: add one per cascade/tile, RGBA8 or RGB10A2 cleared to white. Alpha-blend casters render into it with multiplicative blending (dst *= mix(1, colour, alpha)), writing no depth.
Depth test: the translucency pass depth-tests against the opaque shadow depth, so glass behind a wall doesn't tint.
Transmittance-behind-caster depth: keep the nearest translucent depth, so receivers in front of the glass aren't tinted.
Lookup: light = opaque_visibility * (receiver_depth > translucent_depth ? transmittance.rgb : 1).
Mask casters: stay in the opaque depth map, alpha-tested as now.
Rigged avatars: "Full" in RenderAvatarShadowDetail puts rigged alpha-blend into the translucency pass.

Ray-traced path (RenderVKSunShadows 2, RT projectors)

Ray type: switch the shadow ray query from a terminate-on-first-hit visibility test to a transmittance ray. Candidate hits on translucent instances multiply the throughput by the texel's tinted transmittance, and the ray terminates on an opaque hit or when throughput falls below about 0.01.
Ordering: order is irrelevant for multiplicative transmittance, so no sorting is needed.
Materials: reuse the shared candidate alpha test to fetch base colour and alpha.

Details

Transmittance colour = base colour x vertex tint, with an optional saturation boost setting.
Glow/fullbright surfaces don't emit light.
Cost: one extra colour target per cascade, drawing only the alpha-blend bucket. RT: extra candidate shading on translucent hits only.
Setting: RenderVKTranslucentShadows (on by default for High+), plus Lightbox exposure.

Related: #185, #194, #195.

Today alpha-blend casters either cast a stippled hard shadow at a 0.5 alpha cutoff (non-rigged), or no shadow at all (rigged avatar alpha-blend). Both are wrong for glass, tinted windows, sheer cloth and similar. ## Goal Light through a translucent caster is attenuated by its opacity and tinted by its colour: - More alpha: darker shadow. Less alpha: lighter shadow. - Coloured glass casts coloured light, so stained-glass windows throw colourful patterns. ## Shadow-map path (sun cascades and projector atlas) - **Translucency map:** add one per cascade/tile, RGBA8 or RGB10A2 cleared to white. Alpha-blend casters render into it with multiplicative blending (dst *= mix(1, colour, alpha)), writing no depth. - **Depth test:** the translucency pass depth-tests against the opaque shadow depth, so glass behind a wall doesn't tint. - **Transmittance-behind-caster depth:** keep the nearest translucent depth, so receivers in front of the glass aren't tinted. - **Lookup:** `light = opaque_visibility * (receiver_depth > translucent_depth ? transmittance.rgb : 1)`. - **Mask casters:** stay in the opaque depth map, alpha-tested as now. - **Rigged avatars:** "Full" in `RenderAvatarShadowDetail` puts rigged alpha-blend into the translucency pass. ## Ray-traced path (RenderVKSunShadows 2, RT projectors) - **Ray type:** switch the shadow ray query from a terminate-on-first-hit visibility test to a transmittance ray. Candidate hits on translucent instances multiply the throughput by the texel's tinted transmittance, and the ray terminates on an opaque hit or when throughput falls below about 0.01. - **Ordering:** order is irrelevant for multiplicative transmittance, so no sorting is needed. - **Materials:** reuse the shared candidate alpha test to fetch base colour and alpha. ## Details - Transmittance colour = base colour x vertex tint, with an optional saturation boost setting. - Glow/fullbright surfaces don't emit light. - **Cost:** one extra colour target per cascade, drawing only the alpha-blend bucket. RT: extra candidate shading on translucent hits only. - **Setting:** `RenderVKTranslucentShadows` (on by default for High+), plus Lightbox exposure. Related: #185, #194, #195.

sky_line referenced this issue from a commit

2026-09-30 21:33:37 +00:00

Lightbox shadow filter, avatar shadow detail and terrain hex tiling under Vulkan

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2026-10-01 01:20:10 +00:00

Translucent coloured shadows, RT ray budget from Shadow quality, async parallel pipeline compile

sky_line closed this issue

2026-10-01 01:20:10 +00:00

sky_line referenced this issue from a commit

2026-10-03 01:50:56 +00:00

God rays (#208): ray-traced march for RT sun shadows (RenderVKSunShadows 2)

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