[Epic] Emissive surfaces as light sources (RT emissive lighting) #91

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opened 2026-09-27 06:34:26 +00:00 by backspace119 · 0 comments

backspace119 commented

2026-09-27 06:34:26 +00:00

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Goal: emissive surfaces actually light the world - neon signs, glowing prims, lamp shades, screens, fullbright+glow builds cast light (and soft shadows) onto nearby surfaces, instead of glow being only a screen-space bloom.

Today SL "glow" is a post-process halo; emissive/fullbright surfaces light nothing. With RT we can make them real light sources - a big step for SL's many lit-by-texture builds.

Two tiers

Tier 1 - emissive in GI (falls out of #87, make sure it's in scope there): probe rays that hit emissive surfaces add the surface's emitted radiance (PBR emissive factor x texture, legacy fullbright/glow mapped to an emissive estimate) to probe irradiance. Soft, low-frequency ambient contribution; cheap; no extra ray budget.

Tier 2 - emissive area lights (this ticket's core):

Build an emissive triangle list GPU-side from the instance/material tables (instances whose material has emissive/glow above a threshold), with per-triangle power estimates (area x average emissive texel luminance, e.g. from a low mip).
Light sampling: importance-sample emissive triangles (power-weighted, or a light BVH/alias table), combined with ReSTIR DI (reservoir resampling, temporal + spatial reuse) so thousands of emissive triangles cost a few rays per pixel.
Visibility via the existing ray-query shadow path (alpha-tested, translucent casters).
Denoise with the shared RT denoiser (#85); temporal reuse needs the temporal pipeline (#84).

Design notes

Legacy content mapping: fullbright + glow faces are SL's de-facto "emissive"; define how glow strength / fullbright colour maps to radiance so old builds light up plausibly without blowing out. Setting to scale or disable per class.
Animated emissive textures (signs, screens): sample the current frame's mip; power estimate updated when the texture/anim changes.
Media-on-a-prim screens as light sources (TV lighting a room) - optional stretch.
Budget: cap emissive triangle count / use per-instance aggregation for dense emissive meshes.

Children

Emissive radiance in GI hit shading (tier 1; coordinate with #87).
GPU emissive triangle list + power estimation.
Light sampling + ReSTIR DI resampling pass.
Shading integration in the lit pass + denoiser hookup.
Legacy glow/fullbright mapping + settings + GPU timers.

Size: L. Depends on: RT foundation (done), #84 temporal pipeline, #85 denoiser; tier 1 rides on #87.

**Goal:** emissive surfaces actually light the world - neon signs, glowing prims, lamp shades, screens, fullbright+glow builds cast light (and soft shadows) onto nearby surfaces, instead of glow being only a screen-space bloom. Today SL "glow" is a post-process halo; emissive/fullbright surfaces light nothing. With RT we can make them real light sources - a big step for SL's many lit-by-texture builds. ### Two tiers **Tier 1 - emissive in GI (falls out of #87, make sure it's in scope there):** probe rays that hit emissive surfaces add the surface's emitted radiance (PBR emissive factor x texture, legacy fullbright/glow mapped to an emissive estimate) to probe irradiance. Soft, low-frequency ambient contribution; cheap; no extra ray budget. **Tier 2 - emissive area lights (this ticket's core):** - Build an **emissive triangle list** GPU-side from the instance/material tables (instances whose material has emissive/glow above a threshold), with per-triangle power estimates (area x average emissive texel luminance, e.g. from a low mip). - **Light sampling:** importance-sample emissive triangles (power-weighted, or a light BVH/alias table), combined with **ReSTIR DI** (reservoir resampling, temporal + spatial reuse) so thousands of emissive triangles cost a few rays per pixel. - Visibility via the existing ray-query shadow path (alpha-tested, translucent casters). - Denoise with the shared RT denoiser (#85); temporal reuse needs the temporal pipeline (#84). ### Design notes - **Legacy content mapping:** fullbright + glow faces are SL's de-facto "emissive"; define how glow strength / fullbright colour maps to radiance so old builds light up plausibly without blowing out. Setting to scale or disable per class. - Animated emissive textures (signs, screens): sample the current frame's mip; power estimate updated when the texture/anim changes. - Media-on-a-prim screens as light sources (TV lighting a room) - optional stretch. - Budget: cap emissive triangle count / use per-instance aggregation for dense emissive meshes. ### Children 1. Emissive radiance in GI hit shading (tier 1; coordinate with #87). 2. GPU emissive triangle list + power estimation. 3. Light sampling + ReSTIR DI resampling pass. 4. Shading integration in the lit pass + denoiser hookup. 5. Legacy glow/fullbright mapping + settings + GPU timers. **Size:** L. **Depends on:** RT foundation (done), #84 temporal pipeline, #85 denoiser; tier 1 rides on #87.

backspace119 added this to the Rendering: temporal + RT effects milestone

2026-09-27 06:34:26 +00:00

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2026-09-27 06:34:26 +00:00

backspace119 referenced this issue

2026-09-27 06:34:26 +00:00

[Epic] Ray-traced global illumination #87