areaLighting

The render feature behind SurfaceLight and @builtin::systems.areaLights.surfaceLight. One compute pass runs after lighting, at order 30, adding every registered rectangle's contribution from the G-buffer.

Diffuse

The exact irradiance of a polygon — Lambert's formula, summing the signed solid angle of each edge as seen from the shaded point. For a quad that is four terms.

Being exact matters where the approximations fail: a wide emitter close to a wall, where treating it as a point puts the falloff in the wrong place entirely.

Specular

The representative point: the spot on the rectangle the reflection ray comes closest to, treated as a point source with the roughness widened to cover the solid angle the rectangle spans.

Widening a lobe spreads the same energy further, so the peak is scaled down by the same factor — without that, a large emitter reads brighter than it is rather than broader.

Falloff

Distance uses a smooth cutoff rather than a hard one, so a surface does not switch off as it crosses range. The emitter lights only the side its normal faces.

@scene.material carries the base colour, metallic and roughness. Lighting is summed in linear radiance and the scene target is decoded and re-encoded around it.

Parameters and lights arrive through the area_light_params and area_light_records buffers, packed by @builtin::systems.areaLights.surfaceLight.