glu

A spotlight hits a red wall and only the wall gets lit. Screen-space GI shoots rays from every pixel, reads the colour they hit on screen, and spills that red onto the floor, boxes and ceiling. Watch the room 👇 ▸ The problem Direct lighting stops at the first surface. Real light bounces, so a lit red wall tints everything near it. Baked lightmaps capture that, but only for static lights and geometry. Here the light moves every frame. ▸ How it works The source is a Unity URP renderer feature. It traces screen-space hemisphere rays, not voxel cones, light cascades or baked data: 1. Trace, at half resolution: from each pixel, 2 rays in random cosine-weighted directions around the normal. 2. March each ray against the depth buffer: 4 tiny steps (1.5 cm), 8 medium steps (5 cm), then 40 cm steps, 32 steps in all. A hit counts within 10 cm behind the surface. 3. A hit reads last frame's final image at that point, so light keeps bouncing over frames. Brightness is clamped to cut fireflies. 4. Denoise: reproject the history and average up to 8 frames, then blend 94% history. A 3x3 depth- and normal-aware blur with random spacing follows. 5. Combine: indirect light times the surface colour, added to direct light. ▸ What I hit Two rays per pixel is mostly noise on its own. Where the history can't be reprojected (a cut, or a surface just uncovered), the source switches to 4 rays with a shorter, coarser march. Turning away shows the limit. With the red wall off screen, rays that would hit it leave the frame and find nothing, so the red on the boxes fades. The source falls back to reflection probes and sky there; this room has neither, so misses return black. ▸ My build three.js (r187), WebGL2 fragment passes, 1920 x 1080 with GI at 960 x 540. A G-buffer pass writes colour and normals. I ported the trace, temporal reprojection, spatial blur and combine passes with the source defaults. Two extra denoise passes in the source (a Poisson-disk blur and temporal stabilization) are not ported. Room: 6.4 x 9.2 m, 4 m high, one moving spotlight plus a dim fill light. ▸ Trade-offs + Colour bleeding from moving lights with no baking + Multiple bounces build up over frames - Only surfaces on screen can bounce light - Needs temporal history; fast changes lag and smear - Thin objects leak through the depth test - Rendered on a software GPU, so no timings ▸ When to use it Interiors with moving lights or opening doors, layered over a baked or probe fallback for whatever is off screen.