Voxel cone tracing global illumination

One light moving through a closed room.
Left of the line: direct light only. Right: voxel cone tracing adds the bounce, so the red and green walls tint the floor, ceiling and boxes.

The problem

Direct lighting stops at the first surface the light hits. Real light keeps bouncing, which is why a white floor next to a red wall turns pink. Baked lightmaps capture that, but only for lights that never move. Path tracing every pixel is too slow for most games.

How it works

Voxel cone tracing gives up exact rays for a blurry, cheap version of the scene:

1. Voxelize the scene into a 64³ grid of colour, normal and occupancy.
2. Light every voxel: direct light with a shadow march through the grid, plus the bounce read from last frame's result. Repeat it each frame and the light keeps bouncing.
3. Downsample into 6 more levels (32³ to 1³), so coarse levels hold averaged light.
4. Per pixel, trace a fan of cones over the hemisphere. Each step doubles the cone's width and reads a coarser level, until it is 95% blocked.

A cone that widens and reads a blurrier level costs about the same as one ray, but gathers light from a whole region.

What I hit

My first room was open at the front for the camera, so most of the bounce escaped and the effect was barely visible. I closed it with a front wall that is voxelized but never drawn.

The rotated box got striped self-shadowing until I started each pixel's shadow march 1.75 voxels off the surface.

With the source's default box filter the bounce looked blocky. The source also ships a 3×3×3 Gaussian; I used that, and moved each cone start 1 voxel off the surface instead of 0.5.

My build

three.js 0.170 on WebGL2. The voxel lighting, the downsample and the cone tracing are GLSL ports of Looooong's Unity SRP VXGI. It uses its 32-direction cone table, which keeps 11 to 13 cones per pixel. The grid is 64³, the source's Medium preset; 29,574 voxels are filled. Two voxel lighting passes run per frame.

One big change: the source voxelizes on the GPU every frame. My scene never moves, so I voxelize once on the CPU and only the light moves.

The cone-count panel compares 4, 6 and 9 cones against the source's set, showing the bounce only. With fewer cones the bounce gets patchier and the voxel blocks show more.

Trade-offs

• Bounce light from moving lights, no bake
• One grid serves every pixel; the per-pixel bounce cost depends on cone count, not light count
• 64³ over a room is about 5 cm per voxel, so thin objects leak light
• Up close the bounce shows voxel blocks, and small contacts need SSAO on top
• Moving objects would need the grid rebuilt every frame

When to use it

Rooms and small levels with moving lights, like a torch or a day-night cycle, where bake times hurt. For large open worlds the grid gets too coarse.