Light leaking

Light leaking is light showing up where geometry should block it: a bright seam along the base of a wall, a glow on the inside of a closed room, sky colour in a cave. It almost always comes from global illumination that samples the scene more coarsely than the walls it is lighting. Unity's HDRP manual gives the typical cause: leaks "often occur when geometry receives light from a Light Probe that isn't visible to the geometry" (High Definition Render Pipeline manual: Fix issues with Adaptive Probe Volumes).

#Why it happens

Every real-time GI method simplifies the scene: into probes on a grid, voxels, distance field cells or lightmap texels. When a wall is thinner than that spacing, samples on both sides of the wall end up blended together.

How each system leaks:

• Probe grids. Unity's Adaptive Probe Volumes "use regular grids of Light Probes, so Light Probes might not follow walls" (High Definition Render Pipeline manual: Fix issues with Adaptive Probe Volumes). With default spacings of 1, 3, 9 or 27 m (High Definition Render Pipeline manual: Understanding Adaptive Probe Volumes), a 20 cm wall falls between probes easily.
• Distance fields. In Unreal's Lumen, "Walls should be no thinner than 10 centimeters (cm) to avoid light leaking", and distance fields "cannot represent extremely thin features" (Unreal Engine Documentation: Lumen Technical Details).
• Limited ray distance. In enclosed, cave-like spaces, Lumen limits how far rays travel for performance, so a missed occluder lets light through (Unreal Engine Documentation: Lumen Technical Details).
• Voxels. Godot's VoxelGI documentation warns of leaks after baking and that raising Bias or Normal Bias too far makes "light leaking ... more pronounced" (Godot Engine documentation: Using VoxelGI).
• Sky light indoors. Without an interior setting, reflection probes and GI can pull ambient light from the sky into rooms (Godot Engine documentation: Reflection probes).

#Fixes by engine

Unity notes the limits: a Probe Adjustment Volume "solves most light leak issues, but often not all" (High Definition Render Pipeline manual: Fix issues with Adaptive Probe Volumes).

#Modeling habits that prevent leaks

• Give walls real thickness. Model interiors as closed volumes rather than single planes. See modular kits and grids and metrics.
• Close the backs. Rooms that are only ever seen from inside still need outer faces for GI; Epic notes that one-sided meshes can show artefacts when seen from behind (Unreal Engine Documentation: Lumen Technical Details).
• Seal the level. Godot's advice applies everywhere: keep geometry "fully sealed" (Godot Engine documentation: Using VoxelGI).

Related: baked vs. dynamic lighting, lightmap UVs, real-time lighting.

#Further reading and watching