Global illumination
Simulating indirect light — light that bounces between surfaces and bleeds colour onto them — and not just the direct light straight from the source.
Direct lighting computes only the ray source → surface → eye. In reality the light bounces onward: a red floor tints the wall above it pink, a shadowed corner catches a faint glow instead of going pitch black. Global illumination (GI) captures those bounces. The family spans radiosity, path tracing, photon mapping, light probes and voxel-based methods — all approximations of Kajiya's rendering equation (1986).
Long too expensive in real time, so the light was baked ahead of time into lightmaps for static scenes. Hardware ray tracing (2018) and techniques like Lumen in Unreal Engine 5 (2022) made dynamic GI feasible. Cheaper but limited tricks such as ambient occlusion and screen space reflections each approximate one slice of the effect.