Global illumination (indirect light) has no good rasterization solution and requires ray tracing

Global illumination — lighting that accounts for light bouncing between surfaces (indirect illumination) — is the one rendering effect for which rasterization has no adequate solution. Shadows, reflections, and refractions can all be approximated with rasterization techniques (shadow maps, cube maps, screen-space methods), though imperfectly. Global illumination by contrast requires tracing multiple bounces of light through the scene, which is fundamentally a ray tracing problem. This asymmetry explains why modern games adopt a hybrid: rasterize everything, add ray-traced shadows and reflections where budget allows, and use ray-traced or baked global illumination. Without ray tracing, scenes are lit only by direct light — surfaces in shadow receive no bounce light, which looks artificial.

Examples

A room lit by sunlight through a window: with global illumination, the ceiling bounces light into shadowed corners; without it, the shadowed ceiling stays black. Only ray tracing (or baked lightmaps) solves this correctly at runtime.

Assessment

Explain why global illumination cannot be computed in real-time with rasterization alone. What rasterization-based technique is sometimes used as a workaround, and what are its limitations?