Ambient occlusion

Ambient occlusion (AO) estimates how much of the surrounding environment each point can see. The line where a book meets a shelf, the corner where wall meets ceiling, the space under a sofa all receive less ambient light and darken slightly. AO approximates that effect independently of the actual light sources.

Where and why it is used

In real-time engines and rasterized images, bounced light is not fully computed, so objects can look as if they are floating rather than resting on the floor. AO adds depth at contact points and largely fixes this. Its screen-space version (SSAO) is common in games and in real-time architectural engines.

With methods that compute full GI, such as path tracing, AO is not physically needed, because the darkening already emerges from the real path of light. Many visualizers still output AO as a separate render pass and blend a little of it in post-production to bring out detail.

Things to watch

• Too large a radius leaves a dirty halo around everything.
• In physically based renders, heavy AO can hurt realism rather than help it.
• In clay renders, AO is very effective at revealing form.