This chapter aims to understand how ambient occlusion is computed and shows how to set up a local orthonormal basis at a hit point and describes have implemented ambient occlusion as a light shader. Ambient occlusion can significantly increase the realism of the ambient illumination in ray-traced images. The idea is quite simple: the amount of ambient illumination received at a surface point depends on how much of the hemisphere the point is not blocked by objects. By building ambient occlusion into a new ambient light, it will work for any material that has an ambient-reflection component. As points on the plane get closer to the sphere, the solid angle it subtends increases, more shadow rays hit it, and the ambient illumination that the points receive decreases. A similar situation applies to the sphere. The farther from the top a hit point is, the less ambient illumination it receives.