Occlusion, in brief, is blocking, as in when an object blocks light. Technically, you could say that all regular shadows are kinds of occlusion, but most people reserve the term occlusion for a reference to other kinds of light-blocking that aren't regular shadows from a light. The few examples that follow should shed some light on different kinds of occlusion.
Ambient occlusion is a function designed to darken parts of your scene that are blocked by other geometry or less likely to have a full view of the sky. You can use ambient occlusion as a replacement or supplement to the shadows in your fill lights.
The main idea behind ambient occlusion is hemispheric sampling or looking around the scene from the point of view of each point on a surface. Figure 3.27 shows how rays are sampled in all directions from a point being rendered. The more of these rays hit an object (instead of shooting off into empty space), the darker the ambient occlusion. Ambient occlusion can usually have a maximum distance set for the rays, so that only nearby objects will cause the surface to darken.
Figure 3.27 This diagram illustrates the rays that would be sampled in rendering just one pixel with ambient occlusion. Ambient occlusion looks around the whole scene from the point being rendered, and darkens the point based on how many nearby objects are encountered which might be blocking light.
Ambient occlusion can be a terrific substitute for soft shadows from your fill light. You might still want your main light source, such as the sun, to cast a shadow, but your could turn off shadows from the surrounding fill lights, such as lights simulating illumination from the sky. In Figure 3.28, the brightest light source casts a raytraced shadow. The fill lights do not cast shadows, but ambient occlusion softly darkens the areas underneath and between objects, even in areas that are entirely within the raytraced shadow.
Figure 3.28 A scene with no shadowing on its fill light can look very flat (left), but adding ambient occlusion (right) provides extra shading that darkens areas that would not be exposed to the sky.
It would be possible to use ambient occlusion by itself, with no shadows at all in a scene, but this might produce too uniform a darkening effect in each area where two surfaces came together, and the audience might notice the lack of distinct shadows cast in a specific direction.
Compared to raytraced soft shadows from an area light, ambient occlusion can provide similar looking shading in your scene, but usually takes less time to render.
Occlusion in Global Illumination
Global illumination (abbreviated GI) is an approach to rendering in which indirect light is calculated as it interreflects between surfaces in your scene.
GI is different from ambient occlusion, which functions solely to darken parts of the scene. GI adds light to the scene, to simulate bounced or indirect light, essentially replacing both fill lights and their shadows.
With GI, objects block light by reflecting it away, just as real objects would. In Figure 3.29, occlusion is seen where the ground is darkened underneath the sphere. It looks very similar to a soft shadow from a light, but it is actually a natural part of global illumination, which grows darker in areas from which light has been reflected away.
Figure 3.29 Occlusion is an intrinsic part of global illumination, darkening the area under the sphere where light is reflected away.
It is possible to illuminate a GI scene with absolutely no lights. To prevent the scene from being pure black, you would need to provide something brightly shaded to start it off, such as the white cube in Figure 3.30, which has a high incandescence level on its shader. With global illumination, the bright white cube illuminates the whole scene, as if it were an area light. Where light bounces off the sphere, it does not reach the floor, so parts of the floor are occluded as a natural part of the GI process.
Figure 3.30 With global illumination, any bright object can be a light source and cast its own shadows, even in a scene with no lights.
In theory, productions could be lit entirely with incandescent objects. This would provide interesting options, such as the ability to model any shape you want and use it as a light source. However, this would also make the lighting and rendering process inordinately slow. In productions that use global illumination at all, it is most often used to supplement the direct illumination from conventional lights.
Other Types of Occlusion
Final gathering and image-based lighting are two more algorithms that are related in that they provide extra illumination, usually with built-in occlusion.
Final gathering (sometimes abbreviated FG) serves as a simplified form of global illumination. By itself, it can provide a relatively quick, single-bounce solution to adding both indirect light and soft occlusion to a scene, and it is becoming increasingly popular for this purpose. FG can also be used in conjunction with a full GI solution where it serves to smooth out and improve the quality of the global illumination.
Image-based lighting (abbreviated IBL) is an approach to rendering in which a sky dome surrounding the scene is mapped with an image of an environment, and the colors and tones from that image are used to illuminate the scene. The process of rendering with IBL often uses a similar rendering technique to those used in rendering GI, and it will often include similar built-in occlusion. In renderers that do not provide built-in occlusion in IBL renders, IBL can be combined with ambient occlusion.
Chapter 4, "Lighting Environments and Architecture," will talk more about GI, FG, and IBL.