The main purpose of area light sources is to generate more realistic lighting, resulting in soft shadows. This is achieved by using one of six primitives (rectangles, discs, spheres, cylinders, user, or geometric objects) as light sources with nonzero area. This means that a point on an object may be illuminated by only a part of a light source, which creates accurate soft shadows. Area light sources are based on similar principles as motion blurring.
Area light sources are specified in the .mi file by naming a primitive in a standard light definition. Any of the standard spot and point lights can be turned into an area light source. Infinite light sources ignore any area specifications because the size of a light at infinite distance does not matter, by definition.
There are four different pre-defined shapes of area light sources: rectangles, flat discs, spheres, and cylinders. The orientation of the disc and rectangle shape may be chosen independently of the light direction of spot and directional light sources. Any type of light shader can be used.
mental ray also supports arbitrary geometric scene objects to be turned into an area light sources, and allows user-defined shapes controlled by specialized light shaders.
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