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Shadow mapping
Shadow mapping or projective shadowing is a process by which shadows are added to 3D computer graphics. This concept was introduced by Lance Williams in 1978, in a paper entitled "Casting curved shadows on curved surfaces". Since then, it has been used both in pre-rendered scenes and realtime scenes in many console and PC games.
Shadows are created by testing whether a pixel is visible from the light source, by comparing the pixel to a z-buffer or ''depth'' image of the light source's view, stored in the form of a texture.
==Principle of a shadow and a shadow map==
If you looked out from a source of light, all of the objects you can see would appear in light. Anything behind those objects, however, would be in shadow. This is the basic principle used to create a shadow map. The light's view is rendered, storing the depth of every surface it sees (the shadow map). Next, the regular scene is rendered comparing the depth of every point drawn (as if it were being seen by the light, rather than the eye) to this depth map.
This technique is less accurate than shadow volumes, but the shadow map can be a faster alternative depending on how much fill time is required for either technique in a particular application and therefore may be more suitable to real time applications. In addition, shadow maps do not require the use of an additional stencil buffer, and can be modified to produce shadows with a soft edge. Unlike shadow volumes, however, the accuracy of a shadow map is limited by its resolution.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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