In order to test a point against the depth map, its position in the scene coordinates must be transformed into the equivalent position as seen by the light. Visualization of the depth map projected onto the scene This process has three major components, the first is to find the coordinates of the object as seen from the light, the second is the test which compares that coordinate against the depth map, and finally, once accomplished, the object must be drawn either in shadow or in light. The second step is to draw the scene from the usual camera viewpoint, applying the shadow map. Alternatively, culling front faces and only rendering the back of objects to the shadow map is sometimes used for a similar result. Also, a depth offset which shifts the objects away from the light may be applied to the shadow map rendering in an attempt to resolve stitching problems where the depth map value is close to the depth of a surface being drawn (i.e., the shadow-casting surface) in the next step. In many implementations it is practical to render only a subset of the objects in the scene to the shadow map in order to save some of the time it takes to redraw the map. (If there are multiple lights, a separate depth map must be used for each light.) This depth map must be updated any time there are changes to either the light or the objects in the scene but can be reused in other situations, such as those where only the viewing camera moves. This depth map is often stored as a texture in graphics memory. Because only the depth information is relevant, it is common to avoid updating the color buffers and disable all lighting and texture calculations for this rendering, in order to save drawing time. For directional light (e.g., that from the Sun), an orthographic projection should be used.įrom this rendering, the depth buffer is extracted and saved. For a point light source, the view should be a perspective projection as wide as its desired angle of effect (it will be a sort of square spotlight). The first step renders the scene from the light's point of view. Depending on the implementation (and the number of lights), this may require two or more drawing passes. The first produces the shadow map itself, and the second applies it to the scene. Rendering a shadowed scene involves two major drawing steps. Unlike shadow volumes, however, the accuracy of a shadow map is limited by its resolution. 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. 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.
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. The light's view is rendered, storing the depth of every surface it sees (the shadow map). This is the basic principle used to create a shadow map. Anything behind those objects, however, would be in shadow. If you looked out from a source of light, all of the objects you can see would appear in light. 1 Principle of a shadow and a shadow map.
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