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View Shadow Maps

Shadow Maps (AKA “Depth Buffer Shadows” or “Z Buffer Shadows) are an approximation of an actual raytraced or area shadow.  Shadow maps are typically faster than the other shadow types, though they have unique issues unto themselves.  In the last few years, computers have gotten so much faster, and the other shadowing algorithms have gotten so much better, that shadow maps are almost more trouble than they are worth today.

A shadow map is effectively a texture map that is projected from any light assigned to this type of shadow.  The scene is sampled from the position of the casting light, and then a map is created.  The light uses that map to determine what is in shadow and what is not.  There are numerous settings to dial in the quality of the map to match your time/quality needs, and if you wish to use shadow maps, it makes sense to learn those settings, and how best to use them.  Shadow maps are resolution-dependent; in order to have a better quality map, you will need to increase the resolution of the shadow map.  Doing so will increase the time it takes to create the initial map, as well as increase the memory requirements need to render the scene.  Also, the map is taken from a fixed position in space (the light location), and if your object is large in frame, you may see artifacts on the shadow boundaries.

Shadow map edges are never crisp; that said, most shadows in nature are never knife edge sharp.  Shadow map edges may show pixel blocks is the resolution is too low; higher resolutions will fix this. Shadow maps never defocus, like natural shadows or shadows created with area shadows.  Instead, shadow maps will have the same edge focus top to bottom.  Shadow maps will not motion blur; they will always appear sharp, not matter how much the casting object is in motion. Shadow maps can be noisy, therefore its necessary to adjust the appropriate settings.  Lastly, shadow maps never actually meet the edge of the casting object when the object is sitting on a shadow receiving surface.  There are controls to help mitigate this issue, and it can be made to work well enough.  In short, however, shadow maps are not a “fire and forget” proposition.

 

Category:Glossary

Category:Lighting

Category:Rendering

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