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Virtual Light Sources
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in Radiance
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Radiance now supports virtual light sources in planar surfaces such as
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mirrors. The method of virtual sources is used to create the appearance
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of a new source in a virtual world on the other side of the transferring
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surface, or "relay object." Shadow rays are then sent to these virtual
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sources along with the regular sources, testing not only for occlusion
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but also for the geometric boundaries of the virtual source path.
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This is necessary to yield the correct light patch shape.
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The method used for specifying virtual sources in Radiance is quite
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simple. Certain materials possess the "virtual source" attribute.
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When such a material modifies a planar surface, virtual light
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sources are created. It is an error to use a virtual source material
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on a non-planar surface such as a sphere. Currently, the materials
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"mirror," "prism1" and "prism2" have the virtual light source attribute.
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If multiple facing mirrors appear in a scene, the number of virtual
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sources can multiply quite rapidly. We therefore introduce a limit to
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the number of virtual source "relays" allowed, with the rendering
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option -dr. A setting of -dr 0 means that virtual sources will not
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be considered at all. Another technique that can limit the growth of
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virtual sources is called "virtual source presampling," which is
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controlled with the -dp option. Presampling tests a virtual source
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for visibility before adding it to the calculation, thus avoiding the
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inclusion of virtual sources that would never appear and the shadow
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testing of virtual sources that are never occluded. A presampling
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density of -dp 0 means that all virtual sources will be included and
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fully tested for shadows. This is potentially much more expensive, but it
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is the only way to guarantee absolute shadow accuracy at any resolution.
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Even without presampling, Radiance performs many checks of virtual
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sources before including them in the calculation. In addition to the
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obvious tests to insure that a source is on the correct side of the
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relay object, facing the proper direction and so on, Radiance also
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computes the solid angle that corresponds to the maximum influence of
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each virtual source. This greatly speeds up the direct calculation by
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avoiding virtual source shadow tests that could not possibly pay off.
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Nevertheless, virtual light sources can be quite costly, especially if
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there are many mirror surfaces that see each other. Presampling avoids
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most of the costs associated with fruitless testing, but in scenes with
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mutual reflections, there may still be hundreds or even thousands of
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virtual light sources created. Even with the solid angle limits, each
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virtual source must be considered at least briefly before it is rejected.
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It is therefore very important for efficiency to minimize the number of
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mirror surfaces in a scene as much as possible. In particular, do not
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make relay objects from many small mirror elements. Such elements should
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be consolidated into the largest polygons possible.
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