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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. |
