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<H1><A ID="SECTION00150000000000000000"> The Ray Tree</A> </H1>
<P> When ray tracing a scene, reflected or transmitted rays may strike other reflective or transparent objects. Further reflected or transmitted rays will be spawned, and so on. Taken together, such a family of rays is termed the <EM>ray tree</EM>. Care must be taken to control the depth of this tree: If it is allowed to grow too deeply, one may spend a great deal of time computing rays that contribute little to the final picture; if it is not allowed to grow far enough, this premature tree pruning may be evident in the image.
<P> provides two complementary methods for controlling the depth of the ray tree. One method sets an absolute maximum for the tree. The other allows one to adaptively prune a tree as it grows so that ``unimportant'' rays are not spawned.
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Rays from the eye are of depth zero. The default value for
<EM>level</EM> is 15.
This depth may also be set from the command line through the <TT>-D</TT> option.
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The default value is 0.002. This threshold may also be set from
the command line through the <TT>-T</TT> option.
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