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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.

<P> <BR> <IMG STYLE="height: 2.38ex; vertical-align: 176.40ex; " SRC="img9.png" ALT="
\begin{defkey}{maxdepth}{{\em level}}
Do not spawn rays deeper than those at the given {\em level}.
\end{defkey}
"> <BR> 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.

<P> <BR> <IMG STYLE="height: 2.10ex; vertical-align: 176.68ex; " SRC="img10.png" ALT="
\begin{defkey}{cutoff}{{\em threshold}}
Do not spawn rays whose contribution t...
...as a single floating-point value,
or as a red-green-blue triple.
\end{defkey}
"> <BR> 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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