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<H1><A ID="SECTION00110000000000000000"> Surface Description</A> </H1>

<P> A surface definition consists of a number of component keywords, each of which is usually followed by either a single number or a red-green-blue color triple. Each of the values in the color triple are normalized, with zero indicating zero intensity, and one indicating full intensity.

<P> If any surface component is left unspecified, its value defaults to zero, with the exception of the index of refraction, which is assigned the default index of refraction (normally 1.0).

<P> Surface descriptions are used in to compute the color of a ray that strikes the surface at a point P. The normal to the surface at P, N, is also computed.

<P> <BR> <IMG STYLE="height: 196.25ex; vertical-align: -0.70ex; " SRC="img1.png" ALT="
\begin{defkey}{ambient}{\evec{color}}
Use the given {\em color} to approximate...
...diffuse interreflection) not modeled by the
ray tracing process.
\end{defkey}
"> <BR> A surface's ambient color is always applied to a ray. The color applied is computed by multiplying the ambient color by the intensity of the ambient light source.

<P> If P is in shadow with respect to a given light source, that light source makes no contribution to the shading of P.

<P> <BR> <IMG STYLE="height: 2.38ex; vertical-align: 176.40ex; " SRC="img2.png" ALT="
\begin{defkey}{diffuse}{\evec{color}}
Specifies the diffuse color.
\end{defkey}
"> <BR> The diffuse contribution from each non-shadowed light source at P is equal to the diffuse color of the surface scaled by the cosine of the angle between N and the vector from P to the light source.

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img3.png" ALT="
\begin{defkey}{specular}{\evec{color}}
Specifies the base color of specular reflections.
\end{defkey}
"> <BR>

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img4.png" ALT="
\begin{defkey}{specpow}{{\em exponent}}
Controls the size of the specular high...
...The larger
the {\em exponent}, the smoother the apparent finish.
\end{defkey}
"> <BR> The intensity of specular highlights from light sources are scaled by the specular color of the surface.

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img5.png" ALT="
\begin{defkey}{reflect}{{\em reflectivity}}
Specifies the specular reflectivity of the surface. If non-zero,
reflected rays will be spawned.
\end{defkey}
"> <BR> The intensity of specularly reflected rays will be proportional to the specular color of the surface scaled by the reflectivity.

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.55ex; " SRC="img6.png" ALT="
\begin{defkey}{transp}{{\em transparency}}
Specifies the specular transmissivi...
...ce. If
non-zero,
transmitted (refracted) rays will be spawned.
\end{defkey}
"> <BR>

<P> <BR> <IMG STYLE="height: 2.38ex; vertical-align: 176.40ex; " SRC="img7.png" ALT="
\begin{defkey}{body}{\evec{color}}
Specifies the body color of the object. The...
...fects the color of rays that are transmitted through the
object.
\end{defkey}
"> <BR>

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img8.png" ALT="
\begin{defkey}{extinct}{{\em coefficient}}
Specifies the extinction coefficient of the interior
of the object.
\end{defkey}
"> <BR> The extinction coefficient is raised to a power equal to the distance the transmitted ray travels through the object. The overall intensity of specularly transmitted rays will be proportional to this factor multiplied by the surface's body color multiplied by the transparency of the object.

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img9.png" ALT="
\begin{defkey}{index}{{\em N}}
Specifies the index of refraction. The default .....
.... to the index of refraction of the atmosphere surrounding the eye.
\end{defkey}
"> <BR>

<P> <BR> <IMG STYLE="height: 2.17ex; vertical-align: 176.68ex; " SRC="img10.png" ALT="
\begin{defkey}{translucency}{{\em translu} \evec{color} {\em stexp}}
Specifies...
...d color,
and Phong exponent for transmitted specular highlights.
\end{defkey}
"> <BR> If a light source illuminates a translucent surface from the side opposite that from which a ray approaches, illumination computations are performed, using the given color as the surface's diffuse color, and the given exponent as the Phong highlight exponent. The resulting color is then scaled by the surface's translucency.

<P>

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