Surface Description

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.

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).

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.

$\begin{defkey}{ambient}{\evec{color}} Use the given {\em color} to approximate ... ..., diffuse interreflection) not modeled by the ray tracing process. \end{defkey}$
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.

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

$\begin{defkey}{diffuse}{\evec{color}} Specifies the diffuse color. \end{defkey}$
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.

$\begin{defkey}{specular}{\evec{color}} Specifies the base color of specular reflections. \end{defkey}$

$\begin{defkey}{specpow}{{\em exponent}} Controls the size of the specular highl... .... The larger the {\em exponent}, the smoother the apparent finish. \end{defkey}$
The intensity of specular highlights from light sources are scaled by the specular color of the surface.

$\begin{defkey}{reflect}{{\em reflectivity}} Specifies the specular reflectivity of the surface. If non-zero, reflected rays will be spawned. \end{defkey}$
The intensity of specularly reflected rays will be proportional to the specular color of the surface scaled by the reflectivity.

$\begin{defkey}{transp}{{\em transparency}} Specifies the specular transmissivit... ...rface. If non-zero, transmitted (refracted) rays will be spawned. \end{defkey}$

$\begin{defkey}{body}{\evec{color}} Specifies the body color of the object. The ... ...affects the color of rays that are transmitted through the object. \end{defkey}$

$\begin{defkey}{extinct}{{\em coefficient}} Specifies the extinction coefficient of the interior of the object. \end{defkey}$
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.

$\begin{defkey}{index}{{\em N}} Specifies the index of refraction. The default v... ... to the index of refraction of the atmosphere surrounding the eye. \end{defkey}$

$\begin{defkey}{translucency}{{\em translu} \evec{color} {\em stexp}} Specifies ... ...ted color, and Phong exponent for transmitted specular highlights. \end{defkey}$
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.