Surfaces are used to control the interaction between light sources and objects. A surface specification consists of information about how the light interacts with both the exterior and interior of an object . For non-closed objects, such as polygons, the ``interior'' of an object is the ``other side'' of the object's surface relative to the origin of a ray.
<#545#><#1965#>Rayshade<#1965#><#545#> usually ensures that a primitive's surface normal is pointing towards the origin of the incident ray when performing shading calculations. Exceptions to this rule are transparent primitives, for which <#546#><#1967#>rayshade<#1967#><#546#> uses the direction of the surface normal to determine if the incident ray is entering or exiting the object. All non-transparent primitives will, in effect, be double-sided.