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.
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 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.