||This interface is used to store more information during the computation
Emitter interface is the superinterface for
all entities which emit a quantity of light transport:
Light sources emit
Interior instance represents the interior of a solid
Light instance represents a light source.
Scattering instance represents a scattering entity:
Either a surface
Shader instance represents a surface shader which
calculates the color of an outgoing light ray at a given point
as a function of geometrical and optical properties,
namely local and global point coordinates, canonical uv-coordinates, surface
tangent vectors, outgoing ray direction, index of refraction,
and a list of incident light rays, consisting of color and direction.
||This interface represents a spectrum, i.e., a function f(ν)
of frequency ν of a wave.