Bidirectional Scattering Distribution Function The definition of the BSDF (Bidirectional scattering distribution function) is not well standardised. The term was probably introduced in 1991 by Paul Heckbert. Most often it is used to name the general mathematical function which describes the way in which the light is scattered by a surface. However in practice this phenomenon is usually split into the reflected and transmitted components, which are then treated separately as BRDF (Bidirectional reflectance distribution function) and BTDF (Bidirectional transmittance distribution function).
