The definition of theBSDF (bidirectional scattering distribution function) is not well standardized. The term was probably introduced in 1980 by Bartell, Dereniak, and Wolfe.[1] 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 asBRDF (bidirectional reflectance distribution function) andBTDF (bidirectional transmittance distribution function).
BSDF: BRDF + BTDF
BSDF is a superset and the generalization of theBRDF andBTDF. The concept behind all BxDF functions could be described as a black box with the inputs being any two angles, one for incoming (incident) ray and the second one for the outgoing (reflected or transmitted) ray at a given point of the surface. The output of this black box is the value defining the ratio between the incoming and the outgoing light energy for the given couple of angles. The content of the black box may be a mathematical formula which more or less accurately tries to model and approximate the actual surface behavior or an algorithm which produces the output based on discrete samples of measured data. This implies that the function is 4(+1)-dimensional (4 values for 2 3D angles + 1 optional for wavelength of the light), which means that it cannot be simply represented by 2D and not even by a 3D graph. Each 2D or 3D graph, sometimes seen in the literature, shows only a slice of the function.
Some tend to use the termBSDF simply as a category name covering the whole family of BxDF functions.
The termBSDF is sometimes used in a slightly different context, for the function describing the amount of the scatter (not scattered light), simply as a function of the incident light angle. An example to illustrate this context: for perfectlylambertian surface the BSDF (angle)=const. This approach is used for instance to verify the output quality by the manufacturers of the glossy surfaces.[clarification needed]
Another recent usage of the termBSDF can be seen in some 3D packages, when vendors use it as a 'smart' category to encompass the simple well known cg algorithms likePhong,Blinn–Phong etc.
BTDF (Bidirectional transmittance distribution function)[1] is similar to BRDF but for the opposite side of the surface. (see the top image).
BDF (Bidirectional distribution function) is collectively defined by BRDF and BTDF.
BSSRDF (Bidirectional scattering-surface reflectance distribution function orBidirectional surface scattering RDF)[4][5] describes the relation between outgoing radiance and the incident flux, including the phenomena likesubsurface scattering (SSS). The BSSRDF describes how light is transported between any two rays that hit a surface.
BSSTDF (Bidirectional scattering-surface transmittance distribution function) is like BTDF but with subsurface scattering.
BSSDF (Bidirectional scattering-surface distribution function) is collectively defined by BSSTDF and BSSRDF. Also known asBSDF (Bidirectional scattering distribution function).
^abcDebevec, Paul; Tim Hawkins; Chris Tchou; Haarm-Pieter Duiker; Westley Sarokin; Mark Sagar (2000). "Acquiring the reflectance field of a human face".Proceedings of the 27th annual conference on Computer graphics and interactive techniques - SIGGRAPH '00. ACM. pp. 145–156.doi:10.1145/344779.344855.ISBN978-1581132083.S2CID2860203.
