BRDF (Bidirectional Reflectance Distribution Function)

The Bidirectional Reflectance Distribution Function (BRDF) is a 4D function¹ that describes how light reflects off a surface. It makes up a crucial part of the Rendering Equation.

\[ BRDF(\omega_{o}, \omega_{i}) \]

It takes an incoming light direction and an outgoing view direction and returns the ratio of reflected light (radiance) to incoming light (irradiance).

Raytracing applies the BRDF at surface intersections to calculate precisely how incoming light reflects and defines a material's reflective appearance (matte, shiny, etc.).