Bidirectional reflectance distribution function

The bidirectional reflectance distribution function (BRDF), symbol f r ( ω i , ω r ) {\displaystyle f_{\text{r}}(\omega _{\text{i}},\,\omega _{\text{r}})} , is a function of four real variables that defines how light from a source is reflected off an opaque surface. It is employed in the optics of real-world light, in computer graphics algorithms, and in computer vision algorithms.