bidirectional reflectance distribution function
英[ˌbaidiˈrekʃənəl riˈflektəns ˌdistriˈbju:ʃən ˈfʌŋkʃən]美[ˌbaɪdɪˈrɛkʃənəl rɪˈflɛktəns ˌdɪstrəˈbjuʃən ˈfʌŋkʃən]
The bidirectional reflectance distribution function (BRDF; f_{\text{r}}(\omega_{\text{i}},\, \omega_{\text{r}})?) is a four-dimensional function that defines how light is reflected at an opaque surface. The function takes a negative incoming light direction, \omega_{\text{i}}, and outgoing direction, \omega_{\text{r}}, both defined with respect to the surface normal \mathbf n, and returns the ratio of reflected radiance exiting along \omega_{\text{r}} to the irradiance incident on the surface from direction \omega_{\text{i}}.