The bidirectional reflectance distribution function (BRDF; ) is a four-dimensional function that defines how light is reflected at an opaque surface. The function takes a negative incoming light direction, and outgoing direction, both defined with respect to the surface normal, and returns the ratio of reflected radiance exiting along to the irradiance incident on the surface from direction Note that each direction is itself parameterized by azimuth angle and zenith angle therefore the BRDF as a whole is 4-dimensional. The BRDF has units sr−1, with steradians (sr) being a unit of solid angle.
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