Bidirectional reflectance distribution function
The bidirectional reflectance distribution function (BRDF; 
) is a function of four real variables that defines how light is reflected at an opaque surface. It is employed both in the optics of real-world light, in computer graphics algorithms, and in computer vision algorithms. The function takes an incoming light direction, 
, and outgoing direction, 
(taken in a coordinate system where the surface normal 
lies along the z-axis), and returns the ratio of reflected radiance exiting along to the irradiance incident on the surface from direction . Each direction 
is itself parameterized by azimuth angle 
and zenith angle 
, therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr−1, with steradians (sr) being a unit of solid angle.
Definition
The BRDF was first defined by Fred Nicodemus around 1965. The definition is:
where 
is radiance, or power per unit solid-angle-in-the-direction-of-a-ray per unit projected-area-perpendicular-to-the-ray, 
is irradiance, or power per unit surface area, and 
is the angle between and the surface normal, . The index 
indicates incident light, whereas the index 
indicates reflected light.
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On My Own
by: Brodha VThe first time I ever wrote a rhyme people laughed
My friends called me impetuous and a little brash
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For those who contradict to seek solace in God
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And till this day, I never gave up
I'm On My Own
God never makes mistakes, he is only testing you
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I'm On My Own
And till this day, I never gave up