Definitions:

• I_l,a - Ambient light intensity (same old hack)

• k_a - Ambient surface reflectance (hacks beget hacks)

• I_l,i - Luminous intensity of light source i

• k_s - percentage of light reflected specularly (notice terms sum to one)

• r_l - Diffuse reflectivity

• l_i - vector to light source

• n - average surface normal at point

• D - microfacet distribution function

• G - geometric attenuation Factor

• F_l(q_i) - Fresnel conductance term

• v - vector to viewer

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