Schlick's approximation

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In 3D computer graphics, Schlick's approximation, named after Christophe Schlick, is a formula for approximating the contribution of the Fresnel factor in the specular reflection of light from a non-conducting interface (surface) between two media.

According to Schlick's model, the specular reflection coefficient R can be approximated by:

R(\theta) = R_0 + (1 - R_0)(1 - \cos \theta)^5
R_0 = \left(\frac{n_1-n_2}{n_1+n_2}\right)^2

where \theta is the angle between the viewing direction and the half-angle direction, which is halfway between the incident light direction and the viewing direction, hence \cos\theta=(H\cdot V). And n_1,\,n_2 are the indices of refraction of the two media at the interface and R_0 is the reflection coefficient for light incoming parallel to the normal (i.e., the value of the Fresnel term when \theta = 0 or minimal reflection). In computer graphics, one of the interfaces is usually air, meaning that n_1 very well can be approximated as 1.

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