The input to Schlick's approximation is cos theta, where 2*theta is the angle between the incoming light and reflected light direction.
Hello, greatly appreciate the suggestion. I'm kind of curious whether it's view direction and the halfway vector or if it's the light and normal as you said. I've seen some people suggesting the halfway vector and view direction in their implementation but when I had a look at nvidias whitepaper that describes Fresnel reflection, the article describes it as you do, that you want the angle between the direction to the light and the surface normal. What exactly is the difference between the two?
So 2*theta == angleDiff( in, out ), and we also know that out == reflect( in, N ), and thus angleDiff( in, N ) == angleDiff( out, N ) == theta.
in N out \o|o/ \|/ (o is theta)So from in to out is 2*theta, and from in to N is theta and from N to out is theta. Or alternatively, dot(N,L) == dot(N,reflect(L,N) == cos theta.