# Why need a $h$ parameter in the Geometry Function of a BRDF

Geometry Function in the specular BRDF term is defined as $$G(l, v, h)$$. Why need a $$h$$ since $$h=\frac{l+v}2$$.

$$f_{spec}(l,v) = \frac{F(h,v)G_2(l,v,h)D(h)}{4|n\cdot l||n\cdot v|}$$ from "Real Time Rendering 4th" Chap 9.8 BRDF Models for Surface Reflection.

• @trichoplax I've updated the complete equation. Thanks! – zoujyjs Apr 19 at 2:25