【使用RampTexture实现BRDF效果】
BRDF stands for bidirectional reflectance distribution function. While that is a mouthful, it simply means the way in which light is reflected off an opaque surface from both the view direction and the light direction.
BRDF指光从view-direction、light-direction2个方向散射出去。
首先，需要创建一张适用于BRDF的RampTexture。
使用如下Shader来完成模拟：
效果图如下：
【How it works】
The following image shows the dot product of the view direction and the surface normal.
Since the diffLight calculation and the rimLight calculation both produce a linear range of values from 0 to 1, we can use both the ranges to pick different areas of the ramp texture.
