【Real Time Rendering 2】
1、The light vector l is usually defined pointing in a direction opposite to the direction the light is traveling.
2、The surface irradiance is equal to the irradiance measured perpendicularly to l, times the cosine of the angle θi between l and n.
3、Scattering does not change the amount of light—it just causes it to change direction.
原文：https://www.cnblogs.com/tekkaman/p/8832858.html