Punctual Light

http://www.klayge.org/wiki/index.php/Physically-based_BRDF#Punctual_Light_Sources

These are classic point, directional, and spot lights in compute games. These local light source can be abstracted to a concept named "punctual light sources".

수식으로 잘 설명되어 있다

https://seblagarde.wordpress.com/2012/01/08/pi-or-not-to-pi-in-game-lighting-equation/

위에서는 아래 링크를 추천한다

http://renderwonk.com/publications/s2010-shading-course/

course 전체를 추천했나 싶어서 슬펐는데;

“Background: Physically-Based Shading 의 72 페이지부터 말로 설명되어 있다. (Derivation in the course notes 흑; 맨 처음 링크와 같은 내용인데 밑에 설명이 더길다)