http://blog.selfshadow.com/publications/s2014-shading-course/dupuy/s2014_pbs_leadr_slides.pdf

http://ict.usc.edu/publications.php

http://www.geeks3d.com/20140929/test-particle-rendering-point-sprites-vs-geometry-instancing-based-billboards/

http://www.handprint.com/LS/CVS/color.html

http://graphics.cs.williams.edu/courses/cs371/f14/reading/implicit.pdf