Kirk, David and Arvo, James (1991) Unbiased
sampling techniques for image synthesis; ISBN 0-89791-436-8;
SIGGRAPH ’91 Proceedings of the 18th annual conference on Computer
graphics and interactive techniques; ACM: New York, NY; ACM SIGGRAPH
Computer Graphics; Vol. 25; No. 4; 153-156; 10.1145/127719.122735