Appel, Arthur (1968). "Some techniques for shading machine renderings of solids". Proceedings of the April 30--May 2, 1968, spring joint computer conference on - AFIPS '68 (Spring). p.37. doi:10.1145/1468075.1468082. S2CID207171023.
Appel, Arthur (1968). "Some techniques for shading machine renderings of solids". Proceedings of the April 30--May 2, 1968, spring joint computer conference on - AFIPS '68 (Spring). p.37. doi:10.1145/1468075.1468082. S2CID207171023.