1. Understanding image intensities
2. Horn, B. K.P. 1975.‘Psychology of Computer Vision’Edited by: Winston, P. H. p115New York: McGraw Hill.
3. Intensity, visible-surface, and volumetric representations
4. Mayhew, J. and Frisby, J. 1984.‘Artificial Intelligence: Tools, Techniques and Applications’Edited by: Kealey, T., O'Shea, T. and Eisenstadt, M. 301New York, N.Y. U.S.A.: Harper and Row.