Reference: S. K. Lee, W. Buxton, and K. C. Smith, "A multi-touch three dimensional touch-sensitive tablet," in Proceedings of the SIGCHI conference on Human factors in computing systems San Francisco, California, United States: ACM Press, 1985. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.64.1841&rep=rep1&type=pdf