Rich Gossweiler, Maryam Kamvar, Shumeet Baluja. What s up CAPTCHA?: a CAPTCHA based on image orientation. In Juan Quemada, Gonzalo León, Yoëlle S. Maarek, Wolfgang Nejdl, editors, Proceedings of the 18th International Conference on World Wide Web, WWW 2009, Madrid, Spain, April 20-24, 2009. pages 841-850, ACM, 2009. [doi]
Abstract is missing.