Gabriel Moy Nathan Jones Curt Harkless Randall Potter Distortion Estimation Techniques in Solving Visual CAPTCHAs. 23-28 2004 CVPR (2) http://csdl.computer.org/comp/proceedings/cvpr/2004/2158/02/215820023abs.htm db/conf/cvpr/cvpr2004-2.html#MoyJHP04