Zsófia Ruttkay Han Noot Paul J. W. ten Hagen Emotion Disc and Emotion Squares: Tools to Explore the Facial Expression Space. 49-54 2003 22 Comput. Graph. Forum 1 http://diglib.eg.org/EG/CGF/volume22/issue1/cgf_645.htm db/journals/cgf/cgf22.html#RuttkayNH03