Daniel Wigdor Chia Shen Clifton Forlines Ravin Balakrishnan Perception of elementary graphical elements in tabletop and multi-surface environments. 473-482 2007 conf/chi/2007 CHI http://doi.acm.org/10.1145/1240624.1240701 db/conf/chi/chi2007.html#WigdorSFB07