[1] Microsoft Res, Human Experience & Design Grp, Cambridge, England
[2] Purdue Univ, Sch Ind Engn, W Lafayette, IN 47907 USA
[3] Univ Bristol, Bristol Interact & Graph Grp, Bristol BS8 1TH, Avon, England
[4] UCL, UCLIC, London WC1E 6BT, England
[5] Univ Nottingham, Sch Comp, Nottingham NG7 2RD, England