Rongqing Song Zhongke Wu Mingquan Zhou Xuefeng Ao Free-form Sketching with Ball B-Splines. 583-592 2009 HCI (6) http://dx.doi.org/10.1007/978-3-642-02710-9_65 conf/hci/2009-6 db/conf/hci/hci2009-6.html#SongWZA09