ST Shanghai NSFC-National Natural Science Foundation China State Key Lab for Novel Software Technology, Nanjing University, China INESC-ID - Institute for Computer and Systems Engineering, Lisbon, Portugal CITI - Center of Informatics and Information Technology, FCT/UNL, Lisbon, Portugal LaSIGE - Large Scale Informatic Systems Laboratory, FCUL, Lisbon, Portugal In Cooperation with: SIGGRAPH | SIGCHI | SIGSOFT