1. Pervasive Computing Division, Tsinghua National Laboratory for Information Science, Technology, Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China and Department of Information Art and Design, Tsinghua University, Beijing, 100084, China
2. Pervasive Computing Division, Tsinghua National Laboratory for Information Science and Technology, Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China
3. Pervasive Computing Division, Tsinghua National Laboratory for Information Science and Technology and Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China and Department of Information Art and Design, Tsinghua University, Beijing, 100084, China
4. Pervasive Computing Division, Tsinghua National Laboratory for Information Science and Technology and Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China
5. Pervasive Computing Division, Tsinghua National Laboratory for Information Science and Technology and Department of Computer Science and Technology, Tsinghua University, Beijing, 100084, China and Department of Computer Technology, Qinghai University, Xining, 810016, China
6. Pervasive Computing Division, Tsinghua National Laboratory for Information Science and Technology and Department of Information Art and Design, Tsinghua University, Beijing, 100084, China