A Trajectory-Preserving Synchronization Method for Collaborative Visualization Lewis W.F. Li* Frederick W.B. Li** Rynson W.H. Lau** City University of Hong Kong * **

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 2 Overview Introduction Related Work Methodology Experiment Results Conclusion

Part I Introduction

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 4 Introduction (1/2) Collaborative visualization Geographically separated users to be connected over the network to visualize and manipulate dataset for problem solving Examples Fluid dynamics visualization Volume visualization Medical data visualization

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 5 Introduction (2/2) Characteristics of collaborative visualization User is allowed to interact with the visualization dataset continuously over time Dataset updates should subsequently be distributed to remote users over the network Problems Due to network latency, each remote user may receive updates with a different amount of delay Users ability in performing desirable collaborative tasks will be affected, due to the induced view discrepancy among remote users

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 6 Objectives of This Work Provide a more synchronized view of visualization changes to collaborating users Develop procedures to correct motion trajectories of dynamic objects Prevent discontinuous motion Address false positive and false negative collision detection problems

Part II Related Work

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 8 Related Work Traditional Applications Easy to work well provided that state updates are received by remote sites in a correct order Time gap between two consecutive updates is typically large as compared to network latency Collaborative Applications State updates occurs continuously Unfortunately, updates need to present to remote users timely or at least within a very short time Existing solutions:- User or system side adaptation - Local Lag mechanism

Part III Methodology

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 10 Methodology Relaxed Consistency Control Model Gradual Synchronization Trajectory-Preserving Synchronization

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 11 Relaxed Consistency Control Model Observation: Users generally pay more attention on the trajectory of dynamic objects rather than their individual states Given that the states of a replicated object at two remote sites at time t are s i (t) and s j (t), the state discrepancy D of the object between the two sites during any time period T a and T b should be smaller than an application specific tolerance, ξ. Hence,

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 12 Gradual Synchronization (1/2) ACM Multimedia 2004 Trade accuracy of individual state of a dynamic object for preserving their state trajectory Run a reference simulator on the server for each object in a client-server environment Note:1st order simulator: 2nd order simulator: When a client receive or initiate a new motion update of an object, the client will align the motion of the local object against its reference simulator

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 13 Gradual Synchronization (2/2) ACM Multimedia 2004 Contribution: This method effectively reduces the latency of a client to obtain a state update from a double round-trip time delay to a single one Limitation: High discrepancy occurs between the period when an interaction has just occurred and before the update message reaches a remote client Apparently, such discrepancy appears shortly for each time, but would become serious if interactions occur frequently

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 14 Trajectory-Preserving Synchronization Extends from our gradual synchronization method Consider the characteristics of spatial changes and interactions of dynamic objects are affected by network latency A set of procedures are developed to correct motion trajectory of dynamic objects Handle false positive and false negative collision detection problem

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 15 Client-Server Trajectory-Preserving Synchronization Client A (avatar) and the server

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 16 Client-Client Trajectory-Preserving Synchronization Server and client B (observer)

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 17 Arbitrary Moment Trajectory-Preserving Synchronization Client A (avatar) and the server

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 18 Handling Object Collisions Trajectory-Preserving Synchronization Interpret the collision response as motion commands Resolve inconsistent collision problem into two sets of simpler problems

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 19 Handling Object Collision Trajectory-Preserving Synchronization False negative collisions Collisions detected in the avatar but not in the server (case (b)) Inhabit the avatar to perform collision detection until motion remediation process has finished False positive collisions Collisions detected in the server but not in the observer (case (f)) Inhabit the observer to perform collision detection until motion remediation process has finished

Part IV Experiment Results

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 21 Experiment I (1/4) Demonstrate users navigation at an avatar, the server and an observer Compare the performance of different methods Dead Reckoning Original method New Method Here, focus on comparing dead reckoning and the new method only Full and other Demos

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 22 Experiment I (2/4) Dead Reckoning

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 23 Experiment I (3/4) New Method

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 24 Experiment I (4/4) Focus on comparing several motion changes Dead ReckoningNew Method

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 25 Experiment II (1/5) Focus on the motion of selected object (the green ball) in the virtual environment Compare the position discrepancy in between Client A and the server The server and client B Client A and client B

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 26 Experiment II (2/5) Screen shots of our prototype for collaborative visualization

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 27 Experiment II (3/5) Client A and the server

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 28 Experiment II (4/5) The server and client B

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 29 Experiment II (5/5) Client A and client B

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 30 Experiment III (1/3) Focus on the accuracy of the new method in handling object collisions Compare the position discrepancy between server and four users with different network latencies

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 31 Experiment III (2/3)

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 32 Experiment III (3/3)

Part V Conclusion

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 34 Conclusion (1/2) Propose a trajectory-preserving synchronization method to support collaborative visualization Handle unpredictable user changes Handle collision detection problem

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 35 Conclusion (2/2) Limitations Assume using connection-oriented network Message loss is not considered Future Works Consider difference types of network Support haptic interface and rendering

Lewis Li: Frederick Li: Rynson Lau: Thank you! Questions and Answers Contacts

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 37 Appendix Clock Synchronization Two common approaches Backward correction Forward correction

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 38 Appendix Dead Reckoning Client A and client B

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 39 Appendix Gradual Synchronization For each motion Motion timers T s and T c are maintained at the server and client simulator, respectively Assume position updates in every Δt Estimate the round-trip time, T est Adjust every Δt in client for T c based on T est Synchronized when T c is the same as T s

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 40 Appendix Gradual Synchronization Client A and the server

Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau City University of Hong Kong 41 Appendix Gradual Synchronization Server and client B