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*. **. City University of Hong Kong. A Trajectory-Preserving Synchronization Method for Collaborative Visualization. Lewis W.F. Li* Frederick W.B. Li** Rynson W.H. Lau**. Overview. Introduction Related Work Methodology Experiment Results Conclusion. Part I. Introduction.

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A trajectory preserving synchronization method for collaborative visualization

*

**

City Universityof Hong Kong

A Trajectory-Preserving Synchronization Method for Collaborative Visualization

Lewis W.F. Li* Frederick W.B. Li** Rynson W.H. Lau**


Overview
Overview

  • Introduction

  • Related Work

  • Methodology

  • Experiment Results

  • Conclusion


Part i

Part I

Introduction


Introduction 1 2
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


Introduction 2 2
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

    • User’s ability in performing desirable collaborative tasks will be affected, due to the induced view discrepancy among remote users


Objectives of this work
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

Part II

Related Work


Related work
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

Part III

Methodology


Methodology
Methodology

  • Relaxed Consistency Control Model

  • Gradual Synchronization

  • Trajectory-Preserving Synchronization


Relaxed consistency control model
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 si(t) and sj(t), the state discrepancy D of the object between the two sites during any time period Ta and Tb should be smaller than an application specific tolerance, ξ. Hence,


Gradual synchronization 1 2 acm multimedia 2004
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


Gradual synchronization 2 2 acm multimedia 2004
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


Trajectory preserving synchronization
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


Client server trajectory preserving synchronization
Client-Server Trajectory-Preserving Synchronization

  • Client A (avatar) and the server


Client client trajectory preserving synchronization
Client-ClientTrajectory-Preserving Synchronization

  • Server and client B (observer)


Arbitrary moment trajectory preserving synchronization
Arbitrary MomentTrajectory-Preserving Synchronization

  • Client A (avatar) and the server


Handling object collisions trajectory preserving synchronization
Handling Object CollisionsTrajectory-Preserving Synchronization

  • Interpret the collision response as motion commands

  • Resolve inconsistent collision problem into two sets of simpler problems


Handling object collision trajectory preserving synchronization
Handling Object CollisionTrajectory-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

Part IV

Experiment Results


Experiment i 1 4
Experiment I (1/4)

  • Demonstrate user’s navigation at an avatar, the serverand 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

    • http://www.cs.cityu.edu.hk/~kwfli/vis2006/vis.html


Experiment i 2 4
Experiment I (2/4)

  • Dead Reckoning


Experiment i 3 4
Experiment I (3/4)

  • New Method


Experiment i 4 4
Experiment I (4/4)

  • Focus on comparing several motion changes

Dead Reckoning

New Method


Experiment ii 1 5
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


Experiment ii 2 5
Experiment II (2/5)

  • Screen shots of our prototype for collaborative visualization


Experiment ii 3 5
Experiment II (3/5)

  • Client A and the server


Experiment ii 4 5
Experiment II (4/5)

  • The server and client B


Experiment ii 5 5
Experiment II (5/5)

  • Client A and client B


Experiment iii 1 3
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




Part v

Part V

Conclusion


Conclusion 1 2
Conclusion (1/2)

  • Propose a trajectory-preserving synchronization method to support collaborative visualization

  • Handle unpredictable user changes

  • Handle collision detection problem


Conclusion 2 2
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


Thank you

Thank you!

Contacts

Lewis Li: kwfli@cs.cityu.edu.hk

Frederick Li: Frederick.Li@durham.ac.uk

Rynson Lau: Rynson.Lau@durham.ac.uk

Questions and Answers

http://www.cs.cityu.edu.hk/~kwfli/vis2006/


Appendix clock synchronization
Appendix Clock Synchronization

  • Two common approaches

    • Backward correction

    • Forward correction


Appendix dead reckoning
Appendix Dead Reckoning

  • Client A and client B


Appendix gradual synchronization
AppendixGradual Synchronization

  • For each motion

    • Motion timers Ts and Tc are maintained at the server and client simulator, respectively

    • Assume position updates in every Δt

    • Estimate the round-trip time, Test

    • Adjust every Δt in client for Tc based on Test

  • Synchronized when Tc is the same as Ts


Appendix gradual synchronization1
AppendixGradual Synchronization

  • Client A and the server


Appendix gradual synchronization2
AppendixGradual Synchronization

  • Server and client B