a trajectory-preserving synchronization method for collaborative visualization lewis w.f. li*...

A Trajectory-Preserving Synchronization Method for Collaborative Visualization

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

City Universityof Hong Kong

* **

2Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Overview

IntroductionRelated WorkMethodologyExperiment ResultsConclusion

Part I

Introduction

4Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

5Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

6Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

8Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

10Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Methodology

Relaxed Consistency Control ModelGradual SynchronizationTrajectory-Preserving Synchronization

11Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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,

b

a

T

T ji dttstsD |)()(|

12Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

Vtppnew

2

2

1AtVtppnew

AtVVnew

13Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

14Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

15Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Client-Server Trajectory-Preserving Synchronization

Client A (avatar) and the server

16Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Client-Client Trajectory-Preserving Synchronization

Server and client B (observer)

17Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Arbitrary Moment Trajectory-Preserving Synchronization

Client A (avatar) and the server

18Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Handling Object Collisions Trajectory-Preserving Synchronization

Interpret the collision response as motion commands

Resolve inconsistent collision problem into two sets of simpler problems

Client A and server

Case Client A Server (a) × × (b) O × (c) × O (d) O O

Server and Client B

Case Server Client B (e) × × (f) × O (g) O × (h) O O

19Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

21Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

22Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment I (2/4)

Dead Reckoning

23Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment I (3/4)

New Method

24Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment I (4/4)

Focus on comparing several motion changes

Dead Reckoning New Method

25Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

26Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment II (2/5)

Screen shots of our prototype for collaborative visualization

27Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment II (3/5)

Client A and the server

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New Method

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Dead Reckoning

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28Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment II (4/5)

The server and client B

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29Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment II (5/5)

Client A and client B

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30Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

Latency (ms) Category

Mean Max. Min

LAN (within a department) 5 7 0

Intranet (within a university) 40 57 32

Overseas 1 (modelling two nearby countries) 160 186 132

Overseas 2 (modelling two distant countries) 325 537 294

31Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment III (2/3)

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32Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Experiment III (3/3)

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Part V

Conclusion

34Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Conclusion (1/2)

Propose a trajectory-preserving synchronization method to support collaborative visualization

Handle unpredictable user changes Handle collision detection problem

35Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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: kwfli@cs.cityu.edu.hk

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

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

Thank you!

Questions and Answers

Contacts

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

37Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Appendix Clock Synchronization

Two common approaches• Backward correction• Forward correction

38Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

Appendix Dead Reckoning

Client A and client B

39Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

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

40Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

AppendixGradual Synchronization

Client A and the server

41Lewis W.F. Li Frederick W.B. Li Rynson W.H. Lau

City Universityof Hong Kong

AppendixGradual Synchronization

Server and client B