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


Overview

IntroductionRelated WorkMethodologyExperiment ResultsConclusion

Part I

Introduction



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)

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

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



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



Methodology

Relaxed Consistency Control ModelGradual SynchronizationTrajectory-Preserving Synchronization



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 |)()(|



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



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

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 A (avatar) and the server



Client-Client Trajectory-Preserving Synchronization

Server and client B (observer)



Arbitrary Moment Trajectory-Preserving Synchronization

Client A (avatar) and the server



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



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



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

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



Experiment I (2/4)

Dead Reckoning



Experiment I (3/4)

New Method



Experiment I (4/4)

Focus on comparing several motion changes

Dead Reckoning New Method



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)

Screen shots of our prototype for collaborative visualization



Experiment II (3/5)

Client A and the server

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Experiment II (4/5)

The server and client B

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Experiment II (5/5)

Client A and client B

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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




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Part V

Conclusion



Conclusion (1/2)

Propose a trajectory-preserving synchronization method to support collaborative visualization

Handle unpredictable user changes Handle collision detection problem



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: [email protected]

Frederick Li: [email protected]

Rynson Lau: [email protected]

Thank you!

Questions and Answers

Contacts

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

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



Appendix Clock Synchronization

Two common approaches• Backward correction• Forward correction



Appendix Dead Reckoning

Client A and client B



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




Client A and the server




Server and client B

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

Documents

li rynson

li frederick

introduction slide

synchronization slide

methodology slide

related work slide

visualization dataset

time t