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Interaction Error based Viewpoint Estimation for Continuous Parallax
Error Correction on Interactive Screens
Bastian Migge (ETH Zurich), Andreas Kunz (ETH Zurich), Tim Schmidt (PARC)
Content
n Introduction/Motivation: Interactive surfaces n Problem: Parallax error n Calibration Techniques n Model based viewpoint estimation n Contribution: Empirical observation model for MPC
– User study – Discrete Observation Model
n Conclusion and outlook
Bastian Migge © 06/2011 IWF, ETH Zurich 2
Interactive Surfaces
n Pros – Intuitive operation, easy to learn – Innate interaction (input and output combined)
n Requisition – Good alignment between image plane and
tracking system is essential
3
Digital Desk (Crion table, ETH Zurich)
Single display groupware (Collaboard, ETH Zurich)
Ticketing machine (New York Underground)
Bastian Migge © 06/2011 IWF, ETH Zurich
Parallax Error
4
n Vz : Distance between image and interaction plane
n Vx : Resulting parallax distortion in x-dimension
n ax,z: Distance from user to interaction point in x,z
z
xzx a
aVV ⋅=Parallax Error
Interaction plane and image pane with offset1 (analogous for y)
1 Source: Migge,Kunz: User Model for Predictive Calibration Control on Interactive Screens, IEEE CW 2010
Image plane
Interaction plane4
Vx
User A
23
1
ax
az
Vz
z
xy
User B
5
Bastian Migge © 06/2011 IWF, ETH Zurich
Static Calibration
n Initially to correct geometric distortion n A-priori setup n Is biased by user characteristics
(height, arm length) n Depends on a single viewpoint
n Can not deal with user’s motion n Can not handle multiple users n Can not overcome the parallax error
stemming from changing viewpoints (VP)
5
Static Calibration Process
Bastian Migge © 06/2011 IWF, ETH Zurich
Continuous Calibration with Viewpoint Estimation
n Method1: – Remove sensors - Estimate users viewpoint – Add filter – model the users position and movement – Set the correction for the next (predicted position) interaction – Consider effect of correction actions (user disturbance)
à Model based Predictive Parallax Error Correction under Uncertainty (POMDP)
n Pro – No additional hardware
n Cons – Models needed (a priori) – Interaction on screen needed (at runtime)
6 Bastian Migge © 06/2011 IWF, ETH Zurich
1[Migge, Kunz, Schmidt: POMDP Models for Continuous Calibration of Interactive Surfaces, AAAI SS 2010]
Model based Predictive Parallax Error Correction under Uncertainty
n Parallax correction (-Vx): shift pointing device information n Information sources
– Target can be assumed to be next to Touch point – Viewpoint is not directly observable – VP can be inferred from GUI interactions (ß Vx, Vz)
n Benefits – Correction for all touch points – Increases the pointing accuracy
7
Image plane
Interaction plane
Vx
User's viewpoint
Touch point
Target
xz
Vz
Bastian Migge © 06/2011 IWF, ETH Zurich
Contribution: Viewpoint estimation from interaction error
8
n Contribution: A model of the user’s interaction behavior to estimate the viewpoint of the user
n Static Characteristics: – Display offset Iz
n Dynamic Values: – Relative User position VPx – Relative Interaction Error Ix – VPx ~ Ix
n Observation Model: – Pr(Ix = err |VPx = vp)
Image plane
Interaction plane
User
Target
VPzz
xy
InteractionIz
VPx
Ix
Dependency between relative viewpoint and relative pointing error
Bastian Migge © 06/2011 IWF, ETH Zurich
User Study – Measurement Setup
n Participants: 13(4) (fe)male, avg. age 31.05, avg. height 179mm n Interactive System
– 50“ interactive surface (plasma screen with SMART tracking) – Pixel pitch 0.858 x 0.808 mm, Resolution 1280 x 768 px – On-screen target: 15 x 15 px (13 x 12 mm) – Parallax offset ≥ 13 mm
à Measures the Interaction error (in 2D display coordinates)
9
(b) Assembly of the tracking system on top of the display (a) Schematic measurement setup with test person
10
6.4
652.6
[mm]
Touch sensitive film
Glas plane
Displaybody
Pen
785
mm
1215
mm
Bastian Migge © 06/2011 IWF, ETH Zurich
User Study - Task
n Random clicker – User must click a single button of a full screen application – Button moves after each interaction – Python, QT, X11, Linux system
à Provides global interaction position on display
10 Bastian Migge © 06/2011 IWF, ETH Zurich
Test application
User Study – Tracking Setup
n Tracking System: Qualisys Motion Tracking System – 4 Oqus 300 Camera – IR based out-side in system – Measures Position of reflective marker (passive) – 50 Hz; σ = 0.87 mm
à Measures the 3D Viewpoint and transforms 2D display coordinates to global 3D coordinate system
11
(b) Passive marker on interactive screen (a) Head tracking module 1http://www.qualisys.com
1
Bastian Migge © 06/2011 IWF, ETH Zurich
Vertical measurements
User Study – Results 1/2 Pointing Error
12
n Pointing error n Deviation from
assumed hit point (haptic error)
Boxplots Horizontal measurements
−40 −20 0 20 40 60
0.00
0.05
0.10
0.15
0.20
Error [mm]
Inte
ract
ions
[%]
Pointing ErrorDeviation from assumed hit
−40 −20 0 20 40
0.00
0.05
0.10
0.15
0.20
Error [mm]
Inte
ract
ions
[%]
Pointing ErrorDeviation from assumed hit
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−40
−20
0
20
40
Nor
mal
ized
devi
atio
nPointing error Deviation from assumed hit point
Image plane
Interaction plane4
User
z
xytarget
actual hit point
assumed viewpointassumed hit point
Deviation from assumed hit
Bastian Migge © 06/2011 IWF, ETH Zurich
Pointing error and Haptic error
User Study – Results 2/2 Correlation
13
n Normalized viewpoint interaction error n 5500 interactions
Horizontal measurements Vertical measurements
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−0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6
−1.0
−0.5
0.0
0.5
Pearsons correlation coefficient ( 0.806 )
Viewpoint
Inte
ract
ion
erro
r
Least square fitLowess fit
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−0.6 −0.4 −0.2 0.0 0.2 0.4
−1.0
−0.5
0.0
0.5
Pearsons correlation coefficient ( 0.011 )
Viewpoint
Inte
ract
ion
erro
r
Least square fitLowess fit
Bastian Migge © 06/2011 IWF, ETH Zurich
Data interpretation: Discrete Observation Model
n Measurement Data: Correlation between Interaction error and Viewpoint
n Allows the controller to infer the viewpoint from the interaction error
n Set up the Discrete Model – Observation space O – State space S – P(O|S=s) as probability
distribution
14
inferred viewpoint
image plane
interaction plane
applied correction
actual viewpoint
ohit orightoleft
targetcenter
Observation (O) model for a given viewpoint and target (S) as discrete probability distribution P(O|S)
Bastian Migge © 06/2011 IWF, ETH Zurich
Discrete Observation Model Concrete Example
n Continuous observation space, Discrete states – Click error significantly differs for different viewpoints
n Discrete observation space
15
Horizontal click error for 5 discrete viewpoint states
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[ −1 , −0.55 ) [ −0.15 , 0.15 ) [ 0.55 , 1 )
−1.0
−0.5
0.0
0.5
Viewpoint (State)
Clic
kerro
r (O
bser
vatio
n)
Observation model ( #states= 5 #observations= 5 )Relative click error
[ −0.15 , 0.15 ) mean −0.286 var 0.065
Den
sity
−2 −1 0 1 2
0.0
0.5
1.0
1.5
P(O|s=“left”)
Observation model ( #states= 5 #observations= 5 )Relative click error
[ 0.15 , 0.55 ) mean 0.202 var 0.036
Den
sity
−2 −1 0 1 2
0.0
0.5
1.0
1.5
P(O|s=“right”)
Bastian Migge © 06/2011 IWF, ETH Zurich
Conclusion and Outlook
n Focus was to model the correlation between viewpoint position and interaction error on the screen – Viewpoint can be distinguished
n model the user’s behavior exists
n Complete empirical model – The correlation between pointing accuracy and target size – Effect of correction actions onto the user
n Develop prototype – Prove of concept – Compare controller to classical correction methods and
tracking based correction
16 Bastian Migge © 06/2011 IWF, ETH Zurich
Continuous Calibration with Viewpoint Tracking
n Methods: – Visual marker tracking or – Video image feature extraction
n Pro – High quality viewpoint position
n Cons – Marker at tracked object not applicable – Additional camera hardware – Initial calibration pose needed – Camera lens opening angle critical
18
3D Skelton Tracking with Microsoft Kinect and OpenNI
1http://luka.tnode.com/gallery/projects-and-work/kinect-hackery/skeleton-tracking
Bastian Migge © 06/2011 IWF, ETH Zurich
Model based Predictive Parallax Error Correction under Uncertainty – in detail
n Control: Set the interaction correction parameter Vx, Vy
n Uncertainty: Interaction error on screen (Observations) does indicate the viewpoint under uncertainty
n Predictive: Model the User’s behavior predicting his movement 1
n Method: Partial Observable Markow Decision Process (POMDP) – User’s behavior modeled as time discrete Markow Chain
(state space) – Add control à Markow Decision Process (MDP) – Uncertain measurements indicate states (POMDP)
n Needs a model of the user’s interaction behavior
19 Bastian Migge © 06/2011 IWF, ETH Zurich
1[Migge, Kunz :User Model for Predictive Calibration Control on Interactive Screens, IEEE CW 2010]