Copyright, 1999 © Valerie A. Summers Calibration for Augmented Reality Experimental Testbeds Valerie A. Summers, Kellogg S. Booth, Tom Calvert, Evan Graham, Christine L. MacKenzie

Overview The Problem Physical Configuration Workspace Calibration Point of View Calibration Physical Object Calibration Discussion and Previous Work

The Problem Accurate registration between real and virtual objects is hard: “Current AR systems cannot convincingly meet this requirement. Typically a virtual object appears to swim about as the user moves…’’ (State, 1996) We do not fully understand how calibration errors affect human performance Calibration of a kinematic testbed has requirements not found in other applications

Benefits of Kinematic Experiments Designers of AR systems can predict task error based on varying levels of calibration accuracy Increased knowledge of basic human interaction in augmented environments

Goals Identify calibration requirements of an experimental AR system Merge these requirements with traditional calibration requirements Provide techniques which satisfy both sets of requirements simultaneously

Calibration Requirements Traditional Requirements “Ideally, the calibration methods should be statistically robust, there should be a variety of approaches for different circumstances, and metrology equipment should be sufficiently accurate, convenient to use, and not too expensive”. (Hollerbach and Wampler, 1996) Additional Requirements of Experimental Subsystems: independent (not rely on each other) subject-specific (account for individual differences) avoid residual cues (to prevent subjects using them in unanticipated ways)

Physical Configuration Stereo images drawn on monitor reflect in mirror appear between desktop and mirror

Calibration Components workspace point of view physical objects

Workspace Calibration Markers are aligned with virtual crosses Exactly one position in 3-space eliminates “swim” do NOT need stereo to calibrate

Workspace Calibration Evaluation Maximum variation for any marker (mm) 1.49 (X), 1.02 (Y), 1.34 (Z) Errors in workspace calibration affect placement of virtual objects relative to workspace do not affect relative distance and location of virtual objects do not affect placement of augmented objects (errors cancel)

Point of View Calibration Subject placing eye calibration bars

POV -- Parameter Independence Interpupillary distance correctly computed for each subject Does not assume this distance is evenly divided by nose piece Vertical placement of POV need not be be center of glasses Glasses need not sit levelly on head

Comparison of Calibration and Pupillometer Readings Interpupillary distances (IPDs) measured for 3 subjects Optician’s pupillometer measured IPDs over focal lengths ranging from 35 cm to infinity IPDs obtained via calibration technique were within pupillometer range

Effect of Eye Calibration on Perception Mean Error (mm) X Y Z Max Error (mm) X Y Z 3 1 4

Physical Object Calibration Markers placed anywhere on object Place object in frame so XYZ orientations match

Object Calibration -- Sources of Error tracking error (0.3 mm) physical measurement of object with ruler (< 1 mm) positioning errors (negligible)

Combined Calibrations Stereo image (2 viewpoints) of a physical block augmented with a virtual wire frame

Workspace, point of view and objects can be calibrated in any order Can re-calibrate any component without affecting others Transformations combined off-line for performance prior to real-time execution Coordinate System Transformations

Evaluation robust flexible accurate convenient to use affordable … Plus... independent subject-specific avoid extraneous cues

Previous Work Experimental Virtual Environment Testbeds VRMAT (Pouprev et al., 1997) VEPAB (Lampton et al., 1994) Augmented Reality Calibration magnetic tracker calibration (Ghazisaedy et al., 1995) optical see-through HMDs (Azuma and Bishop, 1994) hybrid tracking systems (State et al., 1996) monitor based augmented reality(Tuceryan et al., 1995)

Conclusions Calibration of experimental systems have requirements beyond those of other applications (independent, subject-specific, eliminate extraneous cues) Can achieve experimental and traditional calibration requirements simultaneously Independence requirement benefits non- experimental systems These techniques are in production use by kinesiology researchers

Financial Support Natural Sciences and Engineering Research Council of Canada Media and Graphics Interdisciplinary Centre at UBC Simon Fraser University BC Advanced Systems Institute

Physical Configuration

Workspace Calibration

Point of View Calibration

Physical Object Calibration Physical objects are placed in the corner of the frame for calibration