Virtual Reality Scientific and Technological Challenges (1995) / Chapter Skim
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5 Position Tracking and Mapping
5 Position Tracking and Mapping
Pages 188-204
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From page 188... ...
body tracking for locomotion and visual displays; (4) body surface mapping for facial expression recognizers, virtual clothiers, and medical telerobots; and (5)
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From page 189... ...
With 1 kHz sampling rates for eye movement, display targets can be well chosen every 1/60th of a second. HEAD TRACKING Head movements can be as fast as 1000 deg/s in yaw, although more usual peak velocities are about 600 deg/s for yaw and 300 deg/s for pitch and roll (Foxlin, 1993~.
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From page 190... ...
Body motion can be considered to have similar measurement requirements to hand motion tracking. By constrast, for body surface mapping or real-environment sensing, the position tracker must be able to scan a volume or surface to yield a dense array of points.
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From page 191... ...
The latter could be employed, for example, to track hand position relative to the body or fingertip position relative to the hand. Then the goniometer may be viewed like a hand controller that is manipulated by the operator; the output is based on calculations from the goniometer's angles, and there is no concern as to how this maps to limb joint angles.
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From page 192... ...
For head tracking, a commercial example is the ADL-1 six degree tracking system by Shooting Star Technology. A related example is the BOOM viewer from Fake Space Labs, in which the visual display is not worn but supported on a pedestal through the BOOM linkage.
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From page 193... ...
Magnetic trackers do not suffer obscuration problems, although they are sensitive to environmental magnetic fields and ferromagnetic materials in the workspace. Multiple trackers can be employed to map whole-body motion and to increase the range of tracked motion to a small room (Badler et al., 1993~.
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From page 194... ...
Below we discuss some major approaches to position tracking and mapping according to the following categories: passive stereo vision systems, marker systems, structured light systems, laser radar systems, and laser interferometric systems. A more complete review of active range imaging sensors may be found in Best (1988~.
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From page 195... ...
A fundamental problem with the use of PSDs is reflections of IRED light from environmental surfaces that move the apparent centroid of the sensed light; the amount of reflected light is high, about 25 percent of the total. The result is that it is very difficult to get camera resolutions beyond 1 part in 4,000; for this reason, Northern Digital has abandoned the Watsmart in favor of the Optotrak, which employs multiple cameras with 2,048-element linear CCD arrays.
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From page 196... ...
This reduces the shadow effect over other laser spot scanners and yields a more compact system. One version of the synchronized scanner is a random access laser scanner, in which the first scanned mirror is a simple two-sided mirror,
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From page 197... ...
Because reflectance is measured, the graphical depiction was reported as being very realistic; during blackout one could hardly tell that a simulation was being used. A second version is a raster scanner that employs as the first scanned mirror a multifacet pyramidal mirror, which is rotated continuously at a high rate.
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From page 198... ...
The retroreflector reflects the beam back to the scanner, where a beam splitter directs the beam to a photodetector for interference fringe counting and to a PSD. Based on the PSD output, the outgoing beam is deflected to the center of the retroreflector for tracking.
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From page 199... ...
Commercial implementations for the VE market include the GP8-3D developed by Science Accessories, the Logitech 3D/6D Mouse, and the Mattel Power Glove. For point tracking at modest accuracies and speeds, ultrasonic trackers are a reasonable and very inexpensive alternative to magnetic sensors: the ranges are larger, and magnetic interference is not a problem.
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From page 200... ...
IC Sensors markets solid-state piezoresistive accelerometers, which employ a micromachined silicon mass suspended by multiple beams to a silicon frame. The GyroChip developed by Systron Donner employs a pair of micromachined tuning forks, which sense angular velocity through the Coriolis forced The GyroEngine developed by Gyration is a miniaturized spinning wheel gyroscope that is even smaller than the GyroChip.
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From page 201... ...
Inertial sensors are also useful in conjunction with other position tracking systems for lead prediction. In the high-end HMD system from CAE Electronics, the outputs of a fast optical head-tracker are combined with angular velocity measurements to predict future head orientation.
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From page 202... ...
Whatever sensory system is employed for limb tracking, there will be difficulties in identifying reliable fiducial points because of the softness of tissue and clothes. To infer joint angles, some calibration procedure must be applied to set up coordinate systems in limb segments; subsequent joint angle inferences will only approximate the true biomechanical angles because of less than ideal joints and measurements.
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From page 203... ...
These drawbacks can be partially ameliorated by using multiple camera placement or target placement. The ability of passive stereo vision systems to process arbitrary environments is a long-range goal of the computer vision community; when eventually developed, stereo vision would represent an extremely attractive method for position tracking and mapping.
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From page 204... ...
The CCD imaging systems such as ISCAN have a reasonably linear response, but the sampling rates are too low and range is limited to about 20 deg. The infrared reflection devices have the bandwidth, but are linear only in a small range; calibration to overcome their nonlinear responses is difficult.
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