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Tracking Systems in VR

Tracking Systems in VR. Performance Accuracy Resolution (precision) Jitter (zero mean) Drift (non-zero mean) Lag Update Rate. Environment Interference Mass, Inertia and Encumbrance (wires) Space (Range) Number of tracked entities Cost Monetary Setup Space. Evaluation Criteria.

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Tracking Systems in VR

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  1. Tracking Systems in VR

  2. Performance Accuracy Resolution (precision) Jitter (zero mean) Drift (non-zero mean) Lag Update Rate Environment Interference Mass, Inertia and Encumbrance (wires) Space (Range) Number of tracked entities Cost Monetary Setup Space Evaluation Criteria Which of these are most important?

  3. Tracking Technologies • Many different tracking approaches and commercial systems available • Goals: • Understand how they work • Understand tradeoffs • Know when to use which

  4. Mechanical Linkage • Measurement of position and orientation via the pose of a set of interconnected rigid bodies • Measure change at joints • Links impose constraints and provide support • Many possible configurations

  5. Mechanical Linkage (Boom) • Earliest tracking system • Used by Ivan Sutherland for the Ultimate Display • Rigid jointed chain-like structure • Each joint provides a transformation relative to parent • Like a scene graph • Each joint has 2-3DOF • Position typically fixed relative to parent joint • Sensors at joints measure the angles • Concatenate translates and rotates to find position and orientation of the distal joint relative to base.

  6. Mechanical Linkage (Exoskeleton) • Attached to articulating body • No range limit • Does not provide position or orientation relative to fixed reference. • Still need to track at least one joint with another system.

  7. Mechanical Linkage Tracking Potential applications severely limited, but if you can use it, great!

  8. Electromagnetic Trackers • One of the earliest tracking systems to be developed • Cheap/Easy to emit a magnetic field • Cheap/Easy to sense a magnetic field • Inexpensive computation • Single circuit controls both emitter and sensors • Emitter • Alternating current through coil stimulates alternating magnetic field • Sensor • Alternating magnetic field through coil stimulates alternating current

  9. Basic Principles of EM Trackers • 1 source with 3 orthonormal emitter coils • N receivers with 3 orthonormal sensor coils each • Pulse the emitter coils in succession • For each pulse, each field sensor measures the strength of the signal (9 total measurements) • Each pulse gives • One column of a 3 x 3 measurement matrix • Measurement matrix linearly related to emitter-sensor transform • Sensor Position • Sensor Orientation • Ambiguity in math means you must choose a hemisphere • See patent for matrix math to decompose measurement matrix

  10. EM Tracker Performance Great performance, and often the only option that avoids occlusion problems. Biggest problem is working range. Excellent at very close ranges, but unusable at long distances (far-field)

  11. Acoustic/Ultrasonic Tracking • Time of Flight Tracking • Emitters • Multiple emitters • In succession, emit sound (record time) • Receiver • Report time of receiving sound • Frequency tuned • Calculate time-of-flight (1000 feet/sec) • Use ultrasonic (high) frequencies • Similar: • EM tracking • Radar/sonar • Phase Coherence tracking • Measure phase offset of sound at reference position relative to emitter

  12. Ultrasonic Tracking System Setup How much data does 1 transmitter provide? How much data do 2 transmitters provide? How much data do 3 transmitters provide? Stationary Origin (receivers) Tracker (transmitters) distance1 distance2 distance3

  13. Acoustic Tracking Performance Really only appropriate for position tracking. Similar to optical but with somewhat less occlusion at the expense of accuracy.

  14. Sourceless Tracking • Leverage “built in” sources and phenomena • Gravity • Earth’s magnetic field • Gyroscopic effect • No external reference frame • Position and orientation are relative to a starting point • May suffer from accumulated error

  15. MARG Tracker • 3-axis Magnetometer • Measures local Magnetic field • 3-axis Gyroscope • Measures Angular Rate • 3-axis Accelerometer • Measures Gravity + acceleration • Accelerometer and Magnetometer yield 3DOF absolute orientation • Gravity + North • Only valid when still (noisy) • Gyroscope predicts new orientation at high rate • Corrected by noisy Acc+Mag • Somewhat possible to integrate accelerometers to get position

  16. MARG Tracker Performance Excellent, but need to be combined with position tracker for most VR applications, e.g. Wiimote, PSMove

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