Tracking Systems in VR

Which of these are most important? Evaluation Criteria 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 Which of these are most important?

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

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

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.

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.

Mechanical Linkage Tracking Variable Quality (Relative to alternatives) Accuracy Excellent (<1 mm, <1 degree) Resolution Excellent (<.1mm, <.1degree) Jitter Excellent (zero) Drift Lag Excellent (<10ms) Update Rate Excellent (>100hz) Interference Poor (Environment obstructions) Encumbrance Poor (many attachments, inertia) Space Poor (Booms severely limited) Tracked Entities Poor (typically 1/system) Calibration Excellent (factory) Cost Okay ($$$-$$$$) Potential applications severely limited, but if you can use it, great!

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

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

EM Tracker Performance Variable Quality (Relative to alternatives) Accuracy Okay ( 1 cm, 1 degree) Resolution Good (1 mm, .1 degree) Jitter Very good Drift Excellent (none) Lag Excellent (<10ms) Update Rate Excellent (>100hz) Interference Poor (magnetic distortions) Encumbrance Okay (wires, wireless possible) Space Poor (practically < 2 meters from source) Tracked Entities Good (>> $$$ for more) Calibration Good (Out of the box, but calibration for higher accuracy) Cost Wide range ($$$-$$$$$) 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)

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

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

Acoustic Tracking Performance Variable Quality (Relative to alternatives) Accuracy Good Pos (<1 cm), Okay Ori (1-2 degrees) Resolution Good (1 mm, .1 degree) Jitter Okay Drift Excellent (none) Lag Good (<20ms) Update Rate Okay (50hz) Interference Okay (echos, line of sight) Encumbrance Okay (wires, wireless possible) Space Good (degrades somewhat with distance) Tracked Entities Okay (many through frequency multiplexing) Calibration None (Out of the box) Cost Okay ($$-$$$$$) Really only appropriate for position tracking. Similar to optical but with somewhat less occlusion at the expense of accuracy.

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

MARG Tracker 3-axis Magnetometer 3-axis Gyroscope 3-axis Accelerometer 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

MARG Tracker Performance Variable Quality (Relative to alternatives) Accuracy Good 3DOF orientation (< 1 degree) Resolution Excellent (<.1 degree) Jitter Good (<.01 degree) Drift Poor (position) -- None (orientation) Lag Excellent (<10ms) Update Rate Excellent (> 100 hz) Interference Very good (magnetic distortion) Encumbrance Very good (wireless options) Space Excellent (sourceless) Tracked Entities Excellent (independent) Calibration Some (local magnetic field) Cost Good ($$-$$$$) Excellent, but need to be combined with position tracker for most VR applications, e.g. Wiimote, PSMove