1. Tracking Systems in VR

2. 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?

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
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!

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
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)

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
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.

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 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

16. 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