Salim Modi, David Nguyen, Mitul Patel

1. Virtual Environments Tracking Systems Salim Modi, David Nguyen, Mitul Patel

2. Outline • Introduction • What is Tracking? • Performance Measures • The Ideal Tracker • Different Technologies: • Technical Overview • Pros / Cons • Hybrids • Conclusions

3. What is Tracking? Trackers are used for five primary purposes: • View Control • Position & Orientation • Virtual Camera • Navigation • Object Selection • Handheld Devices • Grab + Manipulate virtual objects • Instrument Tracking • Physical objects to match virtual representations • Computer-aided surgery • Avatar Animation • Motion Capture

4. Performance How do we measure a good tracker? • Size • Degrees of Freedom (x, y, z, roll, pitch, yaw) • Accuracy (resolution) • Speed (updates & latency) • Occlusion Handling • Resistance to Interference • Range • Cost

5. Performance What are the kinds of tracking errors? • Static Tracked Object • Spatial Distortion (inaccuracy) • Spatial Jitter (noise) • Creep • Dynamic Tracked Object • Lag (time delay, tracker + subsystems  complex relation) • Latency Jitter (variations in latency) • Dynamic Errors (other inaccuracies, e.g. prediction algorithms)

6. The Ideal Tracker Magical, ideal tracker would have these characteristics: • Tiny (transistor size) • Self-Contained • Complete (6 DoF) • Accurate (1mm position, 0.1 degree orientation) • Fast (1000Hz, <1s latency) • Immune to occlusions (no line-of-sight requirement) • Robust (no interference) • No range limitation • Cheap

7. Tracking Technologies • 5 main types: mechanical, inertial, acoustic, optical, magnetic. • Most can be classed as: • Outside-In: user emits signal to indicate its location to the system • Inside-Out: systems emits signal to user which senses location Outside-in Inside-out

8. Mechanical Trackers • First & simplest systems • Use prior knowledge or rigid mechanical pieces and measurements from sensors. • Typically boom-type tracked displays with counterweights.

9. Mechanical Trackers Pros Accurate Low latency Force-feedback No Line of Sight or Magnetic Interference Problems Cons Large & cumbersome Limited range

10. Mechanical Trackers • Some example systems

11. Inertial Trackers • 3 linear accelerometers measure acceleration vector • Rotated using current rotation matrix (orientation) determine by gyroscopes

12. Inertial Trackers • Pros • Small (chip form), self-contained. • Immune to occlusions • No interference • Low latency (typically <2ms) • High sample rate • Cons • Drift is the show stopper • Accelerometer bias of 1 milli-g  4.5m drift after 30s • Close, but no silver-bullet • High potential as part of hybrid systems…

13. Acoustic Trackers • Uses sound waves for transmission and sensing • Involves pulses at intervals • SONAR is best known, determining time of a pulse • Uses ultrasound • Outside-In (microphone sensors) • (Logitech Acoustic Tracker) • (Samba De Amigo Maracas)

14. Acoustic Trackers • Pros • Very small so can be worn • Line of sight less of an issue than with optical systems • Better range than mechanical systems • Cons • Size proportional to range • Environment considerations (temperature, humidity) • Acoustic issues can cause slow update rate (10Hz) (5-100ms) • Attenuation at desirable high frequencies (reduced interference) • Jingling of keys

15. Optical Trackers • Measures reflected or emitted light • Involves a source (active or passive) and sensor • Sensors can be analogue or digital • Photo sensing (light intensity) or Image forming (CCD) • Triangulation with multiple sensors • Possible to be both outside-in and inside-out

16. Optical Trackers • Pros • Analogue sensors with active light source gives high update and spatial precision • Passive with image-forming sensors could be used in an unaffected environment • Image forming sensors provide closed-loop feedback of real environment and tracker • Cons • Line of sight is critical • Target’s orientation harder to determine

17. Magnetic Trackers • Measures changes in the magnetic field • Can be done by magnetometers (for DC) • Or by induced current in an electromagnetic field (for AC) • 3 sensors orthogonally arranged will produce a 3D vector • In tracking, a multi-coil source unit with each coil energised (excited) and when measured results in position and orientation. • Compass: uses the earth’s naturally occurring DC magnetic field to determine heading, can be used here • (Ascension spacePad)

18. Magnetic Trackers • Pros • User-worn component small • No line of sight issues (magnetic fields go through us) • One source unit can excite many sensor units • Very low latency (~5ms) • Ability to track multiple users using a single source unit • Cons • Field distortions (foreign objects, natural magnetic field) • Requires some compensation • Jingling of keys (or anything magnetically conductive) • Need to wait for energised excitation of coil to subside before the next one so update is slow • Jitter increases over distance from emitter/sensor

19. Hybrid Trackers • No ideal solution that suits all applications • Many different approaches, each with advantages and limitations • Can address the limitations by building hybrid systems which combine the advantages of each approach • Inertial sensors have provided the basis for several successful hybrid systems due to their advantages • Example, the AVCATT-A flight simulator uses the InterSenseSimTracker, an acoustic-inertial hybrid

20. Hybrid Trackers • InterSense IS-900 • Tracking system for VR-Walkthrough applications • Inertial (orientation & position) & Ultrasonic (drift correction) hybrid tracker which has highly accurate 6 degree of freedom tracking in a wide area. • Features fast updates, low latency, filtering to reduce jitter and advanced prediction algorithms to reduce latency  very smooth and precise • The four sensors, including a head tracker, a hand tracker, a wand (with four buttons and an integrated joystick), and a stylus (with two buttons). • Used in UCL’s very own ReaCTor

21. Real Summary • Quite a complex and challenging problem • No real ideal solution (“Silver Bullet”) • Several tracking technologies exist with different levels of suitability based on the application in question. All of the technologies display both pros and cons. • The ultimate tracker will probably not be developed from a single technology, but as a hybrid of these technologies. • A VR application should provide the following: • High data rates for accurate mapping without lag • High tolerance to environmentally induced errors • Consistent registration between physical and virtual environments • Good sociability so that multiple users can move freely

22. Reading Material • Motion Tracking: No Silver Bullet, but a Respectable Arsenal • G. Welch and E. Foxlin (2002) • IEEE Computer Graphics and Applications • A Survey of Position Trackers • Kenneth Meyer, Hugh L. Applewhite and Frank A. Biocca (1992) • MIT Press