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

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