1. © Houari Abdallahi, James Lawton, Deniz Ozsen, Christine Dubreu Virtual Environments: Tracking

2. Outline  Introduction History Framework for Suitability 6DOF, Inside-out, Outside-in  Different Technologies: Mechanical Systems Optical Magnetic Acoustic Inertial  Comparison of Technologies  Conclusions Effects of inaccurate Position Tracking Performance Requirements for VR Illusion

3. Position Tracking The real time tracking of the position and orientation of points in space. Tracking the head to control view generation and tracking a pointing device for interaction with the virtual environment.

4. History Egyptian Groma The groma consists of stones hanging from sticks set at right angles to one another. Distant objects could be marked out against the position of the stones in a horizontal plane.

5. History The first head-mounted display system. The Sword of Damocles

6. Framework for Suitability Resolution and accuracy Responsiveness:  Sample Rate  Data Rate  Update Rate  Lag Robustness Registration Sociability  Range of operation  Fitness for Tracking Multiple Objects

7. Tracking Technologies Surging (depth) Swaying (width) Heaving (length) Rolling Pitching Yawing

8. Tracking Technologies The main tracking technologies used are mechanical, magnetic, acoustic, inertial and optical. Most can be classified as: Outside-in Inside-out

9. Mechanical Trackers First systems used Linkages attached to a fixed point Relies on the known geometry of solid linkages Typically measures joint angles

10. Mechanical Trackers Some example systems

11. Mechanical Trackers Sword of Damocles

12. Evaluation Advantages Accurate Reliable (low lag) Force-feedback No Line of Sight or Magnetic Interference Problems Disadvantages Motion Restriction Poor Sociability Mechanical Part Wear-out

13. Optical Trackers Setup Detectors Light Inside/Out Outside/In

14. Fixed Transducer System Relies on Known Distance between Emitters and Sensors Most Common Outside-In & Inside-Out Technologies

15. Advances in Optical Tracking

16. HiBall-3100 An inside-out optical system by 3rdTech Autocalibration 12’x12’ to 40’x40’ setups 2,000Hz tracker update 0.2mm, 0.01° resolution Redundancy avoids occlusions

17. ARTtrack2 Made by A.R.T GmbH Several configurations –Camera with built-in flash –Separate flash (flash can work through walls, if they transmit IR) –Stereo cameras

18. ARTtrack2 Made by A.R.T GmbH Several configurations –Camera with built-in flash –Separate flash (flash can work through walls, if they transmit IR) –Stereo cameras

19. ARTtrack2 Made by A.R.T GmbH Several configurations –Camera with built-in flash –Separate flash (flash can work through walls, if they transmit IR) –Stereo cameras

20. Pattern Recognition System Compares Known Patterns to Sensed Patterns LED Array Symbols Real objects

21. Laser Ranging Light Transmitted onto Object Changes in Reflected Light Sensed Example: Medicine

22. Optical Tracker Evaluation Advantages High Availability High Date Rates Good Sociability No Magnetic Interference Problems High Accuracy Disadvantages Complexity Expense Range Error Weight Line of Sight

23. Mechanical Vs Optical Trackers Mechanical No Line of Sight Problems Poor Sociability Cumbersome No Environmental Error Good Responsiveness Static Accuracy Optical Line of Sight Requirement Potential High Sociability Not Necessarily Cumbersome Possible Errors from Lighting Better Responsiveness Variable Accuracy (Range)

24. Magnetic/Electromagnetic Trackers Emitter: Device that generates a magnetic field Sensor: Device that measures the surrounding magnetic field Advantages –Very low latency (~5ms) –Unaffected by sensor occlusion by non-ferromagnetic objects  Ability to track multiple users  promotes Sociability Problems –Sensitive to interference by magnetic fields from devices in the working area –Distortion of magnetic field by metallic objects –Can make up for this using digital filtering algorithms, however, this lowers the data rate! –Magnetic field diminishes with distance  limited range of operation Two varieties –AC Emitter (e.g. Polhemus) –DC Emitter (e.g. Ascension)  less distortion of magnetic field by metallic objects  better!

25. Magnetic Trackers - Examples Ascension Flock of Birds –DC –Up to 4 sensors –Long range coverage easily added by using “Extended Range Transmitter” –Update Rate: up to 144Hz –Accuracy of position: 1.8mm RMS –Accuracy of orientation: 0.5° RMS Polhemus Fastrak –AC –Single transmitter, up to 4 sensors –Update rate: 120Hz (for single receiver) –Latency: 4ms –Accuracy of position: 1mm RMS –Accuracy of orientation: 0.15° RMS

26. Acoustic Trackers Emitter: Device that generates an acoustic wave of a certain frequency (speaker) –Usually ultrasonic (> 20kHz) Sensor: Device that registers sound (microphone) Advantages –Inexpensive –Can get high data rates, and high accuracy Problems –Sensor occlusion –Interference by acoustic noise, such as keys jingling, and echoes from walls Two varieties Time-of-Flight (TOF) Trackers –Low speed of sound  low data rate –Speed of sound affected by environmental factors such as temperature, pressure Phase Coherent (PC) Trackers –Higher data rate –Can get position wrong by multiples of wavelength when object moves too fast

27. Acoustic Trackers – Examples (1) Component of Intersense Trackers (e.g. IS 900) –TOF tracking –SoniDiscs, mounted on SoniStrips on ceiling, generate 40kHz signals –At the same time, “Tracked Station” starts counter –When signal arrives at “Tracked Station”, counter stops Logitech Red Baron: ultrasonic headtracker Mattel Power Glove

28. Acoustic Trackers – Examples (2) In2Games Gametrak –Accuracy: 1mm –Working volume: 3m cube

29. Inertial Trackers Sensors mounted on object that measure relative changes in position and orientation How it works –Change in position: Accelerometer –Change in orientation: Inclinometer Advantages –Autonomous: tracker on object “knows” where it is Problems –All measurements are relative  Cumulative ranging errors!

30. Inertial Trackers - Example Component of Intersense Trackers (IS 900) –Combined with acoustic tracking

31. Comparison of Technologies (1) MechanicalOpticalMagneticAcoustic Accuracy and Resolutio n Good Decrease as working volume increases (mul- tiple emitters sensors) Good in small working volume Accuracy affec- ted by ferroma- gnetic objects Good Respon- siveness Good Well suited to real time applications Relatively low data rates Filtering can introduce lag TOF : Good in small ranges PC : Respon- siveness unaffected by range

32. Comparison of Technologies (2) MechanicalOpticalMagneticAcoustic Robustness Good Not sensitive to environmen- tally induced errors Good Some systems affected by ambient light Ferromagnetic objects create eddy currents that cause ranging errors TOF : Vulne- rability to ranging errors PC : Excellent. High data rates unaffected by range Sociability Limited range Two systems cannot occupate the same volume Range-accuracy (multiple E/S) Inside out sys- tems more fit for tracking multiple objects Vulnerable to occlusion Small working volume (eddy currents increase with field strength) Multiple emitters Unaffected by non ferromagne- tic occlusions TOF : Small effective volumes PC : Large working volume vulnerables to occlusion

33. Comparison of Technologies (3) Mechanical –Cumbersome, well suited to force feedback –Successful applications in Telerobotics Optical –Compromise between range and accuracy –Successfully used in cockpits Magnetic –Relatively inexpensive, most commonly used in current VR research –Successfully used in cockpits Acoustic –Starting to appear in marketplace

34. Effects of inaccuracy The tracked object can appear to be somewhere it is not –When the tracked object is a part of the body, the illusion of the simulated space tends to break down If a position tracker reports inaccurate data, the user has to construct a mental model of the surrounding space from incompatible data. –Conflict between perceived visual space and perceived proprioceptive space –As the visual information tends to dominate, users can experiment motion sickness.

35. Visual-proprioceptive conflicts Contention between the observed position of a limb and its felt position –Mismatches between the computer generated image and the vestibular system –The user will adjust input values to match with the visual informations Lag in reported body movement –The user will minimize rapid movements that accentuate the visual- proprioceptive conflict –This inhibits natural movements and can interfere with the applications requiring naturalistic simulations Jitter or oscillation of the represented body part –Strongly contribute to motion sickness

36. Performance requirements The performance should be studied in terms of perceptual datas and motion dynamics A gap exists between perceptual understanding and technical practice This gap is not a barrier to current developpement as the user unconsciously ajusts the visual or proprioceptive processes We must understand the performance requirements of the human perceptive system

37. Summary We have discussed several position-tracking technologies and established a framework for suitability. 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 All of the technologies display both strengths ans weaknesses. The ultimate tracker will probably not be developped from a single technology, but as a hybrid of these technologies.