Overview of Virtual Reality

1. Overview of Virtual Reality Tracking Systems

2. Tracking Systems • Background Material • Applied Virtual Reality, SIGGRAPH 1998 Course 14 Notes, Carolina Cruz-Neira (Organizer), pp. 2-14 to 2-18, 2-24 to 2-28. • Review of Virtual Environment Interface Technology, IDA Paper P-3186, March 1996, ch. 3

3. Tracking Systems • Tracking usually involves tracking person’s head and hand(s) • Head position/orientation necessary for correct perspective • Tracking system is the major source of lags and errors in a virtual environment, which degrades performance and induces motion sickness

4. Tracking Systems • Need x,y,z, pitch, roll, yaw • Use position/orientation to correlate visual and auditory inputs to the user’s position • Eye trackers use electroocular, electromagnetic, or optical technologies

5. Tracking Systems • Key Characteristics • Resolution (in position and orientation) • Accuracy (in position and orientation) • System responsiveness • Sample rate • Data rate • Update rate • Latency/Lag • Repeatability • Working Range

6. Sources of Latency • Delays in the tracker signal • Delays in the communication between tracker and the computer system • Delays due to computations required to process the tracker data • Delays due to graphical rendering

7. Seven Technologies • Electromagnetic • Mechanical • Acoustic • Optical • Inertial • Sourceless (non-inertial) • Image Analysis

8. Electromagnetic • Transmitter/source emits EM fields along 3 orthogonal axes • One or more receivers/sensors • Position and Orientation reported – 6DOF • Polhemus Fasttrack and Ascension Flock of Birds are the most popular (note little change since 1995 – see IDA report)

9. Electromagnetic • Advantages • No need for clear line-of-sight (LOS) • Small sensors (1 in cube) • Few restrictions for users • Mature technology • Rather accurate (0.1º resolution at 12 inch) • Rather inexpensive ($8000)

10. Electromagnetic • Advantages • Good noise immunity (? So says IDA report) • Can track multiple objects • Large range – 10 ft (covers small room)

11. Electromagnetic • Limitations/Weaknesses • Sensitivity to ferrous materials • Noise and distortions – noisy and distortions increase as distance between trans/rec increases • Latency – report rates >60Hz, but other delays occur (see details below)

12. Mechanical • Rigid structure with several joints • Joint angles measured to determine position and orientation • Works well for applications that do require sudden change in user’s position • E.g., “The Phantom”

13. Mechanical • Advantages • Low latency/lag • Accurate (probably most accurate trackers) • No line-of-sight problems • No magnetic interference problems • Good for tracking small volumes accurately

14. Mechanical • Limitations/Weaknesses • Restricted mobility • One sensor • Subject to mechanical part wear-out

15. Acoustic • Use ultrasonic sound • A source produces ultrasonic pulses which are received by a set of microphones, usually in a triangular fashion • Position/Orientation determined by different times the pulse reaches each microphone

16. Acoustic • Advantages/Strengths • No interference from metals • Low cost • lightweight

17. Acoustic • Limitations/Weaknesses • Occlusion (need line-of-sight) • Short range • Ultrasonic noise interference (jewelry/teeth cleaner?) • Low accuracy since speed of sound in air varies with environmental conditions • Echoes cause reception of “ghost” pulses

18. Optical • Use combination of light sources (LEDs), video cameras, and image processing techniques • Two approaches • Sources/markers placed on the object to be tracked, with cameras at fixed locations • Cameras placed on user’s head and an array of markers is mounted in a fixed pattern on ceiling • Position/Orientation determined using image processing techniques • Existing systems, but active area of research

19. Optical • Advantages • High accuracy • Extendible to large working volumes • Fast • No magnetic interference problems

20. Optical • Limitations/Weaknesses • Occlusion • Cumbersome • Hard to track more than one object • Expensive

21. Inertial • Use gyroscopes to measure pitch/yaw/roll • Based on principal of conservation of momentum • Use accelerometers to measure acceleration and integrate twice to determine position • Often part of hybrid system

22. Inertial • Advantages • No need for a separate transmitter • Longer range • Fast • No line-of-sight problems • Senses orientation directly • Small size • Low cost

23. Inertial • Limitations/Weaknesses • Accuracy • Only 3 DOF • Drift • Position is relative, not absolute • Not accurate for slow position changes

24. Sourceless (non-inertial) • Use passive magnetic sensors, referenced to earth’s magnetic field, to provide measurement of pitch/roll/yaw • Derive angular acceleration and velocity

25. Sourceless (non-inertial) • Advantages • Inexpensive • No transmitter • Portable • Disadvantages • Only 3 DOF • Difficult to mark movement between magnetic hemispheres

26. Image Analysis • Uses video cameras to capture images of the users • Uses image analysis techniques to identify position of body parts (arms, legs, face)

27. Image Analysis • Advantages • Non-invasive • Disadvantages • Occlusion • Lots of processing required

28. Connecting VR and LODs • Durlach, in a 1994 paper, “contends that head movements can be as fast as 1,000°/sec in yaw, although more usual peak velocities are 600°/sec for yaw and 300°/sec for pitch and roll. The frequency of volitional head motion falls off approximately 1/f2, with most of the energy contained below 8 Hz and nothing detectable above 15 Hz. Tracker to host reporting rates must, therefore, be at least 30 Hz.” • Figure 8.19(b) in LOD book shows that our spatial resolution falls from more than 18 cycles/deg at 0 deg/s to less than 2 cycles/deg above 10 deg/s IDA report, p 50

29. Didn’t notes written on • Some example devices (see IDA report) • Eye tracking (see IDA report) • Research work (see IDA report)