Tracking Systems

1. Tracking Systems INPUT DEVICES Cesar Martinez Internetworked Virtual Reality COMP6461 September 2002

2. INPUT DEVICES Successful human-computer interaction requires efficient transfer of information from human to computers. Such communication is mediated via input devices. While most input devices are conceptually simple a lack of standards, especially in VR interaction, prevents input devices from being interchangeable. Applications are generally designed to work with a fixed set of input devices. While this allows to exploit such devices to the fullest both in terms of function and performance, it has several disadvantages: The application may present a limited choice of input devices. Support for a new device may require changes in the application, or the application may not be able to fully support the features of the new device.

3. INPUT DEVICES Traditionally, computers use input devices ranging from switches and punched cards to keyboards and mice. However, it is obvious that such devices are insufficient for treading the pathways of the virtual world. VR departs from conventional Human-Computer Interaction and naturally requires a different set of user input tools. Rough classification of input devices : discrete, continuous, combination of both. Discrete :Generate one event at a time based on the user action e.g. Keyboard, Pinch Glove, etc. Continuous :• Continuously generate events in isolation or in response to user action e.g. Trackers, Datagloves. Combination/Hybrid Devices :Devices have the ability to generate both discrete and continuous events Classic example : Mouse others : Joysticks, Tablets.

4. INPUT DEVICES Sufficient graphics capabilities have become available on standards PC’s. Add-on high speed graphics processors are inexpensive and give PC’ rendering horsepower that rivals low-to-mid-range graphics workstations. X z Y Computer mouse is the facto standard for interacting with desktop computers. Position Orientation Robust, professional applications have an absolute requirement to coordinate free-moving human motions with image control. In these cases, you need to instantly know not just position but orientational data.

5. INPUT DEVICES Tracking Systems Human exploration in virtual environments requires technology that can accurately measure the location and the orientation of one or several users as they move and interact in the environment. This is referred to as tracking users in the environment. The location and the orientation of each user are measured with respect to the virtual environment coordinate system.

6. INPUT DEVICES Sensor Control Box - Process signal - Communicates Signal Signal Source Computer Source Position N Tracking Systems

7. INPUT DEVICES External sensor (e.g. camera) Artificial source (e.g. reflector) On-body sensors On-body sensor and source Computer Computer Interface controller Artificial source Ultrasound generator Tracking Systems

8. INPUT DEVICES Tracking Systems The usefulness of tracking devices in VR environments depends to a large degree on whether the computer can track the movements of the source fast enough to keep the virtual world synchronized with the user’s actions. Degrees of Freedom (DOF) It is defined as “the number of independent dimensions one must use to define unambiguously the state of the system” Morrison and Newell 1998 This ability is determined by the lag, or latency, of the signal, and the sensor’s update rate. The signal lag is the delay between the change of the position and orientation of the target being tracked and the report of the change to the computer. The update rate is the rate at which measurements are reported to the computer.

9. INPUT DEVICES Tracking Systems Inside-in systems: sensor(s) and source(s) are both worn on the body Inside-out systems: sensor(s) on the body sense(s) external artificial / natural source(s) Outside-in systems: external sensor(s) sense(s) artificial / natural source(s) on the body

10. INPUT DEVICES Tracking Systems POLHEMUS :Fastrack Magnetic Trackers EM trackers have a transmitter that emits EM field along three orthogonal axes that are detected by sensors. The sensors report information about their position and orientation with respect to the source. Advantages . Relatively inexpensive . Minimal shadowing problems . Fairly accurate . Multiple object tracking available . Detectors are small, unobtrusive . Popular, resources are available Disadvantages . Stated performance only occurs in ideal situations . Susceptible to magnetic interference and interference caused by nearby metal objects . Accuracy dependent on distance between emitter and detector . Workspace limited by emitter strength Stylus

11. INPUT DEVICES Tracking Systems Mechanical Trackers Have a rigid structure with several joints. One end is attached to the object to be tracked and the other end is fixed in place. The joint angles are measured and reported to give the objects position and orientation. Advantages . No shadowing problem. . Not affected by external fields or materials . If attached well, can very accurately reflect the user‘s joint angles. . Potential for being less expensive as the technology has been developed elsewhere . Sensors are inherently fast, and will not limit bandwidth or latency . Body based systems are not limited to a confined workspace . No emitter required. Disadvantages . Mechanical device attached to soft tissue: tradeoff between comfort and accuracy . Must be robust enough to fit multiple users of different height, weight, and sex. . Potential to be bulky and heavy, limiting user's movement . Ground based systems limit user's workspace . Need for calibration . A kinematic mismatch can occur, causing relative motion between the person and device

12. INPUT DEVICES Tracking Systems Acoustic Trackers Use ultrasonic sound. A source produces pulses that are received by a set of microphones usually arranged as a triangle. The time each pulse reaches the different microphones gives the source position and orientation. Advantages . Inexpensive . Longer ranges than magnetic trackers . Microphones are small, unobtrusive Disadvantages . Line of sight must be maintained . Latency proportional to distance . Many emitters/receivers required for multiple targets . Phase coherent method must be calibrated, drift problem . Ambient noise interference . Interference cause by echoes from hard surfaces . Atmospheric distortion, temperature effects

13. INPUT DEVICES Optical Trackers Use a combination of LED’s, video cameras, and image processing. The LED’s can be placed on the object to be tracked while the cameras are fixed or the cameras are placed on the object and there is an array of LED’s in the ceiling. The position and orientation are obtained from signal processing techniques. Advantages . Nothing attached to user. . Senses person, not just joints or points; thus, person's body can be virtual icon, rather than being artificially created from limited information Disadvantages . Cameras limit workspace. . Not accurate. . Not robust. . Slow. . Correspondence problem. . Shadowing. . Expensive.

14. INPUT DEVICES Inertial Trackers Use gyroscopes to measure the three orientation angles. They are connected by a cable to the computer system. Advantages . No shadowing problems . No emitter required . Not effected by external fields, materials . Potentially small, unobtrusive sensors Disadvantages . Relatively new VR technology . Drift problems from integration . Need for calibration . Possible nonlinearity

15. INPUT DEVICES Evaluation Criteria . Interference and noise . Mass, Inertia and Encumbrance . Multiple Tracked Points . Price . Registration . Resolution . Lag . Update Rate . Range

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