Salim Modi, David Nguyen, Mitul Patel

Virtual Environments

Tracking Systems

Outline

• Introduction– What is Tracking?– Performance Measures – The Ideal Tracker

• Different Technologies:– Technical Overview– Pros / Cons– Hybrids

• Conclusions

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

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

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)

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

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

Mechanical Trackers

• First & simplest systems

• Use prior knowledge or rigid mechanical pieces and measurements from sensors.

• Typically boom-type tracked displays with counterweights.

Mechanical Trackers

• Pros– Accurate– Low latency– Force-feedback– No Line of Sight or Magnetic Interference Problems

• Cons– Large & cumbersome– Limited range

Mechanical Trackers

• Some example systems

Inertial Trackers

• 3 linear accelerometers measure acceleration vector• Rotated using current rotation matrix (orientation)

determine by gyroscopes

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…

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)

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

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

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

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)

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

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

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

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

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