Jan 9 Jan 9 1 Introduction to Introduction to Tracking Tracking Sherman & Craig, pp. 75-94. Sherman & Craig, pp. 75-94. Welch, Greg and Eric Foxlin (2002). “ Welch, Greg and Eric Foxlin (2002). “ Motion Tracking: No Silver Motion Tracking: No Silver Bullet, but a Respectable Arsenal Bullet, but a Respectable Arsenal ,” IEEE Computer Graphics and ,” IEEE Computer Graphics and Applications, special issue on “Tracking,” November/December 2002, Applications, special issue on “Tracking,” November/December 2002, 22(6): 24–38.. 22(6): 24–38.. (http://www.cs.unc.edu/~tracker/media/pdf/cga02_welch_tracking.pdf) (http://www.cs.unc.edu/~tracker/media/pdf/cga02_welch_tracking.pdf)
Transcript
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Introduction to Introduction to TrackingTracking
Sherman & Craig, pp. 75-94.Sherman & Craig, pp. 75-94.
Welch, Greg and Eric Foxlin (2002). “Welch, Greg and Eric Foxlin (2002). “Motion Tracking: No Silver Bullet, but a Motion Tracking: No Silver Bullet, but a Respectable ArsenalRespectable Arsenal,” IEEE Computer Graphics and Applications, special issue ,” IEEE Computer Graphics and Applications, special issue
on “Tracking,” November/December 2002, 22(6): 24–38.. on “Tracking,” November/December 2002, 22(6): 24–38.. (http://www.cs.unc.edu/~tracker/media/pdf/cga02_welch_tracking.pdf)(http://www.cs.unc.edu/~tracker/media/pdf/cga02_welch_tracking.pdf)
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MotivationMotivation We want to use the human body as an input deviceWe want to use the human body as an input device
– more naturalmore natural– this will lead to higher level of immersionthis will lead to higher level of immersion– to control navigationto control navigation
headhead handhand
– to control interactionto control interaction headhead handhand bodybody
We need two things for this:We need two things for this:– Signaling (button presses, etc.)Signaling (button presses, etc.)– Location. <- this is tracking!Location. <- this is tracking!
ExamplesExamples– Head positionHead position– Hand position (pose)Hand position (pose)– Other body parts (e.g., self-avatars)Other body parts (e.g., self-avatars)– Other objects that also have physical Other objects that also have physical
Trackers provide location and/or position information relative to some coordinate system.
What info would we need?
(x,y,z) (rx,ry,rz)
(0,0,0) Origin for tracker coordinate system
(0,0,0) Receiver coordinate
system
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Degrees of freedomDegrees of freedomThe amount of pose information returned by the tracker
Position (3 degrees)Orientation (3 degrees)
There are trackers that can do:•only position•only orientation•both position and orientation
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QuestionQuestion
Okay, given that I want to track your Okay, given that I want to track your head, I attach a new tracker from head, I attach a new tracker from NewTracker Corp. it returns 6 NewTracker Corp. it returns 6 degrees of freedom (6 floats). What degrees of freedom (6 floats). What questions should you have?questions should you have?
In other words, what are some In other words, what are some evaluation points for a tracking evaluation points for a tracking system?system?
5 minutes to discuss5 minutes to discuss
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Evaluation CriteriaEvaluation Criteria Data returned (3 dof, Data returned (3 dof,
Data returnedData returned Spatial distortion (accuracy)Spatial distortion (accuracy) ResolutionResolution Jitter (precision)Jitter (precision) DriftDrift LagLag Update RateUpdate Rate RangeRange
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Performance MeasuresPerformance Measures Registration (Accuracy)Registration (Accuracy) – Represents the – Represents the
difference between an object’s actual 3D position difference between an object’s actual 3D position and the position reported by the trackerand the position reported by the tracker– LocationLocation– Orientation Orientation
ResolutionResolution – Fineness with which the tracking – Fineness with which the tracking system can distinguish individual points or system can distinguish individual points or orientations in space.orientations in space.
Jitter Jitter – Change in reported position of a – Change in reported position of a stationary object.stationary object.
Drift Drift – Steady increase in tracker error with time.– Steady increase in tracker error with time.
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Performance MeasuresPerformance Measures
Lag (Phase Lag)Lag (Phase Lag) – Difference between when a – Difference between when a sensor first arrives at a point and when the sensor first arrives at a point and when the tracking system first reports that the sensor is at tracking system first reports that the sensor is at that point. Sometimes called that point. Sometimes called latencylatency..– Latency: Latency: The rate (or time delay) at which the The rate (or time delay) at which the
acquisition portion of the system can acquire new data.acquisition portion of the system can acquire new data.– Transmission Lag:Transmission Lag: Time needed to send bits of Time needed to send bits of
information that define position to the computer or information that define position to the computer or graphics engine.graphics engine.
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Update RateUpdate Rate
Number of tracker position/orientation Number of tracker position/orientation samples per second that are transmitted to samples per second that are transmitted to the receiving computer.the receiving computer.– Fast update rate is not the same thing as accurate Fast update rate is not the same thing as accurate
position information.position information.– Poor use of update information may result in more Poor use of update information may result in more
inaccuracy. inaccuracy. – Upper bound is determined by the Upper bound is determined by the
communications rate between tracker and communications rate between tracker and computer and the number of bits it takes to computer and the number of bits it takes to encode position and orientation.encode position and orientation.
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RangeRange
Position range or working volumePosition range or working volume– Sphere (or hemisphere) around the Sphere (or hemisphere) around the
transmitter.transmitter.– Accuracy decreases with distanceAccuracy decreases with distance– Position range is inversely related to Position range is inversely related to
accuracy.accuracy. Orientation Range – set of sensor Orientation Range – set of sensor
orientations that the tracking system orientations that the tracking system can report with a given resolution.can report with a given resolution.
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Interference and NoiseInterference and Noise
Interference is the action of some Interference is the action of some external phenomenon on the tracking external phenomenon on the tracking system that causes the system’s system that causes the system’s performance to degrade in some way.performance to degrade in some way.– Noise – random variation in an otherwise Noise – random variation in an otherwise
constant reading. (Static position constant reading. (Static position resolution)resolution)
– Inaccuracies due to environmental Inaccuracies due to environmental objects.objects.
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Mass, Inertia and Mass, Inertia and EncumbranceEncumbrance
Do you really want to wear this?Do you really want to wear this? Things with no weight on your head Things with no weight on your head
can have inertia.can have inertia. TetheredTethered
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Multiple Tracked PointsMultiple Tracked Points
Ability to track multiple sensors Ability to track multiple sensors within the same working volume.within the same working volume.– Interference between the sensorsInterference between the sensors– MultiplexingMultiplexing
Time Multiplexing – Update rate of S Time Multiplexing – Update rate of S samples per second and N sensors results in samples per second and N sensors results in S/N samples per sensor per secondS/N samples per sensor per second
Frequency Multiplexing – Each sensor Frequency Multiplexing – Each sensor broadcasts on a different frequency. More $broadcasts on a different frequency. More $$$
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PricePrice
You get what you pay for.You get what you pay for. Rich people are a small market.Rich people are a small market.
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Body Tracking TechnologyBody Tracking Technology Position TrackingPosition Tracking
Use the attenuation of oriented Use the attenuation of oriented electromagnetic signals to determine electromagnetic signals to determine the absolute position and orientation the absolute position and orientation of a tracker relative to a source.of a tracker relative to a source.– Polhemus (a.c.)Polhemus (a.c.)– Ascension (d.c.)Ascension (d.c.)
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Basic Principles of EM Basic Principles of EM TrackersTrackers
Source contains 3 orthogonal coils that are pulsed Source contains 3 orthogonal coils that are pulsed in rotation, one after another.in rotation, one after another.
Each pulse transmits a radio frequency Each pulse transmits a radio frequency electromagnetic signal that is detected by a electromagnetic signal that is detected by a sensor.sensor.
The sensor also contains 3 orthogonal coils, which The sensor also contains 3 orthogonal coils, which measure the strength of the signal from the measure the strength of the signal from the current source coil (9 total measurements)current source coil (9 total measurements)
By using the known pulse strength at the source By using the known pulse strength at the source and the known attenuation of the strength with and the known attenuation of the strength with distance, these nine values can be used to distance, these nine values can be used to calculate position and orientation of the sensor calculate position and orientation of the sensor coils.coils.
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Basic EM Principles Basic EM Principles (cont.)(cont.)
Source and sensor are connected to Source and sensor are connected to a box which contains a a box which contains a microcomputer and electronics microcomputer and electronics associated with the pulses.associated with the pulses.
Serial communications (serial port)Serial communications (serial port) A source may be associated with 1 to A source may be associated with 1 to
as many as 18 sensors as many as 18 sensors Problems: Earth’s Magnetism!Problems: Earth’s Magnetism!
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Characteristics of EM Characteristics of EM TrackersTrackers
Measure position and orientation in 3D Measure position and orientation in 3D spacespace
Do not require direct line of sight between Do not require direct line of sight between the source and the sensorthe source and the sensor
Accuracy affected by Accuracy affected by – DC: DC: FerrousFerrous metal and electromagnetic fields. metal and electromagnetic fields. – AC: Metal and electromagnetic fieldsAC: Metal and electromagnetic fields
Operate on only one side of the source (the Operate on only one side of the source (the working hemisphere).working hemisphere).
Working distance of about 3-25? feet from Working distance of about 3-25? feet from source. (Depends on source size, power)source. (Depends on source size, power)
Limited Limited range/resolutionrange/resolution– Tethered (cables to box)Tethered (cables to box)– Metal in environment Metal in environment – No identification problemNo identification problem– 6DOF6DOF
RealtimeRealtime– 30-144 Hz30-144 Hz 13-13-
18 sensors18 sensors
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ExampleExample
6 bytes for position (3 two-byte integers)6 bytes for position (3 two-byte integers) 18 bytes for orientation (9 two-byte integers of a 18 bytes for orientation (9 two-byte integers of a
3x3 orientation matrix).3x3 orientation matrix). 3 byte header3 byte header 8 data bits and 1 stop bit, no start or parity bits (9 8 data bits and 1 stop bit, no start or parity bits (9
bits/byte)bits/byte) Total per data packet: 27*9 = 243 bitsTotal per data packet: 27*9 = 243 bits 19,200 baud19,200 baud
Mechanical LinkageMechanical Linkage Jointed structure that is rigid except Jointed structure that is rigid except
at the joints.at the joints. One end (base) is fixed.One end (base) is fixed. The other (free, distal) end may be The other (free, distal) end may be
moved to an arbitrary position and moved to an arbitrary position and orientation.orientation.
Sensors at the joints, detect the Sensors at the joints, detect the angle of the joints.angle of the joints.
Concatenation of translates and Concatenation of translates and rotates can be used to determine rotates can be used to determine the position and orientation of the the position and orientation of the distal end relative to the base.distal end relative to the base.
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Characteristics of MLCharacteristics of ML
Fast Fast Accurate:Accurate:
– Depends on the physical size of the MLDepends on the physical size of the ML– Depends on quality of rotation sensors at jointsDepends on quality of rotation sensors at joints
Encumbered MovementEncumbered Movement ExpensiveExpensive Can incorporate force feedback Can incorporate force feedback
(PHANToM)(PHANToM) Used on the BOOM display system from Used on the BOOM display system from
Use the time-of-flight of an ultrasonic Use the time-of-flight of an ultrasonic sound pulse from an emitter to a sound pulse from an emitter to a receiver. Either the emitter or the receiver. Either the emitter or the receiver can be fixed, with the other receiver can be fixed, with the other free to move.free to move.– LogitecLogitec– Mattel Power GloveMattel Power Glove– A component of Intersense A component of Intersense – Inertial + Ultrasonic systemsInertial + Ultrasonic systems
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Basic Principles of UTBasic Principles of UT
Based on measurement of time-of-Based on measurement of time-of-flight of a sound signal. 1000 feet/Secflight of a sound signal. 1000 feet/Sec
Source component contains Source component contains transmitters that produce a short transmitters that produce a short burst of sound at a fixed ultrasonic burst of sound at a fixed ultrasonic frequency.frequency.
The sensor component contains The sensor component contains microphones that are tuned to the microphones that are tuned to the frequency of the sources.frequency of the sources.
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UT CharacteristicsUT Characteristics
Inexpensive (Used in Mattell Inexpensive (Used in Mattell Powerglove $100).Powerglove $100).
Inaccurate.Inaccurate.– Echoes and other ambient noiseEchoes and other ambient noise
Require a clear line-of-sight between Require a clear line-of-sight between the emitter and the receiver.the emitter and the receiver.
Sometimes used for head-tracking Sometimes used for head-tracking for CRT displays.for CRT displays.
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Basic UT SetupBasic UT Setup
Stationary Origin(receivers)
Tracker(transmitters)distance1
distance2
distance3
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UT Position and Orientation UT Position and Orientation InformationInformation
1 transmitter, 3 receivers : 3D 1 transmitter, 3 receivers : 3D position relative to fixed originposition relative to fixed origin
2 transmitters, 3 receivers : 3D 2 transmitters, 3 receivers : 3D position and orientation up to a roll position and orientation up to a roll around a line through the two around a line through the two transmitterstransmitters
3 transmitters, 3 receivers : complete 3 transmitters, 3 receivers : complete position and orientation informationposition and orientation information
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Inertial TrackingInertial Tracking
Uses electromechanical devices to Uses electromechanical devices to detect the relative motion of sensors detect the relative motion of sensors by measuring change in:by measuring change in:– AccelerationAcceleration– Gyroscopic forcesGyroscopic forces– Inclination Inclination
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AccelerometersAccelerometers
Mounted on a body part to detect Mounted on a body part to detect acceleration of that body part.acceleration of that body part.
Acceleration is integrated to find the Acceleration is integrated to find the velocity which is then integrated to velocity which is then integrated to find position.find position.
Unencumbered and large area Unencumbered and large area tracking possibletracking possible
Suppose the acceleration is measured Suppose the acceleration is measured with a constant error with a constant error ii, so that , so that measured acceleration is ameasured acceleration is aii(t)+ (t)+ II
Errors accumulate since each position is Errors accumulate since each position is measured relative to the last positionmeasured relative to the last position
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Inertial TrackingInertial Tracking
Inclinometer – Inclinometer – measures measures inclination relative inclination relative to some “level” to some “level” positionposition
– Cameras carried by Cameras carried by participant, tracking participant, tracking makers (typically fixed) in makers (typically fixed) in the environmentthe environment
Outside Looking In Outside Looking In Optical TrackingOptical Tracking
Precision Point Precision Point Tracking by WorldVizTracking by WorldViz
IR Filtered Cameras are IR Filtered Cameras are calibratedcalibrated
Each frame:Each frame:– Get latest images of Get latest images of
pointpoint– Generate a ray (in world Generate a ray (in world
coordinates) through the coordinates) through the point on the image planepoint on the image plane
– Triangulate to get Triangulate to get positionposition
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Outside Looking In Outside Looking In Optical TrackingOptical Tracking
What factors play a What factors play a role in O-L-I tracking?role in O-L-I tracking?– Camera resolutionCamera resolution– Frame rateFrame rate– Camera calibrationCamera calibration– OcclusionOcclusion– CCD QualityCCD Quality
How does it do for:How does it do for:– PositionPosition
stable, very goodstable, very good– OrientationOrientation
Unstable, poorUnstable, poor– LatencyLatency
Cameras are 60HzCameras are 60Hz
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OrientationOrientation Since orientation Since orientation
is poor, you can is poor, you can get an orientation get an orientation only sensor (ex. only sensor (ex. Intersense’s Intersense’s InertiaCube)InertiaCube)
Called a ‘hybrid Called a ‘hybrid tracker’ or ‘multi-tracker’ or ‘multi-modal tracker’modal tracker’
The tracking device The tracking device carries the camera carries the camera that tracks markers that tracks markers in the in the environment.environment.
Optical SensorsOptical Sensors– Have an emitter on one end and a Have an emitter on one end and a
receiver on the other. receiver on the other. – As the sensor is bent, the amount of As the sensor is bent, the amount of
light that gets from the emitter to the light that gets from the emitter to the receiver is attenuated in a way that is receiver is attenuated in a way that is determined by the angle of the bend.determined by the angle of the bend.
Exoskeletal StructuresExoskeletal Structures– Sensors which attach a rigid jointed Sensors which attach a rigid jointed
structure to the body segments on structure to the body segments on either side of a joint.either side of a joint.
– As the joint bends, the angle between As the joint bends, the angle between the body segments is measured via the body segments is measured via potentiometers or optical encoders in potentiometers or optical encoders in the joints of the exoskeleton.the joints of the exoskeleton.
– Exos Dexterous Hand MasterExos Dexterous Hand Master
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Other TechniquesOther Techniques
Pinch GlovesPinch Gloves– Have sensor Have sensor
contacts on the contacts on the ends of each fingerends of each finger
– TetheredTethered– No identification problemNo identification problem– RealtimeRealtime - 500Hz- 500Hz– No range limitNo range limit - Fit- Fit– Rigid body approximationRigid body approximation
