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Tracking

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http://www.sv.vt.edu/future/vt-cave/apps/#detour. Pose. Position. Orientation ... a new tracker from NewTracker Corp. it returns 6 degrees of freedom (6 floats) ... – PowerPoint PPT presentation

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Title: Tracking


1
Tracking
  • Sherman Craig, pp. 75-94.
  • Welch, Greg and Eric Foxlin (2002). Motion
    Tracking No Silver Bullet, but a Respectable
    Arsenal, IEEE Computer Graphics and
    Applications, special issue on Tracking,
    November/December 2002, 22(6) 2438..
    (http//www.cs.unc.edu/tracker/media/pdf/cga02_we
    lch_tracking.pdf)

2
Motivation
  • Methods to interact with the virtual world
  • More natural
  • Higher level of immersion
  • Task performance
  • Control navigation
  • Control interaction
  • Ex. Training soldiers w/ a gun
  • How do we track the gun?
  • How do we determine what the user sees?
  • This requires
  • Signaling (button presses, etc.)
  • Location. lt- this is tracking!

3
Tracking
  • http//www.sv.vt.edu/future/vt-cave/apps/detour
  • Pose
  • Position
  • Orientation
  • What do we want to track?
  • Head pose
  • Hand pose
  • Other body part
  • Other objects (e.g. spider)
  • So what does it mean if a tracking system reports
    your head at 2.5,3.3, 1.9?

4
Common Tracking Methods
  • GPS
  • Wii

http//www.directionsmag.com/images/articles/GPS_a
rticles/realtime_diff_GPS.jpg
5
Basic Idea
Trackers provide location and/or position
information relative to some coordinate
system. (x,y,z) (rx,ry,rz)
(0,0,0) Receiver coordinate system
(0,0,0) Origin for tracker coordinate system
6
Degrees of freedom
  • The 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

7
Question
  • Given that I want to track your head, I attach a
    new tracker from NewTracker Corp. it returns 6
    degrees of freedom (6 floats). What questions
    should you have?
  • What are some evaluation points for a tracking
    system?

8
Evaluation Criteria
  • Data returned
  • Spatial distortion (accuracy)
  • Resolution
  • Jitter (precision)
  • Drift
  • Lag
  • Update Rate
  • Range
  • Interference and noise
  • Mass, Inertia and Encumbrance
  • Number of Tracked Points Durability
  • Wireless
  • Price

Which of these are most important?
9
Performance Measures
  • Registration (Accuracy)
  • Difference between an objects pose and the
    reported pose
  • Location
  • Orientation
  • What are determining factors?
  • Resolution
  • Granularity that the tracking system can
    distinguish individual points or orientations
  • What are determining factors?
  • Jitter
  • Change in reported position of a stationary
    object
  • What are determining factors?
  • Drift
  • Steady increase in error with time
  • What are determining factors?

10
Performance Measures
  • t0 time when sensor is at point p
  • t1 time when sensor reports p
  • Lag or Latency t1 - t0
  • What makes up latency?
  • Acquisition
  • Transmission
  • Filtering

11
Performance Measures
  • t0 time when sensor is at point p
  • t1 time when sensor reports p
  • Lag or Latency t1 - t0
  • What makes up latency?
  • Acquisition
  • Transmission
  • Filtering
  • What is a minimum?

12
Update Rate
  • Number of tracker position/orientation samples
    per second
  • High update rate ! accuracy
  • Poor use of update information may result in more
    inaccuracy
  • Communication pathways and data packet size are
    important

13
Range
  • Working volume
  • What is the shape?
  • Accuracy decreases with distance
  • Range is inversely related to accuracy
  • Position and orientation range could be different
  • Sensitivity not uniform across all axis

14
Interference and Noise
  • Interference - external phenomenon that degrades
    systems performance
  • Each type of tracker has different causes of
    interference/noise
  • Occlusion
  • Metal
  • Noise
  • Environmental (e.g. door slamming, air
    conditioner)

15
Mass, Inertia and Encumbrance
  • Do you really want to wear this?
  • Inertia
  • Tethered

16
Multiple Tracked Points
  • Number of potentially tracked points
  • Unique
  • Simultaneous
  • Difficulties
  • Interference between the sensors
  • Multiplexing
  • Time Multiplexing Update rate of S samples per
    second and N sensors results in S/N samples per
    sensor per second
  • Frequency Multiplexing Each sensor broadcasts
    on a different frequency. More

17
Price
  • You get what you pay for. (30-100k)
  • Rich people are a small market.

18
Tracking Technologies
  • Different Tracking Technologies
  • Goals
  • Understand how they work
  • Understand tradeoffs
  • Know when to use which
  • Future directions

19
Mechanical Linkage
  • Rigid jointed structure
  • One end (base) is fixed
  • The other (distal) is free
  • Distal is user controlled to an arbitrary
    position and orientation.
  • Sensors at the joints detect the angle
  • Concatenate translates and rotates
  • Determine the position and orientation of the
    distal relative to the base.

20
Mechanical Tracking
  • Data returned 6 DOF
  • Spatial distortion 0.3381 mm
  • Resolution very high
  • Jitter (precision) very low
  • Drift - none
  • Lag gt5ms
  • Update Rate - 300 Hz
  • Range - 8 ft
  • Number of Tracked Points 1
  • Wireless - no
  • Interference and noise metal, earth
  • Mass, Inertia and Encumbrance substantial
  • Durability low
  • Price high
  • Pros
  • Accurate
  • Fast
  • Low lag
  • Minimal environmental interference
  • No calibration
  • Can incorporate force feedback
  • Cons
  • Low range (effectively 5 does not scale well)
  • Cost
  • 1 tracked point (body/others are hard to track)

21
Mechanical Tracking Products
  • Fake Space Labs BOOM Display (discontinued)
  • Sensible Phantom

22
Electromagnetic Trackers
  • Emitter
  • Apply current through coil
  • Magnetic field formed
  • 3 orthonormal coils to generate fields
  • Sensor
  • Strength attenuated by distance
  • 3 orthonormal magnetic-field-strength sensors
  • Determine the absolute position and orientation
    of a tracker relative to a source.
  • Polhemus (a.c.)
  • Ascension (d.c.)

23
Basic Principles of EM Trackers
  • Pulse the emitter coils in succession
  • Sensor contains 3 orthogonal coils
  • For each pulse, sensor measures the strength of
    the signal its 3 coils (9 total measurements)
  • Known
  • Pulse strength at the source
  • Attenuation rate of field strength with distance
  • Calculate position and orientation of the sensor
    coils

24
EM Trackers
  • Data returned 6 DOF
  • Spatial distortion 0.6 mm, 0.025
  • Resolution 0.00508 mm, 0.025 / inch from
    receiver
  • Jitter (precision) mm to cm
  • Drift - none
  • Lag reported 4 ms
  • Update Rate - 120 Hz
  • Range - 5 ft
  • Number of Tracked Points 16 (divides update
    rate)
  • Wireless - yes
  • Interference and noise metal, earth
  • Mass, Inertia and Encumbrance - minimal
  • Durability - high
  • Price - 4000

25
EM Trackers
  • Data returned 6 DOF
  • Spatial distortion 0.6 mm, 0.025
  • Resolution 0.00508 mm, 0.025 / inch from
    receiver
  • Jitter (precision) mm to cm
  • Drift - none
  • Lag reported 4 ms
  • Update Rate - 120 Hz
  • Range - 5 ft
  • Number of Tracked Points 16 (divides update
    rate)
  • Wireless - yes
  • Interference and noise metal, earth
  • Mass, Inertia and Encumbrance - minimal
  • Durability - high
  • Price - 4000
  • Pros
  • Measure position and orientation in 3D space
  • Does not require direct line of sight
  • Low encumbrance
  • Cost
  • Good performance close to emitter
  • Lag
  • Can be built into devices
  • Earth magnetic field good for 3DOF
  • Cons
  • Accuracy affected by
  • DC Ferrous metal and electromagnetic fields.
  • AC Metal and electromagnetic fields
  • Operate on only one side of the source (the
    working hemisphere)
  • Low range (effectively 5 does not scale well)
  • Calibration

26
EM Tracking
  • Ascension Flock of Birds
  • Polhemus Fastrak
  • Extremely popular
  • Good for many applications
  • CAVEs (remove metal)
  • HMDs
  • Projection displays
  • Fishtank

27
Acoustic/Ultrasonic Tracking
  • Time of Flight Tracking
  • Emitters
  • Multiple emitters
  • In succession, emit sound (record time)
  • Receiver
  • Report time of receiving sound
  • Frequency tuned
  • Calculate time-of-flight (1000 feet/sec)
  • Use ultrasonic (high) frequencies
  • Similar
  • EM tracking
  • Radar/sonar
  • Phase Coherence tracking
  • Orientation only
  • Check phase of received signal

28
Ultrasonic Tracking System Setup
How much data does 1 transmitter provide? How
much data do 2 transmitters provide? How much
data do 3 transmitters provide?
Stationary Origin (receivers)
Tracker (transmitters)
distance1
distance2
distance3
29
Acoustic/Ultrasonic Tracking Characteristics
  • Pros
  • Inexpensive
  • Wide area
  • Encumbrance
  • Cons
  • Inaccurate
  • Interference
  • Requires line-of-sight
  • Data returned 3 or 6 DOF
  • Spatial distortion low (good accuracy)
  • Resolution good
  • Jitter (precision) mm to cm
  • Drift - none
  • Lag very slow
  • Update Rate - 120 Hz
  • Range 40 (scaling issues)
  • Number of Tracked Points numerous
    (spread-spectrum)
  • Wireless - yes
  • Interference and noise medium, noise,
    environment
  • Mass, Inertia and Encumbrance - minimal
  • Durability - high
  • Price cheap to 12000

30
Ultrasonic Tracking Devices
  • Logitech
  • Mattel Power Glove
  • Intersense
  • Used as part of hybrid systems

31
Inertial Tracking
  • Electromechanical devices
  • Detect the relative motion of sensors
  • Measuring change
  • Acceleration (accelerometers)
  • Gyroscopic forces (electronic gyroscopes piezo
    electric)
  • Inclination (inclinometer)
  • Frameless tracking
  • Known start
  • Each reading updates current position

32
Accelerometers
  • Mounted on to body parts
  • Detects acceleration
  • Acceleration is integrated to find the velocity
  • Velocity is integrated to find position
  • Unencumbered and large area tracking possible
  • Difficult to factor out gravity

33
Accelerometer Tracking Errors
  • Suppose a constant error ?i, so that measured
    acceleration is ai(t) ?i
  • vi(t) ?(ai(t) ?i)dt ? ai(t)dt ?it
  • xi(t) ? vi(t)dt ???(? ai(t)dt ?t)dt
  • xi(t) ?? ai(t)dtdt 1/2 ?it2
  • Errors accumulate since each position is measured
    relative to the last position
  • Estimated 10 degrees per minute. How is this
    related to drift?

34
Inertial Tracking
  • Inclinometer
  • Measures inclination
  • Relative to some level position
  • Gyroscopes
  • Resist rotation
  • Measure resistance

35
Inertial Tracking Systems Characteristics
  • Data returned 3 or 6 DOF
  • Spatial distortion low (good accuracy)
  • Resolution good
  • Jitter (precision) low
  • Drift - high
  • Lag very low
  • Update Rate - high
  • Range very large
  • Number of Tracked Points 1
  • Wireless - yes
  • Interference and noise gravity
  • Mass, Inertia and Encumbrance - minimal
  • Durability - high
  • Price cheap
  • Pros
  • Inexpensive
  • Wide area
  • Orientation very accurate
  • Minimal interference
  • Encumbrance
  • Cons
  • Position poor
  • Need to recenter
  • Calibration
  • Inaccurate over time
  • Drift

36
Optical Trackers
  • Use vision based systems to track sensors
  • Outside-Looking In
  • Cameras (typically fixed) in the environment
  • Track a marked point
  • PPT tracker from WorldViz (www.worldviz.com)
  • Older optical trackers
  • Inside-Looking Out
  • Cameras carried by participant
  • Track makers (typically fixed) in the environment
  • Intersense Optical Tracker
  • 3rdTech HiBall Tracker

Image from High-Performance Wide- Area Optical
Tracking The HiBall Tracking System, Welch, et.
al. 1999.
37
Outside Looking In Optical Tracking
  • Precision Point Tracking by WorldViz
  • IR Filtered Cameras are calibrated
  • Intrinsics
  • Focal length, Center of projection, aspect ratio
  • Extrinicis
  • Position and orientation in world space
  • Each frame
  • Get latest images of point
  • Generate a ray (in world coordinates) through the
    point on the image plane
  • Triangulate to get position

38
Outside Looking In Optical Tracking
  • What factors play a role in O-L-I tracking?
  • Camera resolution
  • Frame rate
  • Camera calibration
  • Occlusion
  • CCD Quality
  • How does it do for
  • Position
  • stable, very good
  • Orientation
  • Unstable, poor
  • Latency
  • Cameras are 60Hz

39
Orientation
  • How to compensate for poor orientation?
  • Combine with orientation only sensor (ex.
    Intersenses InertiaCube)
  • Also known as
  • Hybrid tracker
  • Multi-modal tracker
  • Position vision
  • Orientation inertial

40
Inside-Looking-OutOptical Tracking
  • Tracking device carries the camera
  • Tracks markers in the environment
  • Intersense Tracker
  • 3rdTech HiBall Tracker

Images from High-Performance Wide- Area Optical
Tracking The HiBall Tracking System, Welch, et.
al. 1999.
41
HiBall Tracker
  • Position
  • Pretty good
  • Orientation
  • Very good
  • Latency
  • LEPDs can operate at 1500 Hz

Six Lateral Effect Photo Dioides (LEPDs)
in HiBall. Think 6 cameras.
42
LED Optical Trackers
  • Sensors
  • Webcameras
  • Photodiodes
  • Track
  • LEDs
  • Reflected LED light
  • Why LEDs?
  • Easy to track
  • Grab your webcam and point a remote at it
  • Super cheap
  • P5 Glove
  • Nintendo Wii
  • WorldViz PPT
  • Virtual Patients

43
Optical Tracking Review
  • Data returned 6 DOF
  • Spatial distortion very low (very good
    accuracy)
  • Resolution very good
  • Jitter (precision) very good
  • Drift - none
  • Lag moderate
  • Update Rate low - high
  • Range very large (40 x 40 )
  • Number of Tracked Points 4
  • Wireless - yes
  • Interference and noise occlusion
  • Mass, Inertia and Encumbrance - moderate
  • Durability low - high
  • Price cheap to very expensive
  • Pros
  • Inexpensive
  • Wide area
  • Very accurate
  • Cons
  • High quality is very expensive
  • Occlusion
  • Calibration

44
Hybrid Approaches
  • Nintendo Wii

45
Angle Measurement
  • Measurement of the bend of various joints in the
    users body
  • Used for
  • Reconstruction of the position of various body
    parts (hand, torso).
  • Measurement of the motion of the human body
    (medical)
  • Gestural Interfaces
  • Sign language

46
Angle Measurement Technology
  • Optical Sensors
  • Emitter and receiver on ends of sensor
  • As sensor is bent, the amount of light from
    emitter to receiver is attenuated
  • Attenuation is determined by bend angle
  • Examples Flexible hollow tubes, optical fibers
  • VPL Data Glove

47
Angle Measurement Technology (cont.)
  • Strain Sensors
  • Measure the mechanical strain as the sensor is
    bent.
  • May be mechanical or electrical in nature.
  • P5 Glove 25 (!)
  • Cyberglove (Virtual Technologies)

48
Joints and Cyberglove Sensors
Proximal Inter- phalangeal Joint (PIP)
Interphalangeal Joint (IP)
Metacarpophalangeal Joint (MCP)
Metacarpophalangeal Joint (MCP)
Abduction Sensors
Thumb Rotation Sensor
49
Angle Measurement Technology (cont.)
  • Exoskeletal Structures
  • Sensors mimic joint structure
  • Potentiometers or optical encoders in joints
    report bend
  • Exos Dexterous Hand Master

50
Other Techniques
  • Pinch Gloves
  • Have sensor contacts on the ends of each finger

51
Technology
Mechanical motion capture
  • Dataglove
  • Low accuracy
  • Focused resolution
  • Monkey
  • High accuracy
  • High data rate
  • Not realistic motion
  • No paid actor

52
Technology
  • Exoskeleton angle sensors
  • Analogous
  • Tethered
  • No identification problem
  • Realtime
  • No range limit
  • Rigid body approximation

53
Body Tracking Technology
  • Position Tracking
  • Orthogonal Electromagnetic Fields
  • Measurement of Mechanical Linkages
  • Ultrasonic Signals
  • Inertial Tracking
  • Optical Tracking
  • Inside Looking Out
  • Outside Looking In
  • Angle Measurement
  • Optical Sensors
  • Strain Sensors
  • Exoskeletal Structures
  • http//www.measurand.com/videos/ShapeTapeTheMovie.
    m1v

54
Recap Tracking Table
  • Focusing on Head and Hand Tracking
  • Data returned
  • Magnetic 6 DOF
  • Acoustic 3 DOF per sensor (need 3 to get 6 DOF)
  • Inertial 3 DOF
  • Optical 6 DOF
  • Spatial distortion (accuracy)
  • Magnetic good close to emitter, degrades quickly
  • Acoustic okay close to emitter
  • Inertial short time very good, poor due to drift
  • Optical okay (webcam) to very good accuracy
  • Resolution
  • Magnetic good close to emitter, degrades quickly
  • Acoustic okay close to emitter
  • Inertial very good
  • Optical okay (webcam) to very good accuracy
  • Jitter (precision)
  • Magnetic good close to emitter, degrades quickly
  • Acoustic okay close to emitter

55
Recap Tracking Table
  • Drift
  • Magnetic none
  • Acoustic none
  • Inertial substantial
  • Optical none
  • Lag
  • Magnetic low
  • Acoustic moderate
  • Inertial low
  • Optical low to moderate
  • Update Rate
  • Magnetic good
  • Acoustic poor
  • Inertial good
  • Optical poor to very good
  • Range
  • Magnetic 5
  • Acoustic 15
  • Inertial excellent

56
Recap Tracking Table
  • Number of Tracked Points
  • Magnetic 16
  • Acoustic 16
  • Inertial 1
  • Optical lt4
  • Wireless
  • Magnetic yes
  • Acoustic yes
  • Inertial yes
  • Optical yes
  • Interference and noise
  • Magnetic metal, Earth
  • Acoustic environment, occlusion
  • Inertial none
  • Optical occlusion

57
Recap Tracking Table
  • Mass, Inertia and Encumbrance
  • Magnetic low
  • Acoustic low
  • Inertial low
  • Optical low to high
  • Durability
  • Magnetic high
  • Acoustic high
  • Inertial high
  • Optical low
  • Price
  • Magnetic 4000
  • Acoustic 4000
  • Inertial very cheap
  • Optical cheap (wecams) - 180k (motion capture
    systems)
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