Introduction to Tracking

1
Introduction to 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

• We want to use the human body as an input device
• more natural
• this will lead to higher level of immersion
• to control navigation
• head
• hand
• to control interaction
• head
• hand
• body
• We need two things for this
• Signaling (button presses, etc.)
• Location. lt- this is tracking!

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Tracking

• Position
• Location
• Orientation
• Pose
• Examples
• Head position
• Hand position (pose)
• Other body parts (e.g., self-avatars)
• Other objects that also have physical
  representations (spider).

4
Basic Idea
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) Receiver coordinate system
(0,0,0) Origin for tracker coordinate system
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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

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Question

• Okay, 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?
• In other words, what are some evaluation points
  for a tracking system?
• 5 minutes to discuss

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Evaluation Criteria

• Data returned (3 dof, 6 dof, gt6 dof)
• Spatial distortion (accuracy) (sub mm)
• Resolution (sub mm)
• Jitter (precision) (sub mm)
• Drift
• Lag (1 ms)
• Update Rate (2000 Hz)
• Range (40x40 GPS)

• Interference and noise
• Mass, Inertia and Encumbrance
• Multiple Tracked Points (1-4, 128)
• Durability (self-contained?)
• Wireless (yes)
• Price (1800 3dof - 40,000, 180k mocap)

Which of these are most important?
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Performance Measures

• Data returned
• Spatial distortion (accuracy)
• Resolution
• Jitter (precision)
• Drift
• Lag
• Update Rate
• Range

Reportable location and orientation based on
resolution
Jitter
Drift
Registration
Actual Object Position
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Performance Measures

• Registration (Accuracy) Represents the
  difference between an objects actual 3D position
  and the position reported by the tracker
• Location
• Orientation
• Resolution Fineness with which the tracking
  system can distinguish individual points or
  orientations in space.
• Jitter Change in reported position of a
  stationary object.
• Drift Steady increase in tracker error with
  time.

10
Performance Measures

• Lag (Phase Lag) Difference between when a
  sensor first arrives at a point and when the
  tracking system first reports that the sensor is
  at that point. Sometimes called latency.
• Latency The rate (or time delay) at which the
  acquisition portion of the system can acquire new
  data.
• Transmission Lag Time needed to send bits of
  information that define position to the computer
  or graphics engine.

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Update Rate

• Number of tracker position/orientation samples
  per second that are transmitted to the receiving
  computer.
• Fast update rate is not the same thing as
  accurate position information.
• Poor use of update information may result in more
  inaccuracy.
• Upper bound is determined by the communications
  rate between tracker and computer and the number
  of bits it takes to encode position and
  orientation.

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Range

• Position range or working volume
• Sphere (or hemisphere) around the transmitter.
• Accuracy decreases with distance
• Position range is inversely related to accuracy.
• Orientation Range set of sensor orientations
  that the tracking system can report with a given
  resolution.

13
Interference and Noise

• Interference is the action of some external
  phenomenon on the tracking system that causes the
  systems performance to degrade in some way.
• Noise random variation in an otherwise constant
  reading. (Static position resolution)
• Inaccuracies due to environmental objects.

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Mass, Inertia and Encumbrance

• Do you really want to wear this?
• Things with no weight on your head can have
  inertia.
• Tethered

15
Multiple Tracked Points

• Ability to track multiple sensors within the same
  working volume.
• 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

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Price

• You get what you pay for.
• Rich people are a small market.

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Body Tracking Technology

• Position Tracking
• Orthogonal Electromagnetic Fields
• Measurement of Mechanical Linkages
• Ultrasonic Signals
• Inertial Tracking
• Optical Tracking
• Inside Looking Out (Videometric)
• Outside Looking In

• Angle Measurement
• Optical Sensors
• Strain Sensors
• Exoskeletal Structures

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Electromagnetic Trackers

• Use the attenuation of oriented electromagnetic
  signals to determine the absolute position and
  orientation of a tracker relative to a source.
• Polhemus (a.c.)
• Ascension (d.c.)

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Basic Principles of EM Trackers

• Source contains 3 orthogonal coils that are
  pulsed in rotation, one after another.
• Each pulse transmits a radio frequency
  electromagnetic signal that is detected by a
  sensor.
• The sensor also contains 3 orthogonal coils,
  which measure the strength of the signal from the
  current source coil (9 total measurements)
• By using the known pulse strength at the source
  and the known attenuation of the strength with
  distance, these nine values can be used to
  calculate position and orientation of the sensor
  coils.

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Basic EM Principles (cont.)

• Source and sensor are connected to a box which
  contains a microcomputer and electronics
  associated with the pulses.
• Serial communications (serial port)
• A source may be associated with 1 to as many as
  18 sensors
• Problems Earths Magnetism!

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Characteristics of EM Trackers

• Measure position and orientation in 3D space
• Do not require direct line of sight between the
  source and the sensor
• 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).
• Working distance of about 3-25? feet from source.
  (Depends on source size, power)

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Output of EM Trackers

• Polhemus (AC)
• Position 3 Integers
• Orientation Euler angles,Directional Cosines,
  Quaternions
• Ascension (DC)
• Position 3 Integers
• Orientation Euler angles, 3x3 Rotation Matrices

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Technology

• Electromagnetic Transducers
• Ascension Flock of Birds, etc
• Polhemus Fastrak, etc
• Limited range/resolution
• Tethered (cables to box)
• Metal in environment
• No identification problem
• 6DOF Realtime
• 30-144 Hz 13-18 sensors

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Example

• 6 bytes for position (3 two-byte integers)
• 18 bytes for orientation (9 two-byte integers of
  a 3x3 orientation matrix).
• 3 byte header
• 8 data bits and 1 stop bit, no start or parity
  bits (9 bits/byte)
• Total per data packet 279 243 bits
• 19,200 baud
• 13 millisecond transmission time
• 79 packets/second
• Now all USB

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Lag between actual and rendered position

• Time to acquire and compute position and
  orientation
• Transmission time (0.013 seconds for example for
  one sensor).
• Graphics Frame rate (10-60 frames/sec)

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Mechanical Linkage

• Jointed structure that is rigid except at the
  joints.
• One end (base) is fixed.
• The other (free, distal) end may be moved to an
  arbitrary position and orientation.
• Sensors at the joints, detect the angle of the
  joints.
• Concatenation of translates and rotates can be
  used to determine the position and orientation of
  the distal end relative to the base.

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Characteristics of ML

• Fast
• Accurate
• Depends on the physical size of the ML
• Depends on quality of rotation sensors at joints
• Encumbered Movement
• Expensive
• Can incorporate force feedback (PHANToM)
• Used on the BOOM display system from Fake Space
  Labs

28
Sensible Technolgies Phantom
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Ultrasonic Tracking

• Use the time-of-flight of an ultrasonic sound
  pulse from an emitter to a receiver. Either the
  emitter or the receiver can be fixed, with the
  other free to move.
• Logitec
• Mattel Power Glove
• A component of Intersense
• Inertial Ultrasonic systems

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Basic Principles of UT

• Based on measurement of time-of-flight of a sound
  signal. 1000 feet/Sec
• Source component contains transmitters that
  produce a short burst of sound at a fixed
  ultrasonic frequency.
• The sensor component contains microphones that
  are tuned to the frequency of the sources.

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UT Characteristics

• Inexpensive (Used in Mattell Powerglove 100).
• Inaccurate.
• Echoes and other ambient noise
• Require a clear line-of-sight between the emitter
  and the receiver.
• Sometimes used for head-tracking for CRT displays.

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Basic UT Setup
Stationary Origin (receivers)
Tracker (transmitters)
distance1
distance2
distance3
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UT Position and Orientation Information

• 1 transmitter, 3 receivers 3D position relative
  to fixed origin
• 2 transmitters, 3 receivers 3D position and
  orientation up to a roll around a line through
  the two transmitters
• 3 transmitters, 3 receivers complete position
  and orientation information

34
Inertial Tracking

• Uses electromechanical devices to detect the
  relative motion of sensors by measuring change
  in
• Acceleration
• Gyroscopic forces
• Inclination

35
Accelerometers

• Mounted on a body part to detect acceleration of
  that body part.
• Acceleration is integrated to find the velocity
  which is then integrated to find position.
• Unencumbered and large area tracking possible

36
Accelerometer Tracking Errors

• Suppose the acceleration is measured with 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

37
Inertial Tracking

• Inclinometer measures inclination relative to
  some level position

• Gyroscopes

38
Optical Trackers

• 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, tracking 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.
39
Outside Looking In Optical Tracking

• Precision Point Tracking by WorldViz
• IR Filtered Cameras are calibrated
• Each frame
• Get latest images of point
• Generate a ray (in world coordinates) through the
  point on the image plane
• Triangulate to get position

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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

41
Orientation

• Since orientation is poor, you can get an
  orientation only sensor (ex. Intersenses
  InertiaCube)
• Called a hybrid tracker or multi-modal
  tracker
• Position vision
• Orientation inertial

42
Inside-Looking-OutOptical Tracking

• The tracking device carries the camera that
  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.
43
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.
44
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

45
Angle Measurement Technology

• Optical Sensors
• Have an emitter on one end and a receiver on the
  other.
• As the sensor is bent, the amount of light that
  gets from the emitter to the receiver is
  attenuated in a way that is determined by the
  angle of the bend.
• Examples Flexible hollow tubes, optical fibers
• VPL Data Glove

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Angle Measurement Technology (cont.)

• Strain Sensors
• Measure the mechanical strain as the sensor is
  bent.
• May be mechanical or electrical in nature.
• Cyberglove (Virtual Technologies)

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Joints and Cyberglove Sensors
Proximal Inter- phalangeal Joint (PIP)
Interphalangeal Joint (IP)
Metacarpophalangeal Joint (MCP)
Metacarpophalangeal Joint (MCP)
Abduction Sensors
Thumb Rotation Sensor
48
Cyberglove Accuracy
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Cyberglove Accuracy (Adj.)
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Angle Measurement Technology (cont.)

• Exoskeletal Structures
• Sensors which attach a rigid jointed structure to
  the body segments on either side of a joint.
• As the joint bends, the angle between the body
  segments is measured via potentiometers or
  optical encoders in the joints of the
  exoskeleton.
• Exos Dexterous Hand Master

51
Other Techniques

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

52
Technology
Mechanical motion capture

• Dataglove
• Low accuracy
• Focused resolution

• Monkey
• High accuracy
• High data rate
• Not realistic motion
• No paid actor

53
Technology

• Exoskeleton angle sensors
• Analogous
• Tethered
• No identification problem
• Realtime - 500Hz
• No range limit - Fit
• Rigid body approximation