Tracking

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Tracking

• 6 Degrees of Freedom (dof)
• Evaluation Criteria
• Technologies

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Tracking

• Head position and orientation tracking
• Hand position, orientation and pose
• Other body parts (e.g., avatars)
• Other objects that also have physical
  representations (spider).

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Six degrees of freedom
Position in 3D space (x,y,z) Yaw (azimuth) Pitch
(elevation) Roll
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Evaluation Criteria

• Resolution
• Registration
• Lag
• Update Rate
• Range

• Interference and noise
• Mass, Inertia and Encumbrance
• Multiple Tracked Points
• Price

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Resolution

• Fineness with which the tracking system can
  distinguish individual points in space.
• Measurement resolution the ability of the
  tracker to measure different points
• Numerical accuracy bits of precision
• Related to accuracy
• Static accuracy
• Dynamic accuracy

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Accuracy/Registration

• Correspondence between actual position and
  orientation and reported position and
  orientation.
• Calibration processes are used to measure and
  adjust for the differences between reported and
  actual position.
• Crucial for Augmented Reality applications

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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.
• Data generation (latency). The rate at which the
  acquisition portion of the system can acquire new
  data.
• Transmission lag. Sending the 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.

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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

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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
• Video Signal Processing
• Video Sensing of LEDs

• 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
  16 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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Performance of EM Tracking

• To calculate the size of one data packet
• Number of bits associated with each data byte
• (Number of data bitsnumber of start/stop
  bitsParity checking bit) (number of bytes in a
  data record)
• Divide by baud rate of the serial port

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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 (treadmill, Rebar!)
• No identification problem
• 6DOF Realtime
• 30-144 Hz 13-18 markers

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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

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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 (PHAMToM)
• Used on the BOOM display system from Fake Space
  Labs

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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.
• Often 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

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Inertial Tracking

• Use 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

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Inertial 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

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Other Tracking Methods

• Video Signal Processing
• Process a video image of the body to determine
  the position of various body parts.
• Works for 2D
• Usually requires the user to wear special markers
  on the body

• Video Sensing of (infrared) Light Emitting Diodes
  (LEDs)
• LEDs are monitored by several video cameras
• Positions are inferred through signal processing
  techniques.

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Technology

• Passive reflection Peak Performance Tech
• Hand or semi-automatically digitized
• Video
• Time consuming
• Issues
• No glossy or reflective materials
• Tight clothing
• Marker occlusion by props
• High frames/sec

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Technology

• Passive reflection --Acclaim, Motion Analysis
• Automatically digitized
• 240Hz
• Not real-time, Correspondence
• 3 markers/body part
• 2 cameras for 3D position data

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Technology

• Active light sources -- Optotrak
• Automatically digitized
• 256 markers
• 3500 marker/sec
• Real-time
• Specialized cameras

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Technology

• Technology issues
• Resolution/range of motion
• Calibration
• Accuracy
• Occlusion/Correspondence

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Animation Issues

• Style
• Scaling
• Generalization

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Resolution

• Positioning of camera

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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

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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
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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

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Other Techniques

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

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Technology
Mechanical motion capture

• Dataglove
• Low accuracy
• Focused resolution

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

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Technology

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