Download Tracking - CENG METU

Ceng 701 - Tracking
Gökhan Tekkaya
Gürkan Vural
Can Eroğul
METU, 2008
1
Outline
– Overview
– Tracking Devices
Electromagnetic
Mechanical
Acoustic
Optical
Inertial
Combinations
– Eye Tracking
– Finger/Hand Tracking
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Tracking - Overview
Purpose:
– The primary purpose of tracking is to
update the visual display based on the
viewers head position and orientation.
– Instead of tracking the viewer's eyes
directly, we track the position and
orientation of the user's head. From this
we determine the position and
orientation of the two eyes.
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Tracking - Overview
We may also be tracking the user's
hand(s), fingers, legs or other interface
devices.
Want tracking to be as 'invisible' as
possible to the user.
Want the user to be able to move freely
with few encumberances
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Tracking - Overview
Want to be able to have multiple 'guests'
nearby
Want to track as many objects as necessary
Want to have minimal delay between
movement of an object and the detection of
the objects new position / orientation (< 50
msec total)
Want tracking to be accurate (1mm and 1
degree)
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Outline
– Overview
– Tracking Devices
Electromagnetic
Mechanical
Acoustic
Optical
Inertial
Combinations
– Eye Tracking
– Finger/Hand Tracking
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Tracking Devices - Electromagnetic
large transmitter and one or more small
sensors.
transmitter emits an electromagnetic field.
sensors report the strength of that field at
their location to a computer
sensors can be polled specifically by the
computer or transmit continuously.
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Tracking Devices - Electromagnetic
uses:
– projection based systems, HMDs
advantages are:
–
–
–
–
large tracking volume (approx 10 feet / 3 meters)
no line-of-sight restriction
sensors are small and light
technology has been around for a while
disadvantages are:
–
–
–
–
affected by metal in the nearby area
latency can be high (0.1 seconds)
accuracy is low in large volumes
somewhat expensive
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Tracking Devices - Electromagnetic
Polhemus:
– Motion Tracking:
LIBERTY LATUS
LIBERTY
PATRIOT
FASTRAK
MINUTEMAN
– Scanning:
FastSCAN
– Eye Tracking:
VisionTrak
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Tracking Devices - Electromagnetic
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Tracking Devices - Mechanical
rigid structures with
multiple joints
one end is fixed, the other
is the object being
tracked
could be tracking users
head, or their hand
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Tracking Devices - Mechanical
physically measure the rotation about
joints in the armature to compute position
and orientation
structure is counter-weighted - movements
are slow and smooth
Knowing the length of each joint and the
rotation at each joint, location and
orientation of the end point is easy to
compute.
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Tracking Devices - Mechanical
uses:
– BOOMs, Phantom
advantages are:
–
–
–
–
low latency
high accuracy
sensors are small and light
technology has been around for a while
disadvantages are:
– small volume
– only track one object at a time
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Tracking Devices - Acoustic
small transmitter and one medium sized
sensor
each transmitter emits ultrasonic pulses
which are received by microphones on the
sensor (usually arranged in a triangle)
as the pulses will reach the different
microphones at slightly different times, the
position and orientation of the transmitter
can be determined
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Tracking Devices - Acoustic
Transmit acoustic pulse
Listen for arrival (or echo)
Inside-out or outside-in
– Outside-in
Measure position
Sound wave field, maybe
Multipath interference
– Inside-out
Measure position and orientation
Must wait for receipt of signal
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Tracking Devices - Acoustic
advantages are:
– Small and lightweight
– Not magnetic
– Relatively little infrastructure
disadvantages are:
– Line-of-sight requirement
– Acoustic interference (e.g. jingle)
– Ambient temperature concerns
– PC wave ambiguity
– PC error accumulation
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Tracking Devices - Acoustic
Logitech:
– 6 degrees of freedom
– 3D and 2D modes
– Ultrasonic Technology
– High level of ruggedness
– PC and Unix compatibility
– 250 dpi resolution/ 3D mode
– 400 dpi resolution/ 2D mode
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Tracking Devices - Optical
LEDs or reflective
materials are placed
on the object to be
tracked
video cameras at
fixed locations
capture the scene
image processing
techniques are used
to locate the object
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Tracking Devices - Optical
Inside looking out
– beacons/landmarks are fixed in the environment
– user-mounted sensors
– accurate orientation
Outside looking in
– sensors are fixed in the environment
– user-mounted beacons/landmarks
– orientation can be a problem as distance to target grows
Hybrid or cascaded
– head tracked inside-out from room
– hands tracked outside in from head
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Tracking Devices - Optical
A. R. T. system from
Germany:
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Tracking Devices - Inertial
Sense rate, integrate once
Sense acceleration, integrate twice
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Tracking Devices - Inertial
self-contained
gyroscopes / accelerometers used
knowing where the object was and its
change in position / orientation the device
and 'know' where it now is
tend to work for limited periods of time
then drift.
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Tracking Devices - Inertial
advantages are:
– Truly sourceless
– Very very fast
– Robust
disadvantages are:
– Bias, scale, and alignment errors
– Bias integration results in drift
– gravity vector kills you
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Tracking Devices - Combinations
Intersense uses a combination on Acoustic and
Inertial. Inertial can deal with fast movements
and acoustic keeps the inertial from drifting
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Outline
– Overview
– Tracking Devices
Electromagnetic
Mechanical
Acoustic
Optical
Inertial
Combinations
– Eye Tracking
– Finger/Hand Tracking
25
Eye Tracking
Eye tracking is the
process of
measuring either
the point of gaze
"where we are
looking" or the
motion of an eye
relative to the head.
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Eye Tracker
Eye tracker is a device for measuring eye
positions and eye movements
Eye movements are typically divided into
two episodes:
– fixation: the eye gaze pauses in a certain
position
– saccades: the eye moves to another position
The resulting series of fixations and
saccades is called a scanpath
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Scanpath of Eye
An example of fixations
and saccades over text
The typical pattern of eye
movements during
reading
Never move smoothly
over still text
Most information is made
available during a fixation
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Application Areas
of Eye Trackers
Psychology (notably psycholinguistics, the
visual world paradigm)
Cognitive science
Marketing research
Medical research (neurological diagnosis)
Human-computer interaction
Vehicle Simulators
Communication systems for disabled
...
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Types of Eye Trackers - I
Optical - Using
video images
– the eye position is
extracted from video
images
– a camera focuses
on one or both eyes
and records their
movement as the
viewer looks at
some kind of
stimulus
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Optical Eye Trackers - I
modern eyetrackers use
– contrast to locate
the center of the
pupil
– infrared and nearinfrared noncollimated light to
create a corneal
reflection (CR).
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Optical Eye Trackers - II
two general techniques based on the
location of the illumination source
– Bright Pupil
creates greater iris/pupil contrast allowing for more
robust eye tracking with all iris pigmentation
allows for tracking in lighting conditions ranging from
total darkness to very bright
– Dark Pupil
not effective for tracking outdoors as extraneous IR
sources interfere with monitoring
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Optical Eye Trackers - III
use a sampling rate of at least 30 Hz
although 50/60 Hz is most common
modern video-based eye trackers run at
240, 350 or even 1000/1250 Hz
– needed in order to capture the detail of the very
rapid eye movements during reading, or during
studies of neurology
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Types of Eye Trackers - II
Search coils
– known as magnetic
eye trackers
– uses coils that are
embedded into a
tightly-fitting contact
lens or a rubber ring
that adheres to eye
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Magnetic Eye Trackers
Alternating magnetic field with horizontal
and vertical components is used to induce
electric currents in these coils
These currents depend on the eye position
– on the angle by which the eye is rotated
– thus provide a measure of the eye position
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Types of Eye Trackers - III
Electrooculogram
– pairs of electrodes
are placed either
above and below the
eye or to the left and
right of the eye
– a potential difference
occurs between the
electrodes
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Electrooculogram
If the eye is moved from the center
position towards one electrode, this
electrode "sees" the positive side of the
retina and the opposite electrode "sees"
the negative side of the retina
A potential difference occurs between the
electrodes
Assuming that the resting potential is
constant, the recorded potential is a
measure for the eye position
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Setups of Eye Trackers
Setups vary greatly
– some are headmounted
– some require the
head to be stable
(for example, with a
chin rest)
– some function
remotely and
automatically track
the head during
motion
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Sample Devices - eyebox2
attached to store
displays or billboards
to count how many
times viewed by
passersby
uses video images
infrared leds create
red eyes
from up to 10 meters
price $1500
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Sample Devices - Lexus LS460
the vehicle with the
capacity to assess in
real-time the visual
behavior of the driver
automobiles with the
ability to monitor
drowsiness,
inattention, and
cognitive engagement
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Outline
– Overview
– Tracking Devices
Electromagnetic
Mechanical
Acoustic
Optical
Inertial
Combinations
– Eye Tracking
– Finger/Hand Tracking
41