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Research Background:Depth Exam Presentation
Susan Kolakowski
March 20, 2006
Committee:
Juan Cockburn, Chair
Jeff Pelz, Adviser
Andrew Herbert
Mitchell Rosen
Carl Salvaggio
Research Background
• Introduction
• Human Visual System
• Eye Movements
• Eye Trackers– RIT Wearable Eye Tracker
• My Research
Introduction
• Why are eye trackers used?– Objective measure of where people look– Interest in Human Visual System
• Examples:– Understanding Behaviors: How do humans read?– Improving Skill: Train people to move their eyes as
an expert would.– Improving Quality: What parts of an image are
important to the image’s overall quality?
Human Visual System
• What we see is determined by– How our rods and cones are connected and
distributed– How our brain processes this information– What we already accept as truth (previous
knowledge)– How we move our eyes throughout a scene
The Retina• Contains two types of photoreceptors
– Rods that offer wide field of view (and night vision)– Cones that provide high acuity (and color vision)
Affect of Previous Knowledge
Rotating Mask
The Fovea
• At its center: contains only cones (no rods)
• Perceive greatest detail and color vision– To get the most detailed representation of a
scene, must move your eyes rapidly so that different areas of the scene fall on your fovea
• Along visual axis - lowest potential for aberrations
Eye Movements…• Saccades
• Smooth Pursuit
• Optokinesis (OKN)
• Vestibular-Ocular Reflex (VOR)
• Fixations
… and lack thereof
Fixations• Stabilizations of the eye for higher acuity
at a given point
• Drifts and tremors of the eye occur during fixations such that the view is always changing slightly
Saccades• Rapid ballistic movement of eye from one
position to another
• Shift point of gaze such that a new region falls on the fovea
Eye Movements
X X X
Smooth Pursuit• Smooth eye movement to track a moving
target
• Involuntary - can’t be produced without a moving object
Eye Movements
X X
Optokinesis• Invoked to stabilize an image on the retina
• Eye rotates with large object or with its field-of-view
Eye Movements
Vestibular-Ocular Reflex• Invoked to stabilize an image on the retina
• Stabilizes an image as the head or body moves relative to the image
X
Eye Trackers
• Invasive– Painful devices which discomfort subject’s eye
• Restrictive– Devices that require strict stabilization of
subject’s head, not allowing for natural movement
• Modern Video-Based Trackers– Remote - constrained to 2D stimuli– Head-mounted - allows natural movement
My Research
• Objective: Improve the performance of video-based eye trackers in the processing stage.– Compensate for camera movement with
respect to the subject’s head– Reduce noise
LOWER PRECISION
R.I.T. Wearable Eye Tracker• Advantage:
– Subject is less constrained, can perform more natural tasks
• Disadvantage:– Camera (eye tracker)
not stabilized - need to account for any movement of camera relative to head
Lower Precision• Need to account for movement of camera
with respect to the head requires additional data: corneal reflection
• Corneal Reflection data is not as precise as Pupil data.
Analysis of Disadvantages
Too bad we can’t just use the Pupil data
Oversimplifying Assumption• Assumption: When the camera moves
with respect to the head, the pupil and corneal reflection move the same amount.
• To account for camera movement:
Analysis of Disadvantages
€
P −CR
Why this assumption is wrong• Corneal Reflection data comes from the
center of the reflection off the curved outer surface of the eye
• Pupil data comes from the center of the flat virtual image of the pupil inside the eye.
DON’T MOVE THE SAME AMOUNT
WHEN THE CAMERA MOVES
Result of Oversimplification
• P-CR vector difference changes with camera movement– Artifacts in final data
X X X
The Solution
• Determine the actual relationship between the pupil and corneal reflection during BOTH:– Camera movements– Eye movements
• Use these relationships to develop a new equation in terms of pupil and corneal reflection position
Camera and Eye Gains• Eye Gain: amount corneal reflection moves when
pupil moves 1 degree during an eye movement
• Camera Gain: amount corneal reflection moves when pupil moves 1 degree during a camera movement€
eye_ gain =ΔCR
ΔP
⎛
⎝ ⎜
⎞
⎠ ⎟eye
≅ 0.5146
€
cam_ gain =ΔC
ΔP
⎛
⎝ ⎜
⎞
⎠ ⎟camera
≅ 0.8524
€
Pcam = Ptrack − Peye
The Equations
€
Pcam =Ptrack ⋅E −CRtrack
E −C
€
Pcam ⋅E = Ptrack ⋅E −CReye
€
Pcam ⋅E −CRcam = Ptrack ⋅E −CReye −CRcam
€
Pcam (E −C) = Ptrack ⋅E −CRtrack
4 Initial Equations
€
Peye, Pcam,CReye,CRcam4 Unknowns:€
Ptrack = Pcam + Peye(1)
€
CRtrack =CRcam +CReye(2)
€
E =CReyePeye
(3)
€
C =CRcamPcam
(4)
Added Benefit
• Can smooth Camera array without loss of information from Pupil array:
• Assuming camera moves more slowly than eye moves.
• Result is on same level as Pupil only data€
Peye = Ptrack − Pcam
Determining the Gains• Eye Gain: (Instruct subject to…)
– Look at center of field-of-view.– Keep camera and head perfectly still.– Look through calibration points.
• Cam Gain: (Instruct subject to…)– Look at center of field-of-view.– Keep eye fixated while moving the camera on
nose.
Testing the Algorithm• Collect data:
– 5 subjects look through 9 calibration points while moving the eye tracker’s headgear
• Extract eye movements:– Use average gains to calculate Camera array– Smoothed Camera array– Subtracted smoothed Camera array from
Pupil array Eye array
Conclusions
• Successful application to head-mounted video-based eye trackers– Use same gain values for all subjects
• Final Eye array precision is on the order of the Pupil array precision– Noise due to Corneal Reflection data is
reduced
Next Steps• Calibration - Eye array represents eye movement in head - need to map this to the world (via scene camera)
Next Steps• Calibration - Eye array represents
eye movement in head - need to map
this to the world (via scene camera)• Investigate realistic camera movements and
alternative smoothing options for Camera array
Next Steps• Calibration - Eye array represents
eye movement in head - need to map
this to the world (via scene camera)• Investigate realistic camera movements and
alternative smoothing options for Camera array• Obtain gain values for larger group of subjects
Next Steps• Calibration - Eye array represents
eye movement in head - need to map
this to the world (via scene camera)• Investigate realistic camera movements and
alternative smoothing options for Camera array• Obtain gain values for larger group of subjects• Test on larger eye movements
Next Steps• Calibration - Eye array represents
eye movement in head - need to map
this to the world (via scene camera)• Investigate realistic camera movements and
alternative smoothing options for Camera array• Obtain gain values for larger group of subjects• Test on larger eye movements• Revision for remote trackers