Polarization- Beneficial for
Visibility Enhancement?
Tali Treibitz, Yoav Y.
Schechner
Technion, Israel
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Visibility in Haze
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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total
intensity
I
L object
airlight
A
D
direct
transmission
object
radiance
scattering
D( x, y)
I (x, y)  Lobject (x, y) t( x, y)  A( x, y)
A
1
0
z
0
z
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Visibility in Haze
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
Haze Through a Polarizer
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best
polarized
image
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Haze Through a Polarizer
best
polarized
image
increased
exposure time
single frame- used by photographers
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Haze Through a Polarizer
best
polarized
image
worst
polarized
image
post-processing
2 frames
two frames- Schechner et al.
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
Is it worth using a polarizer?
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best polarized
image
unpolarized image
post-processing
2 frames
 goal: object detection
 local contrast stretch- OK
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Method-Independent Visibility Limits
 Noise induces a resolution cutoff, without imaging blur
 Independent of post processing
detection
no detection
system
input SNR
frequency
“Recovery Limits in Pointwise Degradations”, Treibitz & Schechner, ICCP ’09
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Method-Dependent Performance
gain SNR?
increase
acquisition time?
constraint: equal acquisition time
related works:
Optimization goal
Constraint
Hasinoff & Kutulakos ’08
acquisition time
depth of field & intensity
Levin et al. ’08
SNR in lightfield acquisition
number of sensors
Agrawal & Raskar ’09
(Wednesday)
SNR in motion deblurring
captured light
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Noise Model
I
 2 2  I / g
2
2
  200
2
I
photon noise dominates
courtesy of Brian Curless
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Degradations in Haze
deterministic
random
I (x, y)  Lobject (x, y) t( x, y)  A( x, y)  n( x, y )
reduce SNR
even if known
object -background
SNRhaze=
 A( x, y )  Lobject ( x, y )  t ( x, y )  / g
I/g
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Using a Single Polarized Image
Ibest
Best
polarized
image
Ibest (x, y ) 
D ( x, y )
1 p
 A( x, y)
2
2
p  0,1
degree of polarization
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SNR Comparison
I unpolarized
Ibest


equal acquisition time
acquisition time = exposure time X number of frames
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a single saturated image
SNRpolarized
SNRunpolarized
p
maximal value
in nature
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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same total acquisition time
SNRpolarized
SNRunpolarized
p
p<0.4
in our experiments
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
maximal value
in nature
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Experiment
larger FOV
I best
I unpolarized
- average of 2 images
same total acquisition time
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Polarization-Based Dehazing
Ibest
Best
polarized
image
I best (x, y ) 
D ( x, y )
1 p
 A( x, y )
2
2
p  0,1
degree of polarization
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Polarization-Based Dehazing
Iworst
Worst
polarized
image
I worst (x, y ) 
D ( x, y )
1 p
 A( x, y )
2
2
p  0,1
degree of polarization
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SNR Comparison
I unpolarized
dehazing
 1 p 


 p 


 1 p 


 p 


equal acquisition time
optimal
exposures
tbest
tworst
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“In defense of contrast stretch”
Our analysis: higher noise in dehazed image
I unpolarized
D( x )  I ( x )  A( x )
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
dehazing
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Advantages of Polarization-Based Dehazing
• contrast stretch in non-uniform distances
• color
• compensate for attenuation
• Goal is object detection
• Local contrast stretch on areas with similar distances
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Single Image Dehazing
• Tan, CVPR ‘08
• Fattal, SIGGRAPH ‘08
• Kopf et al., SIGGRAPH Asia ’08
• He et al., CVPR ‘09 (tomorrow- best paper session)
our context:
 Polarization does not give SNR advantage
 Puts these methods in a more favorable position
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?
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Summary
• Case study of method comparison
• Performance trade-offs
• Unpolarized images are best (equal acquisition time)
Treibitz & Schechner, Polarization- Beneficial for Visibility Enhancement?