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Removing Weather Effects fromMonochrome Images
Srinivasa Narasimhan and Shree Nayar
Computer Science DepartmentColumbia University
IEEE CVPR ConferenceDecember 2001, Hawaii, USA
SponsorsDARPA HID, NSF
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How does scene contrast degrade in bad weather ?
How can scene contrast be restored from bad weather images ?
Contrast Degradation in Bad Weather
RainFog
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Weather Effects are Depth Dependent
Image Processing Does Not Suffice
Histogram Equalized Images
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Prior Methods for Contrast Restoration
Yitzhaky, Kopeika [98]
Oakley, Tan, Satherley[98,01]
Nayar, Narasimhan [99]Narasimhan, Nayar [00]
Scene DepthWeather
Information
RequiredPredicted Weather PSF
Required
ComputedWavelengthIndependentScattering
Not Required
MethodClear-day SceneIntensity/Color
Computed
Gaussiandistribution
assumed
Computed(Color Images Required)
OUR GOAL : ComputedNot Required Computed
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Scattering Models : Attenuation and Airlight
Object
Observer
d
Attenuation
d
SunlightDiffuseSkylight
DiffuseGround Light
Airlight
E
d
deEE )1( deEE
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Contrast Degradation in Bad Weather
Irradiance = Attenuation + Airlight
)(dE += deE )1( deE
HorizonBrightness
Depth Reflectance ScatteringCoefficient
Contrast Decay : Exponential in Scene Depth
)1(2)2()1(
)2()1(
)2()1(
)2()1(
12
deEE
EEC
Contrast between Iso-Depth points , P and P :(1) (2)
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Mild Fog Denser Fog
Depth Edge Reflectance Edge
Depth Edges vs. Reflectance Edges
( 2) ( 2)Normalized SSD of
Reflectance Edge NeighborhoodNormalized SSD of
Depth Edge Neighborhood
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Edge Classification from Weather Changes
Mild Fog Denser Fog Edge Classification
Reflectance Edge : Depth Edge :
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Scene Structure from Weather Changes
Irradiance under versus Irradiance under : Linear 1 21E 2E
2
1
2
1 lnln)(2
112
E
E
EE
EEdScaled Depth :
1E
2E
1E
2E
)1( )()( 12
2
12
1
2 dd eEeE
E
1E2E
All Scene pointsat Depth 1
All Scene pointsat Depth 2
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Depth Map from Two Weather Conditions
Mild Fog, 5 PM (Input)
Denser Fog , 5: 30 PM (Input)
Computed Depth Map (Output)
Comparing with Prior Methods:
• Color Images Not Needed
• Works for Wider Range of Weather Conditions
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Weather Removal Using Scene Structure
Contrast Restored Image (Output)Dense Fog, 5:30 PM (Input)
Computed Depth Map (Input) Histogram Equalized Image (For comparison)
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Different amounts of Fog removed from different Depths
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A De-Weathering System
DetectSignificantWeatherChange
Video Frame (Weather 1)
Video Frame(Weather 2)
Scene Structure
System Initialization : Computing Scene Structure
Continuous De-Weathering Using Scene Structure
Scene Structure
ContrastRestored
Video Frame