Introduction to Light Scattering:

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• Introduction to Light Scattering
• An Imaging Sciences Perspective
• Lecture 19

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• First, lets look at some pretty pictures

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Distant objects appear Bright !
Mountains
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De-hazed
Haze
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Bad Weather
Mist
Haze
Rain
Fog
Images Courtesy Steve and Carol Sheldon
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More Weather
Non-uniform Fog
Rain Drops and Rain Streaks
Snow Flakes and Snow Streaks
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How often do we see Bad Weather?
Clear Sunny (77)
Bad Weather (23)
Manhattan, Every Hour, 12 Months
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Natural illumination in Scattering Media
Narasimhan and Nayar, 99 - 03, Schechner et al,
01, 04
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Glows of Light Sources
Mist
Fog
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Active illumination in Scattering Media
Levoy et al., Narasimhan-Nayar, Kocak-Caimi,
Jaffe et al., Schechner et al., Negahdaripour et
al.
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Floodlighting is Bad in Scattering Media
Remember Driving in Fog at Night?
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Is BRDF sufficient for Translucent
Objects ?
Jensen et al., 2001
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Translucent Objects
Koenderink and van Doorn, 2001
Clouds
Milk
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Rendering Milk
Jensen et al., 2001
Whole Milk
Skim Milk
Diffuse BRDF
BSSRDF Model
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Rendering a Marble Bust
Jensen et al., 2001
BRDF
BSSRDF Model
Full Monte Carlo Simulation of Radiative
transport
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Rendering Moon
Jensen et al., 2001
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Computer Vision
Optics
Underwater Imaging
Computer Graphics
Satellite Imaging
Medical Imaging
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Scattering in different fields
Art - 500-600 years Physics - 250
years Astrophysics/Astronomy - 80-100
years Atmospheric Optics - 80-100
years Medical Imaging - 30 years Remote
Sensing - 30 years Oceanic Engineering - 30
years Computer Graphics - 20 years Computer
Vision - 5-10 years
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The Fundamental Assumption in Vision
Lighting
No Change in Radiance
Surface
Camera
Assumption We live in Vacuum!
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Radiation Fog
Advection Fog
Simulated Foggy Image
Actual Clear Day Image
Simulated Foggy Image
Actual Clear Day Image
Dense Aerosols with Drizzle
Haze
Simulated Hazy Image
Actual Clear Day Image
Simulated Foggy Image
Actual Clear Day Image
Urban Aerosol with Moderate Rain
Fog with Cumulus Clouds
Simulated Foggy Image
Simulated Foggy Image
Actual Clear Day Image
Actual Clear Day Image
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Driving in Bad Weather
People tend to drive fast in fog!! Nature, 1998
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Weather Conditions and Particles
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CONDITION PARTICLE TYPE RADIUS (?m)
CONCENTRATION(cm )
AIR HAZE FOG CLOUD RAIN
Molecule Aerosol Water Droplet Water
Droplet Water Drop
( Mie 1908, McCartney 1975 )
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Particle Scattering Mechanisms
( Mie 1908 )
Single Scattering
Incident Beam

Size 0.01
Size 0.1
Size 1
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Multiple Scattering in the Atmosphere
Incident Beam
Particle
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Properties of Scattering Media
Scattering Coefficient Fractional loss in
intensity due to scattering per
unit cross section Absorption Coefficient
Fractional loss in intensity due to
absorption per unit cross
section Extinction Coefficient Scattering
Coefficient Absorption Coefficient
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Phase Function

• Probability of light getting scattered in a
  single direction
• Phase function integrates to 1
• Light Scattered in any direction
• Phase function Scattering Coefficient

Exiting Direction
Incident Direction
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Attenuation Model Zeroth Order Scattering
Scattering Medium
Attenuated Exiting Light
Incident Light
X d
Unit Cross Section
dx
X 0
Brightness at Distance d
( Bouguers Law, 1729 )
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Direct Transmission
Attenuation of Diverging Beams
( Allards Law, 1876 )
Optical Thickness
Scaled Depth
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Airlight Model First Order Scattering
( Koschmeider, 1924 )
Sunlight
Diffuse Skylight
dV
Observer
Object
d
Diffuse Ground Light
Brightness due to a Path of Length d
Horizon Brightness
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Distant objects appear Bright !
Mountains
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Structure from Airlight
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How does Brightness/Color vary with Distance?
Object
Observer
d
Color
Color
Distance
Distance
Attenuation
Airlight
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Contrast Degradation in Bad Weather
Irradiance Attenuation
Airlight


Scattering Coefficient
Reflectance
Horizon Brightness
Depth
(1)
(2)
Contrast between Iso-Depth points , P
and P
Contrast Decay Exponential in Scene Depth
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Depth Edges vs. Reflectance Edges
Mild Fog
Denser Fog
Reflectance Edge
Depth Edge
Normalized SSD of Depth Edge Neighborhood
Normalized SSD of Reflectance Edge
Neighborhood
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Edge Classification from Weather Changes
Edge Classification
Mild Fog
Denser Fog
Reflectance Edge Depth Edge
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Scene Structure from Weather Changes
Irradiance under versus
Irradiance under Linear
All Scene points at Depth 1
All Scene points at Depth 2
Scaled Depth
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Gray World Contrast Restoration and Structure
3D Visualization
Deweathering
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Contrast Restoration and 3D Structure
Dense Fog, 530 PM
Mild Fog, 5 PM
Contrast Restored Image
Computed Depth Map (20 levels)
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Defogging Videos
Foggy Video
Defogged Video
Histogram Equalized Video
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Scattering and Wavelength
constant
Rayleighs Law
(0 4)
Smaller the particles, larger the dependence on
wavelength Blue skies through pure air (small
particles) Fog looks greyish (whitish) larger
water droplets.
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Clear Day from Hazy Days
Unknown Hazy Conditions
Time 300 PM
Time 330 PM
( Narasimhan et. al, IJCV 2002)
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Clear Day from Hazy Day Using Polarizing Filters
De-hazed
Haze
Airlight is Partially Polarized
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Model
I
A
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Model
camera
2 input images
I
A
transmission
airlight
_
polarization degree
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Scattering from Near-Field Sources
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Scattering from Near-Field Sources
Loss of contrast
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Scattering from Near-Field Sources
Dimming and blur
Loss of contrast
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Scattering from Near-Field Sources
Glows
Dimming and blur
Lost of contrast
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Light Transport in Clear Day
Point Source
Viewer
Surface Point
Near-Field Divergent Sources
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Light Transport in Scattering Media
Point Source
Viewer
Surface Point
Clear Day
Foggy Day
Clear Day
Foggy Day
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Complexity of Rendering Scattering Media
Objects
Virtual Viewpoint
Virtual Screen
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Complexity of Rendering Scattering Media
Objects
Virtual Viewpoint
Virtual Screen
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Complexity of Rendering Scattering Media
Objects
Virtual Viewpoint
Virtual Screen
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Complexity of Rendering Scattering Media
Objects
Virtual Viewpoint
Virtual Screen
640 x 480 (image) x 4 (lights) x 50 (steps)
100 ( directions ) x 50 (steps) x 30
(intersect) ?
1.9 Trillion Calculations 3.0 GHz
CPU?
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