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Comp 790 - Computational Photography
Spatially Varying White Balance
Megha PandeySept. 16, 2008
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Color Constancy• Color Constancy – interpretation of material colors
independent of surrounding illumination.
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Color Constancy• Color Constancy – interpretation of material colors
independent of surrounding illumination.
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Digital Images under Varying Illumination• Cameras can not adapt to varying illumination as
humans do – images have a color cast depending on the light source.
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• Cameras can not adapt to varying illumination as humans do – images have a color cast depending on the light source.
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Color Temperature• Color temperature of a
light source is the temperature of an ideal black body radiator at which the color of the color of the light source and the black body are identical.
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Incandescent Light – Orange Color Cast
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Moonlight – Blue Color Cast
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Fluorescent Light : Green Color Cast
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Color Balance• Color Balance – adjusting the color components to
eliminate color casts.• Chromatic Adaptation : estimation of representation of object as it
would appear under a different light source than the one in which it was recorded.
• White Balance – aims to render neutral colors correctly to emulate the property of color constancy
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• Color Balance – adjusting the color components to eliminate color casts.
• White Balance – aims to render neutral casts correctly to render visually pleasing images.
• white balanced image
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White Balance Tools
Digital Cameras
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Auto White Balance
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White Balance Caps
Gray Cards
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• Take a picture of a neutral object (white or gray)
• Deduce the weight of each channel– If the object is recoded as R’w,
G’w, B’wuse weights 1/R’w, 1/G’w, 1/B’w
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Auto WB Custom WB
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Color Correction Filters
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Mixed Lighting
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Light Filters
Gel Filters
Light Filters
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White Balance under Mixed Lighting• Barnard [1997] – adaptation of gamut-based color constancy technique,Assumes smooth illumination
• Kawakami [2005] – outdoor scenes with hard shadows, illuminants restricted to black-body radiators
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• Lischinski [2006] – user scribbles, correct localized color casts
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• Ebner [2004] – local color shifts, “Gray World Assumption”
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Local Color Shift
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Light Mixture Estimation forSpatially Varying White Balance
Eugene HsuTom MertensSylvain ParisShai AvidanFredo Durand
(Several slides from Eugene Hsu)
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Algorithm Overview• Recovers the dominant material colors and uses them to
estimate the relative proportion of the two light colors at each of the pixels.
Input image illuminated by two light types
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Voting scheme to recover dominant material colors inthe scene.
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Estimate light mixture at reliable pixels and interpolate missing values.
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Estimated light mixture is used to achievespatially varying white balance.
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Assumptions• Two light sources – specified by the user
• Interaction of light can be described using RGB channels only
• Surfaces are Lambertian and non-fluorescent - which implies that the image color is the product of illumination and reflectance.
• Color bleeding due to indirect illumination can be ignored
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Image Formation Model
Observed pixel color is material color multipliedby scaled light color.
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White Balance
Proper white balance is achieved by inverting the effect of the light source color.
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Proper white balance is achieved by inverting the effect of the light source color.
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Image model with two light sources
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Proper white balance can be achieved if the relativeproportion of the two light sources is known.
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Solving for α is under-constrained since the actualmaterial colors are not given.
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Material Color Estimation• Assume scene is dominated by a small set of
material colors, hence reflectance spectra is sparse.
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Material Color Estimation• Assume scene is dominated by a small set of
material colors, hence reflectance spectra is sparse.
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Material Color Estimation• Assume scene is dominated by a small set of
material colors, hence reflectance spectra is sparse.
Scene viewed in white light
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Material Color Estimation• Assume scene is dominated by a small set of
material colors, hence reflectance spectra is sparse.
Scene viewed in mixed light
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Sample material colors and find the one that accountsfor the observed color of most pixels.
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Given a candidate material color…
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…a pixel votes for a material color only if the observedcolor can be explained by a combination of given lightsources.
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If this expression holds, we say that the pixel votes for the material color.
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48%
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48 %
16 %
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Light mixture estimation for reliable pixels
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Mixture Interpolation
Assume L1B and L2B are 1, divide out the blue channels.
This looks exactly like Image Matting.
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Interpolation is performed using Matting Laplacian[Levin et al. 2006]
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Scene shot with multiple exposures so that ground truthis available.
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Constraint the marked points and interpolate the rest
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Smooth interpolation is pretty bad.
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Edge-aware interpolation doesn’t work satisfactorilyeither.
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Matting Laplacian gives much better result.
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Experiments – Synthetic Data
Input
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Output
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Input Output
Ground Truth Comparison
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Experiments – Real Data
Input Alpha Map Output
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Input
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Output
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Input
LME Local Color Shift
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LME Local Color Shift
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Scene Relighting
Separate the two lighting contributions from the whiteBalanced image
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• The observed scene is a blend of two images as seen by either of the light sources in proportions αand 1- α.
Multiply the white balanced image by α for the firstcontribution
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Multiply the white balanced image by α for the firstContribution and by 1- α for the second contribution
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We can choose new lights and add desired effects.
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Input
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Output
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Discussion• Works best for raw image data
• Better results for indoor scenes
• Handles specularities and inter-reflections
• Material colors should exhibit enough color variation for the voting to work.
• Accurate specification of light sources is required.
• Scalability Issues