Colour Constancy

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Colour Constancy

• T.W. Hung

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Colour Constancy Human

• A mechanism enables human to perceive constant
  colour of a surface over a wide range of lighting
  conditions.
• Land studied and attempted to explain the
  mechanism, Retinex theory.

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Colour Histogram 2 (picture 4) Image under biased
lighting condition
Colour Histogram 1 (picture 3) Image under
reference lighting condition
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Colour Constancy - Machine

• Problem
• Model
• What form of a transform used in the algorithm
  is sufficient to discount the effect of change of
  illuminant
• Mechanism
• How to compute the parameters of the desired
  transformation

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Common Assumption

• A set of squared colour patches
• Reasonable number of different coloured patches
• Surface property is Lambertian
• Single source of illumination
• Even illumination over the scene

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Model

• Coefficient Model Von Kries, Finlayson

Ci,j,1 T Ci,j,2
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Mechanism

• White patch algorithm
• Maximum to predefined maximum
• Grey world algorithm
• Average to predefined average
• Gamut mapping method
• Feasible mapping with Surface colour
  constraints, illumination constraints
• Specular highlight detection method
• Detect the presence of the specular highlight
• Retinex algorithm
• Average of logorithmic ratio of surface i and
  its neighborhood colours

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• All objects that are known to us from
  experience, or that we regard as familiar by
  their colour, we see through the spectacles of
  memory colour
• Hering 1878

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Problem

• To compute a transformation to match the colours
  in the image to a database of known colours.
• Form of transformation
• Fitness measure for the matching criteria
• Matching method

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Mahanobis eqn.
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Experimental setting
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Graph of lighting
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Picture
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Conclusion

• Description of the colour constancy problem
  model and mechanism
• Description of some of the existing algorithms
• Simple linear transformation
• Minimum error matching criteria
• Match to normalised colour model
• Adaptation to relative colour match, seems
  robust