ECE 449CS 443: COMPUTER VISON LECTURE 11

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ECE 449/CS 443 COMPUTER VISON LECTURE 11

• Colour II
• Colour Matching
• Colour Models
• Colour Interpretation
• Reading
• Colour (Chapter 4)
• For next time Linear Filters and Convolution
  (Ch. 8)
• Solution of problem 2.5
• See www-cvr.ai.uiuc.edu/ponce/spring02/sol1.ps
• Lecture notes
• See www-cvr.ai.uiuc.edu/ponce/spring02/lect.pp
  t

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Colour Matching Experiments
Adjust the knobs on the primaries until the split
field looks uniform.
T w1P1w2P2 wkPk
Notation only!
Subtractive matching
Tw1P1 wnPn wn1Pn1 wkPk
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Trichromacy
Three primaries are sufficient to match any test
light!

• Note
• This requires allowing subtractive matching.
• For a given set of primaries and test light,
  people
• tend to pick the same weights.
• This is related to our three types of cones.

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Linearity of Colour Matching ( Grassmans Laws)
Ta wa1P1wa2P2wa3P3 and Tb
wb1P1wb2P2wb3P3
l Tam Tb (l wa1m wb1)P1(l wa2m wb2)P2 (l
wa3m wb3)P3
Ta w1P1w2P2w3P3 and Tb w1P1w2P2w3P3
Ta Tb
Note this does not mean that Ta and Tb have the
same spectrum.
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Principle of Univariance

• The eye photoreceptors do not directly measure
• spectral radiance.
• Two lights will match if they produce the same
• photoreceptor responses even if they have quite
• different spectral radiances.
• Because of linearity, the response of each
• photoreceptor can be modeled as
• PksL sk(l)E(l)dl.
• The sensitivity sk can be measured
• experimentally.

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Spectral sensitivity of the three types of cones
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Colour Matching Functions

• Problem given a set of primaries, what are the
  weights
• matching a given spectral radiance?

• Experiments

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l
Colour matching functions
L(l) f1(l)P1f2(l)P2f3(l)P3

• To match S(l), use linearity

S(l)(sL f1(l)S(l)dl)P1(sL f1(l)S(l)dl)P2(sL
f1(l)S(l)dl)P3
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RGB Colour Matching Functions
R645.16nm G526.32nm B444.44nm
Negative weights
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A rather poor reproduction of the RGB
colour cube..
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CIE XYZ Colour Matching Functions
Note there are no physical XYZ primaries!
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CIE XYZ and xy spaces
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Simple Colour Models for Images
Geometric terms
C(P) Gd(P) D(P) Gs(P) S(P) others..
Image color
Color terms
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Colour Interpretation (Klinker and Shafer, 1987)
Metal
Dielectric material
Assume a single dielectric object with uniform
reflectance is observed.
C(P) Gd(P) D Gs(P) S

• Diffuse component Line through the origin
• Specular component Second line

Dog-leg pattern
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The dog-leg pattern
Dichromatic plane
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The structure of dog-leg patterns