Color

1
Color

• Jim Rehg
• CS 4495/7495 Computer Vision
• Lecture 22-24
• Wed Oct 9 (11 16) 2002

2
Outline

• Role of color vision
• Radiometry of color
• Examples of color spectra
• Color mixing
• Color space models

3
What is color?

• A perceptual attribute of objects and scenes
  constructed by the visual system
• A quantity related to the wavelength of light in
  the visible spectrum
• A box of Crayola crayons
• A significant industry with conferences,
  standards bodies, etc.
• A challenge
• There are no second-rate brains in color
  vision Edwin Land

4
Why is color a useful attribute?

• Distinguish food from nonfood
• Recognize predators and prey
• Find a persons skin
• Check health, fitness, etc. of other individuals.
• Segment (group together) pixel regions belonging
  to the same object.

5
Human Photoreceptors
Fovea
Periphery
Distribution of cones as a function of distance
from the fovea
6
Human Cone Sensitivities

• Spectral sensitivity of L, M, S cones in human
  eye
• Wavelength loss due to cornea, lens, inert
  pigments plays a role.

7
Reflectance Model
8
Transmittance Model
9
Illumination Spectra
10
Reflectance Spectra
11
Color Names for Cartoon Spectra
12
Additive Color Mixing
13
Subtractive Color Mixing
14
Color Matching Process
Basis for industrial color standards and
pointwise color models.
15
Color Matching Experiment 1
Image courtesy Bill Freeman
16
Color Matching Experiment 1
Image courtesy Bill Freeman
17
Color Matching Experiment 1
Image courtesy Bill Freeman
18
Color Matching Experiment 1
Image courtesy Bill Freeman
19
Color Matching Experiment 2
Image courtesy Bill Freeman
20
Color Matching Experiment 2
Image courtesy Bill Freeman
21
Color Matching Experiment 2
Image courtesy Bill Freeman
22
Color Matching Experiment 2
Image courtesy Bill Freeman
23
Principle of Trichromaticity
24
Superposition in Color Matching
25
Grassmans Laws
26
Color Spaces

• Use color matching functions to define a
  coordinate system for color.
• Each color can be assigned a triple of
  coordinates with respect to some color space
  (e.g. RGB).
• Devices (monitors, printers, projectors) and
  computers can communicate colors precisely.

27
A qualitative rendering of the CIE (x,y) space.
The blobby region represents visible colors.
There are sets of (x, y) coordinates that dont
represent real colors, because the primaries are
not real lights (so that the color matching
functions could be positive everywhere).
Slide courtesy Forsyth and Ponce
28
A plot of the CIE (x,y) space. We show the
spectral locus (the colors of monochromatic
lights) and the black-body locus (the colors of
heated black-bodies). I have also plotted the
range of typical incandescent lighting.
Slide courtesy Forsyth and Ponce
29
Finding Linear Color Models