ASAM Image Processing 20082009

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ASAM - Image Processing2008/2009

• Lecture 11
• Colour spaces

Ioannis Ivrissimtzis 29-Jan-2009
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Overview

• RGB
• CMY - CMYK
• HSV
• HSI

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RGB

• The RGB colour space is related to human vision
  through the
• tristimulus theory of colour vision.
• The RGB is an additive colour model. The primary
  colours red, green
• and blue are combined to reproduce other colours.
• In the RGB colour space, a colour is represented
  by a triplet (r,g,b)
• r gives the intensity of the red component
• g gives the intensity of the green component
• b gives the intensity of the blue component
• Here we assume that r,g,b are real numbers in the
  interval 0,1.
• You will often see the values of r,g,b as
  integers in the interval 0,255.

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RGB

• As a colour space, RGB is represented by a cube.
  The black is the
• (0,0,0) while the white is the (1,1,1).

0,1,1 Cyan
1,1,1 White
0,1,0 Green
1,1,0 Yellow
G
B
0,0,1 Blue
1,0,1 Magenta
R
0,0,0 Black
1,0,0 Red
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RGB

• Example 11.1 Working in the RGB space, create
  squares coloured with a mix of blue and white in
  proportions
• (1,0), (3/4,1/4), (1/2,1/2), (1/4,3/4),
  (0,1).

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Overview

• RGB
• CMY - CMYK
• HSV
• HSI

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CMY

• The CMY is a subtractive colour model based on
  mixing pigments of
• Cyan, Magenta and Yellow to create other colours.
  In CMY, the colours
• that are seen are the part of the visible
  spectrum that is not absorbed by
• the pigments.
• For example, yellow pigments are seen as yellow
  because absorb blue
• and reflect red and green, while magenta plus
  yellow pigments are seen
• as red because absorb green and blue and reflect
  red.
• CMY is used when applying pigments on a white
  background, as for
• example ink on paper.

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CMY

• CMY cartridges for colour printers.
• http//www.usa.canon.com

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CMY

• The CMY colour space is also represented by a
  unit cube. The white is the (0,0,0) and the black
  is the (1,1,1).

0,1,1 Red
1,1,1 Black
1,1,0 Blue
0,1,0 Magenta
M
Y
0,0,1 Yellow
1,0,1 Green
C
0,0,0 White
1,0,0 Cyan
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CMY

• The conversion from RGB to CMY is given by the
  formula
• Example 11.2 The red colour is written in RGB as
  (1,0,0). In CMY it is written as
• that is, magenta and yellow.

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CMY

• Example 11.3 The magenta is written in CMY as
  (0,1,0). In RGB it is
• written as
• giving,
• that is, red and blue.

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CMYK

• The CMYK colour space is an extension of the CMY.
• CMYK is derived from CMY with the addition of a
  fourth component K,
• which is pure black and absorbs all the light in
  the visible spectrum.
• In CMY the black is represented by (1,1,1) and
  generated by mixing
• cyan, magenta and yellow. But, in practice, due
  to pigment
• imperfections, it is not as dark as the genuine
  black ink.
• Instead, four-colour CMYK printing gives visually
  superior results with
• greater contrast.

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CMYK

• CMYK cartridges for colour printers.

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Overview

• RGB
• CMY - CMYK
• HSV
• HSI

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HSV

• The components of the HSV colour space are Hue,
  Saturation and Value.
• Colour is the result of the perception of light
  at different wavelengths. Usually, we do not
  experience light at a single wavelength but a
  blend of waves at different wavelengths. The hue
  corresponds to the dominant wavelength and
  determines the type of the colour, for example
  red, yellow, or blue.
• The saturation determines the purity of the
  colour. High saturation gives
• pure colours (narrow wavelength band), while low
  saturation means
• colours mixed with a lot of white (white light
  combines all the visible
• wavelengths).
• The value determines the brightness. A value
  equal to zero represents
• absence of light, while a high value gives bright
  colours.

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Hue

• The HSV spectrum of bright pure colours ( s 1,
  v 1 ).
• Notice the cyclicality of the hue. The red
  corresponds both to h0
• and h1.

h 0
h 1
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Saturation

• Bright colours (v1) with decreasing values of
  saturation.

s 1
s 0.25
s 0.75
s 0.5
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Saturation
The Red Roomby Henri Matisse In the early
years of the century, the so-called FAUVE artists
("wild beasts") in France, led by Henri Matisse,
experimented with vivid, highly saturated colours
and bold brushwork to evoke intense emotional
responses.

http//www.nga.gov/collection/gallery/20cent.shtm
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Saturation
The Yosemite Valley California by Frederick
Schafer The painting is very bright, using
unsaturated colours throughout except in the
green foliage of a few conifers in the middle
distance.
http//ffscat.csail.mit.edu/ffshtml/descrs/FFSd033
6.htm
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Saturation
The Sower by Vincent van Gogh. Although the
effective areas of yellow and blue are different,
neither colour dominates over the other. By
breaking up the blues with unsaturated yellows in
the field, the artist has made the conversation
much more dynamic and interesting.
http//www.physics.utoledo.edu/lsa/_color/24_oppo
nent.htm
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Value

• Pure colours (s1) with decreasing values of
  value (brightness).

v 1
v 0.5
v 0.25
v 0.75
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HSV space

• In HSV, the hue is mapped cyclically. Red
  corresponds to both h0 and h1.

The HSV space can be described by a cylinder.
h 0
h 1
http//edoc.hu-berlin.de
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HSV space

• When we map the RGB cube on the HSV cylinder we
  get a subset of this cylinder in the shape of a
  pyramid with a hexagonal base.

http//edoc.hu-berlin.de
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HSV

• Example 11.4 Check the colour space Java applets
• http//olli.informatik.uni-oldenburg.de/Grafiti3/g
  rafiti/flow12/page5.html
• http//www.cs.rit.edu/ncs/color/a_spaces.html

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Overview

• RGB
• CMY - CMYK
• HSV
• HSI

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HSI

• HSI is a colour space similar to the HSV space.
• Its three components are Hue, Saturation and
  Intensity.

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HSI space (description)

• The HSI colour space can be represented
  geometrically by two pyramids connected at their
  triangular bases.

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HSI space (description)

• The segment connecting the apices is called the
  intensity axis and contains the grey values.

The intersection of a plane perpendicular to the
intensity axis and the HSI space is a triangle
with constant intensity. The three vertices of
these triangles are shades of red, green and
blue.
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HSI space (description)

• The saturation of a colour P, corresponds to the
  distance between P and the centre of the triangle
  W.
• The hue corresponds to the angle between WR and
  WP where R is the red vertex of the triangle.

P
W
R
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RGB to HSI (interpretation)

• The HSI colour space was constructed by a natural
  mapping of the RGB cube on the dipyramid.

The greyscale colours of the RGB cube are on the
diagonal between the black and the white. They
are mapped on the intensity axis. All colours
on the intersection of the cube with a plane
perpendicular to that diagonal will have constant
intensity.
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RGB to HSI (interpretation)

• Consider a triangle with vertices the black the
  white and a third colour (let say cyan).
• All the colours inside the triangle will have
  cyan hue because they are linear combination of
  cyan with black and white.

To obtain a different hue we rotate the shaded
plane around the intensity axis. The hue is
defined by the angle of such a rotation.
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RGB to HSI (definition)

• The RGB colour (R,G,B) is mapped to the HSI
  colour (H,S,I) with

with
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RGB to HSI (definition)
and
and
This mapping can be seen as the definition of the
HSI space. http//turing.iimas.unam.mx/elena/PD
I-Lic/hsi-to-rgb-conversion.pdf