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(0,255,255)
a)
b)
(255,255,255)
(Cyan)
(White)
(0,255,0)
Green
(Green)
(255,255,0)
Yellow
Cyan
Green
(Yellow)
Colour
wheel
White
Magenta
(Blue)
(0,0,255)
Red
Blue
B lu e
(Magenta)
(255,0,255)
(Black)
Green
(0,0,0)
(Red)
Re d
c)
(255,0,0)
e
Saturatio n
Grey
Hue
Red
Blue
-
-
-
-
Figure 2.31 Hue, saturation and intensity. (a) The primary and secondary colours of light. (b) The red
green
blue (RGB) and hue
saturation
intensity (HSI) colour models. Redrawn, with permission, from Drury ( 1987 ) . (c) Colour wheel showing changes in tone across a circle
whose axis is the intensity axis. Hue changes around the circumference, saturation varies from pure spectral colour at the circumference
through pastel colours to grey at the centre, and intensity changes along the central axis.
( Fig. 2.31a ) . The HSI model is based on the polar coord-
inate system whose origin is that of the RGB colour
space. Colours are represented by hue and saturation,
which are de ned in terms of a colour wheel centred
on the intensity axis of the RGB colour model. Hue varies
around the circumference with the primary and second-
ary colours occurring at equally spaced intervals
( Fig. 2.31c ) . Saturation is the radial distance from the
axis, so lines of equal saturation form concentric circles
about the intensity axis. Zero saturation occurs on the
intensity axis. Hue can be thought of as the dominant
colour itself
2.8.2.2 Look-up tables
The human eye can perceive many thousands of different
colours; however, the enormous number of colours avail-
able in the 24-bit RGB colour model is unnecessary in
practice as the human eye is unable to resolve them all.
Instead, this can be reduced to 256 selected colours, as
there is normally only minor visible difference between
an image displayed using the full 24-bit colour spectrum
and one using 256 colours selected from across the full
spectrum. This also signi
cantly reduces the computa-
tional resources needed to produce the image. The 256
colours together constitute a colour map or look-up table
(LUT).
The LUT is simply a table containing the colours de ned
by their coordinates in the RGB colour space, with each
colour being identi ed by its position in the table (0 to
255). Numerous LUTs are in common use, with many
image processing systems allowing the user to design their
own. A simple example is a grey-scale look-up table, where
the colours vary from black to white via increasingly lighter
shades of grey ( Fig. 2.32a ) . In terms of the RGB colour
space, the 256 grey-scale values all lie on the intensity axis.
-
-
orange, red, purple etc.
whilst saturation
represents the
of the colour. For example, a
pure red colour has high saturation, but pastel shades or
lavender or pink have a lower saturation. Moving the
colour wheel down the axis reduces intensity, so the
colours darken but are otherwise unaltered, whilst
moving up the axis produces brighter colours. Note that
although a pixel
'
whiteness
'
s colour may be defined in terms of HSI,
it has to be converted to the equivalent coordinates in the
RGB space for display, since RGB is the basis on which
the hardware creates the colours.
'
 
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