Image Processing Reference
In-Depth Information
Fig. 3.9 The RGB color
cube. Each dot corresponds to
a particular pixel value.
Multiple dots on the same
lineallhavethesamecolor,
but different levels of
illumination
Fig. 3.10 ( a ) The triangle
where all color vectors pass
through. The value of a point
on the triangle is defined
using normalized RGB
coordinates. ( b )The
chromaticity plane
3.2.3 The Normalized RGB Color Representation
If we have the following three RGB pixel values ( 0 , 50 , 0 ) , ( 0 , 100 , 0 ) , and
( 0 , 223 , 0 ) in the RGB color cube, we can see that they all lie on the same vec-
tor, namely the one spanned by ( 0 , 0 , 0 ) and ( 0 , 255 , 0 ) . We say that all values are a
shade of green and go even further and say that they all have the same color (green),
but different levels of illumination. This also applies to the rest of the color cube.
For example, the points ( 40 , 20 , 50 ) , ( 100 , 50 , 125 ) and ( 200 , 100 , 250 ) all lie on
the same vector and therefore have the same color, but just different illumination
levels. This is illustrated in Fig. 3.9 .
If we generalize this idea of different points on the same line having the same
color, then we can see that all possible lines pass through the triangle defined by
the points ( 1 , 0 , 0 ) , ( 0 , 1 , 0 ) and ( 0 , 0 , 1 ) , see Fig. 3.10 (a). The actual point (r,g,b)
where a line intersects the triangle is found as 2 :
R
R + G + B ,
G
R + G + B ,
B
R + G + B
(r,g,b) =
(3.5)
These values are named normalized RGB and denoted (r,g,b) . In Table 3.3 the
rgb values of some RGB values are shown. Note that each value is in the interval
[
0 , 1
]
and that r
+
g
+
b
=
1. This means that if we know two of the normalized
2 Note that the formula is undefined for (R,G,B) = ( 0 , 0 , 0 ) . We therefore make the following
definition: (r,g,b) ( 0 , 0 , 0 ) when (R,G,B) = ( 0 , 0 , 0 ) .
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