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The chromatic space defined in that way introduces some losses (it is not possible
to revert to the full RGB space of colours). However, simultaneously the space does
not include the information about luminance which causes an object moving in an
unevenly illuminated scene to have identical description in the
R
c
G
c
space all the
time. For example, for
R
c
=
3, it is known that all the compound
colours have the same share in the original RGB space, although it is not possible to
determine whether the colour is black, grey, or white. Another advantage of the
R
c
G
c
colour space is the simplicity of its specification and lower dimension of histograms
based on it.
In order to limit dimensions of the histogram descriptors and to reduce their
internal redundancy, they are calculated for each colour channel independently. Fur-
thermore, histograms bin size is equal to 4, so each colour channel is represented by
64 bins in the histogram (assuming 8 bits per channel, i.e. 256 levels). Therefore, the
HistFull
descriptor contains 192 elements and
Hist
—128 elements.
1
/
3 and
G
c
=
1
/
12.4.2 Vertical Trace
One of the methods to describe an object is by its colour characteristics. This can
be achieved by calculating the image colour trace along a defined direction [
26
]. In
case of images representing people, the most reasonable direction is vertical, since
more information about colour distribution can be acquired (Fig.
12.4
). However,
this descriptor can be utilized for other objects as well, assuming that all of them
share the same orientation. Trace can be calculated in several ways, for example as
the mean or median of slices along the orientation.
As the trace length is dependent on the described object dimensions, during
the preprocessing stage, the image needs to be normalized. With this approach,
192-element feature vector is built, containing 64 values for each of the colour
channels (
VertTrace
descriptor). To introduce description invariance, feature
extraction procedure is performed for transformed colour representation, defined
by the following equation:
Average colour value
for each descriptor field
Fig. 12.4
Vertical trace calculation
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