Bio-medical Ontologies Maintenance and Change Management

Multimedia Medical Databases 91
differentiate colors excepting the blue and green. The cons can differentiate the
colors and increase the image clarity, but cannot work in low light. That is the
reason why in twilit light we cannot see clear, the colors “disappear” and
everything have bluish and greenish hues. In these conditions works only the rods.
If the light is very bright, it works only the cons. There are three types of cons,
each with its own spectral sensibility. They are red, green and blue depending to
their sensibility to these colors. The cons are grouped in the posterior side of the
retina, in the so-called yellow spot. Most of the rods are positioned outside the
yellow spot, along with few rare cons [96].
In colors perception, the color is not only a wavelengths collection but it is also
influenced by the adjacent colors. This process if called chromatic induction.
As we have presented above, the perception of color images is a complex
process. To simplify this, several suppositions are being made. The first
supposition is that the color is a process that involves a single pixel. The colors
perception is not influenced by the surrounding colors.
It is supposed that some factors can be ignored: the distance to the color, the
display quality and the ambient light. In general, these parameters are hard to be
controlled in the content-based retrieval applications.
3.4 Color Systems
Taking into consideration that the content-based visual query process must use
color systems with certain properties, some of them will be presented in detail.
The studies made on color images showed that these color systems are the best
candidates for usage in this process. It is also presented the RGB color system
because the other color spaces result by linear or non-linear transformations of it.
3.4.1 RGB Color System
The first color space that is presented is the RGB color space (figure 3.2) [29, 36].
This color space is built on a cube with Cartesian coordinates. Each dimension of
the cube represents a basic color. Each point inside the cube represents a certain
hue. The coordinates of a point specify the contribution of each basic color to
the final color. The values of the coordinates for each point must be between
0 and 255.
The pure red has the coordinates (255, 0, 0), the pure green has the coordinates
(0, 255, 0) and the blue’s coordinates are (0, 0, 255). The yellow is (255, 255, 0)
and the orange is between red and yellow and has the coordinates 255, 127, 0).
The diagonal between black (0, 0, 0) and white (255, 255, 255) gives different
grey hues.
This model has the possibility to represent 256 3
meaning more than 16 million
colors.
This color system has the following characteristics [29]: device dependent, not
perceptual uniform (that is why adjacent colors in RGB space don’t imply color