Bio-medical Ontologies Maintenance and Change Management

94 L. Stanescu, D. Dan Burdescu, and M. Brezovan
The distance between two stimulus is calculated using [86]:
ΔE* uv = [(ΔL* ) 2
+ (Δu* ) 2
+ (Δv* ) 2
] ½
(3.7)
CIE L*a*b* (CIELAB) is the most complete color model used conventionally
to describe all the colors visible to the human eye. It was developed for this specific
purpose by C.I.E. The three basic features represent the lightness of the color
(L*, L* = 0 yields black and L* = 100 indicates white), its position between magenta
and green (a*, negative values indicate green while positive values indicate
magenta) and its position between yellow and blue (b*, negative values indicate
blue and positive values indicate yellow). The L*a*b* color model has been created
to serve as a device independent model to be used as a reference [52].
The values for a* and b* are resulted from [52]:
a* = 500 [ (X/X ) n 1/2 - (Y/Y ) n 1/2 ]
b* = 200 [ (Y/Y ) n 1/2 - (Z/Z ) n 1/2 (3.8)
]
The color distance between two color stimulus is calculated using [82]:
ΔE* = [(ΔL* ) ab 2 + (Δa* ) 2 + (Δb* ) 2 ] 1/2 (3.9)
These two color systems have the following characteristics [29]: device independent,
perceptual uniform, not intuitive, nonlinear transformation and dependent
on viewing direction, object geometry, direction, intensity or color of the illumination
and highlights.
3.4.6 HSV Color System
Alvey Ray Smith created the HSV color space in 1978. The HSV name is from the
initials Hue, Saturation and Value. The HSV space describes the colors in more
natural terms, close to the artists’ concepts. The HSV color space is cylindrical, as
presented in figure 3.3 [84].
The Hue or color is provided by the angle around the axis. The hue with the 0
value is red. The green is for an angle of 120° and the blue is for an angle of 240°.
w c = T c v c
v c = (r,g,b) w c = (h,s,v)
Fig. 3.3. T c transformation from RGB to HSV