Re: Re: Color saturation and contrast

[P3D Joerg Meyer Inf.] <jmeyer@xxxxxxxxxxxxxx>]

On Sat, 24 May 1997 17:41:37 -0400 John Ohrt <johrt@xxxxxxx> wrote:

> If the intensity is 50%, the hue is red, then with no saturation the
> pixel will appear  red.  With 100% saturation, the pixel will appear
> white regardless of the hue, and somewhere in between, the pixel will
> appear pink.  If the intensity is 0%, the pixel will appear black
> regardless of the hue and saturation, and if the intensity is 100% the
> color will be the most intense red available.

I think you're mixing up two models here: HSV (hue, saturation, value)
and HLS (hue, lightness, saturation). If you project the RGB cube
along the diagonal axis from white (1,1,1) to black (0,0,0), you'll
obtain a pyramid with a hexagon as the base plane. H (hue) refers to
the angle from the center of the plane, S (saturation) is the distance
from the center, and V (value) is the line between the tip of the
pyramid and the center of the base plane. This means that the vertical
axis (S = 0) is a grey scale from black (tip) to white (center of the
base plane). A pure color can be selected using H = alpha, V = S = 1.
White can be added by reducing S, black can be added by reducing V.

Similar to HSV, the HLS model uses a color circle to describe H (hue).
L = 0.5 is the center of the circle, L = 0 and L = 1 are the opposite
ends of two pyramids. S (saturation) again is the distance from the
center line. A pure color (H = alpha, S = 1) thus must be located
on the base plane of the opponent pyramids (L = 0.5). The vertical axis
(S = 0) is a grey scale from one tip (L = 0, black) to the other (L = 1,
white).

Increasing saturation not necessarily increases contrast, since contrast
is a term which is not included in one of these color theories. Contrast
refers to a difference between two colors, and it depends on the measure
(RGB distance, luminosity, etc.) which describes it. For example, if
you have two colors with L = 0.3 and L = 0.4 respectively (difference:
C = 0.1), and you double the lightness (L = 0.6 and L =0.8 resp.),
the contrast also doubles (C = 0.2). 

To find out what happens if you increase saturation, you could convert
an HSV or HLS color to the RGB model and calculate the RGB distance.


Joerg

    _V_     | Joerg Meyer ----- University of Kaiserslautern, Germany
   /   \    | Department of Computer Science, Computer Graphics Group
   |O O|    | mailto:jmeyer@xxxxxxxxxxxxxx --------------------------
ooO--U--Ooo | http://davinci.informatik.uni-kl.de/~jmeyer -----------


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