User Guide

GNU Image Manipulation Program
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RGB is a color model which has components for Red, Green and Blue. These colors are emitted by screen elements and
not reflected as they are with paint. The resulting color is a combination of the three primary RGB colors, with different
degrees of lightness. If you look closely at your television screen, whose pitch is less than that of a computer screen, you
can see the red, green and blue elements lit with different intensities. The RGB color model is additive.
GIMP uses eight bits per channel for each primary color. That means there are 256 intensities (Values) available, resulting
in 256×256×256 = 16,777,216 colors.
It is not obvious why a given combination of primary colors produces a particular color. Why, for instance, does
229R+205G+229B give a shade of pink? This depends upon the human eye and brain. There is no color in nature,
only a continuous spectrum of wavelengths of light. There are three kinds of cones in the retina. The same wavelength
of light acting upon the three types of cones stimulates each of them differently, and the mind has learned, after several
million years of evolution, how to recognize a color from these differences.
It is easy to see that no light (0R+0G+0B) produces complete darkness, black, and that full light (255R+255G+255B)
produces white. Equal intensity on all color channels produces a level of gray. That is why there can only be 256 gray
levels in GIMP.
Mixing two Primary colors in RGB mode gives a Secondary color, that is, a color in the CMY model. Thus combining
Red and Green gives Yellow, Green and Blue give Cyan, Blue and Red give Magenta. Don’t confuse secondary colors
with Complementary colors which are directly opposite a primary color in the chromatic circle:
Le mélange des couleurs primaires deux à deux en mode RVB donne les couleurs secondaires qui sont les couleurs du
mode CMJ : la combinaison du rouge et du vert donne du jaune, le vert et le bleu donnent du cyan (bleu clair), le bleu et le
rouge donnent du magenta (violet). Ne pas confondre les couleurs secondaires avec les couleurs complémentaires qui sont
diamétralement opposées aux couleurs primaires dans le cercle chromatique :
Figure 17.3: Colorcircle
Mixing a primary color with its complementary color gives gray (a neutral color).
It is important to know what happens when you are dealing with colors in GIMP. The most important rule to remember
is that decreasing the intensity of a primary color results in increasing the intensity of the complementary color (and vice
versa). This is because when you decrease the value of a channel, for instance Green, you automatically increase the
relative importance of the other two, here Red and Blue. The combination of these two channels gives the secondary color,
Magenta, which is the complementary color of Green.
Exercise : You can check this out. Create a new image with only a white background (255R+255G+255B). Open the Tools
→ Color Tools → Levels dialog and select the Red channel. If necessary, check the preview box. Move the white slider to
the left to decrease the Red value. You will notice that the background of your image gets closer and closer to Cyan. Now,
decrease the Blue channel: only the Green will remain. For practice, go backwards, add a color and try to guess what hue
will appear.
The Color Picker tool lets you find out the RGB values of a pixel and the hextriplet for the color.
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