1.3.3 Other colour mixture models

In the previous sections we have covered mixture by addition,
where a compound stimulus is created through the addition at
source of two stimuli; observing the mixture produces a sensation
distinct in character from the two mixed stimuli. There is a second
way in which mixtures can be produced: instead of having a mixed
stimulus, the visual system itself produces a mixture by averaging
different stimuli arriving at the eye.
The first way in which this can happen is through colour mixing by
averaging over an area, where regions of different colour are
amalgamated into a single colour. The pointillism and divisionism
styles of painting use this feature: many small dots of varied colours
are painted, which when viewed from a distance meld into an area
of a single colour. Pointillism in particular makes an interesting use
of this averaging, by using individual dots of strongly contrasting
colours.
This technique, using the eye to mix colours, sees applications in
digital displays and in printing. Digital displays with 24bit
colour
(8 bits for each of a red, green and blue primary) are considered to
be adequate in the colour space resolution, and are now common;
displays with lower colour resolution, however, are still in existence,
and can be used to reproduce images with a higher colour resolution
by dithering: to reproduce a colour not exactly representable in the
lowerresolution
colour space, pixels in the region are coloured so
that their average is close to the desired colour.
In printing, a similar technique called halftoning is used: the dots
printed by a halftoning process can be of different sizes, and are on
grids with different orientations. The result is that different amounts
of pigments of different colours arrive on the paper, some dots
overprinting others wholly or partially and some not; the visual
system produces the desired colour by averaging. This phenomenon
is also used to make printed colours lighter; assuming that the ink is
printed onto white paper, by using less ink (smaller dots) more light
of all colours is reflected from that area, which will lead to a lighter
average colour.
Learning activity
Use the following Processing code as a basis for investigating the colours achievable
by dithering.
colorMode(RGB,1.0);
for (int x = 0; x < 100; x++) {
for (int y = 0; y < 100; y++) {
int parity = (x + y) % 2;
stroke(parity,1,(1parity));
point(x,y);
}
}