A Method of Simulating Paint Mixing on Computer Monitors
Ferdinand Carabott & Garth Lewis
St Martins School of Fasfion & Design
The London Institute, U.K.
Mixing coloured pigments is fundamental to an artist's practice. New colours
are obtained from combinations of two or more colours e.g. mixing orange with
blue produces green in the middle of a progressive mix.
Computer programs like Adobe Photoshop can generate a mixture of two 'computer'
colours by using the 'gradient' control in the toolbar. This blends one colour
progressively to another. However, if one creates such a gradient in Photoshop
using orange and blue as parent colours, no green is produced - instead a muddy
brownish hue is produced in the middle of the gradient. Moreover, the
Photoshop gradient mix produces a middle colour which is significantly lighter
than what one would get with real pigments.
Colour for computer painting programs is organised around a conventional colour
space. Adobe Photoshop utilises the Lab colour space whereby any colour can be
located by reference to three figures: L, a and b. As colours approach white
or black, the circle of possible colours is ever diminishing, eventually
converging to a single point of pure white or black when L=100 or 0
respectively.
If we generate a gradient going from, for example, orange to blue, with the
"Gradient' tool, Photoshop will create a straight line path joining these
parent colours within the Lab colour space. It can be shown that this path
passes through brown territory with no identifiable green.
If, on the other hand, a swatch of colours made from a progressive paint mix
from orange to blue is measured with a spectrophotometer and mapped on the Lab
space, the path of the paint mixture follows a curve bending into green
territory.
Apart from orange and blue, ten other commonly used artists' colours were mixed
in stepped swatches with each other. The intervening mixtures were all mapped
into Lab space using a spectrophotometer. It became evident that most colour
combinations follow curves with only very few following relatively straight
lines. It's clear from these findings that gradients created in Photoshop will
not be able to simulate paint mixes.
We have devised a method whereby the straight Photoshop gradient line joining
two parent colours is 'bent' to follow the 'real' paint mixes closely not just
in hue but in value as well as mentioned earlier.
It was also evident from paint tests that when mixed with other colours, white
and black changed the hue as well as value - in effect black and white needed
to be treated as 'paint colours' and had to be mixed with all the other 11 hues
and their curves plotted into Lab space as well.
All the colour combinations together with the black and white mixes are made
available for use with Photoshop and thus comprise a colour library in two
parts:
1. Colours imported into Photoshop as "Swatches".
Such palettes can be saved permanently in the Photoshop folder resulting in
over a thousand spot colours.
2. The entire palette of simulated paint gradients can be stored in the
Photoshop folder.
Any colour on the gradient can be spot-sampled and a mask can be switched on
ond off to view the colours as steps, helping hue discrimination.
Any colour-combination file or number of files can therefore be opened and
viewed alongside artwork.
In addition, a stand alone computer interface has been designed to allow
simulation of colour mixing: all thirteen colours are shown at either end of
the interface allowing choice of two parent colours to generate a stepped
simulated mixture. This is useful short cut device to allow rapid experimental
mixtures facilitating colour choice when working with computer graphics.
The educational applications of this work range from the simple interface to a
sophisticated colour course. Illustrators, graphic designers, computer artists
and architects would also find this a useful tool bridging the seemingly
disparate disciplines of analogue and digital paint mixing.