Tutorial - Additive colour synthesis (RGB).
Each coloured point in an image is obtained by mixing a few basic colours, usually just three. When the image is displayed on a screen (television, computer, camera, video) the three basic colours are red, green and blue. This is known as additive synthesis or RGB synthesis.
This choice of colours is not necessarily the same for all peripherals: printers, for example, can use other colours and work in subtractive synthesis (see "Subtractive synthesis">CMYB).
Typically, these three values can vary between 0 and 255. If the three basic colours are 0, we have black. If they are all 255, the result is white. When the values of the three colours are identical, the result is grey, more or less dark. As soon as there is a difference between the values, a dominant colour appears, the greater the difference.
The RGB colour simulator.
To familiarise yourself with additive synthesis in RGB mode, play with the three sliders and observe the resulting colour.
| decimal | hexa | |||
|---|---|---|---|---|
| Red | ||||
| Green | ||||
| Blue |
Getting to grips with RGB.
Here are a few steps to help you understand colour synthesis in RGB mode:
- Place all the sliders in the middle. The result is a medium grey, about halfway between black and white.
- Gradually increase the value of the three sliders, but keep them equal. The result is lighter and lighter greys: grey 1, grey 2, grey 3, and finally white.
- We could obtain darker and darker greys, to black, by moving the sliders to the left while keeping them equal. As long as the sliders are equal, no colour will appear, just grey.
- From the middle position, move one of the sliders to the right. You will notice that the grey is no longer neutral: a dominant colour has appeared, corresponding to the slider that has been moved.
- Start again from the middle position, and move one of the sliders this time to the left.
The dominant colour that appears corresponds to the complementary colour of the slider that has been moved.
Attention ! The distribution of colours in computing is not the same as that commonly used in the graphic arts. As a result, the complementary colours are not exactly the same either.
Concept of colour space.
The simulator works in a neutral (or theoretical) colour space, i.e. it is assumed that colours are rendered without any distortion. This, of course, is not possible: all peripherals produce inaccuracies in colour reproduction. Theoretical spaces are, however, used by all graphics software for their internal calculations, with any device-related corrections being applied at the last moment.
The most common colour spaces are sRGB, AdobeRGB, ProfotoRGB. For more information on colour spaces, please consult the "Colorspaces" tutorial.
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