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Colour representations

In order to be able to handle colours correctly and to exchange colorimetric information it is necessary to have the means to categorize them and to choose them. Thus, it is not uncommon to have the choice of the colour of a product before it is even manufactured. In this case, a palette is presented, in which the appropriate colour is selected. Most of the time, the product (vehicle, building, etc) has a colour which corresponds to that selected.

Likewise in IT, it is essential to have a means of choosing a colour among all those that are usable. However, the possible colour range is very vast and the image processing chain passes through various peripheral devices: for example, a digitizer (scanner) followed by image improvement software, and finally a printer. It is thus necessary to be able to represent the colour in a reliable way in order to ensure coherence between these various peripheral devices.

The mathematical representation of a set of colours is thus called the "colour space". There are several, among which the best known are:

• RGB Coding (Red, Green, Blue).
• HSL Coding (Hue, Saturation, Luminance).
• CMYK Coding.
• CIE Coding.
• YUV Coding.
• YIQ Coding.

The colour spectrum that a peripheral display device allows to be shown is called a gamut or colorimetric space. The colours not belonging to the gamut are called out-of-range colours.

Selection of colours in software

Most graphic software offers means of selecting a colour in an interactive way. The main one is often the swatch chart, that is, the colours are presented in a table in which they are classified by nuances:

More and more software is integrating more powerful tools however, allowing the choice of a colour from a vast range. Thus, in the colour selector below, the hue is represented by a chromatic disc, while luminance is represented by a vertical selector offering the colour nuances ranging from black to white.

In the selector below on the other hand, the hue is presented as an X-coordinate of the selector on the left, and the saturation is presented as a Y-coordinate. The selector on the right-hand side makes it possible to regulate the luminosity:

Gamma factor

The criterion that defines the nonlinear character of the luminous intensity of an element is called the facteur gamma.

Thus, the luminance of a computerscreen is nonlinear insofar as  :

• the luminous intensity which it emits is not linearly proportional to the voltage applied, but corresponds to a function curve of the screen gamma (usually between 2.3 and 2.6):

  I ~ Vgamma

• the luminous intensity perceived by the eye is not proportional to the quantity of light actually emitted

In order to overcome this and to obtain a satisfactory reproduction of the luminous intensity, it is possible to compensate for luminance by applying a transformation called a "gamma correction".

Thus, there is a gamma transformation corresponding to each peripheral display device, which is itself capable of being adapted to the perception of the user.

Colour management

It is easy to understand the interest regarding image colours when passing through several peripheral devices (a digital chain made up for example of a scanner, an image processing software and then a printer) in order to make sure that the image at the end of the processing chain has colours close to those of the original image. The group of operations that are necessary to guarantee the good conservation of the colours of an image is known as "colour management".

In order to be able to guarantee colour coherence, it is essential to calibrate (or gauge) all the devices or tools of the digital chain. The calibration (or gauging) of a device or tool thus consists in describing all the colours which it is capable of acquiring or producing (i.e. its gamut) in an independent colour space (for example CIE Lab or CIE XYZ) in a file called an ICC profile (International Color Consortium).

This ICC profile is integrated in the image and conveys the entire set of transformations which it underwent along the image processing chain, as a follow-up log.
This document entitled « Video and digital imaging - Digital representation of colour » from Kioskea (en.kioskea.net) is made available under the Creative Commons license. You can copy, modify copies of this page, under the conditions stipulated by the licence, as this note appears clearly.