HSL coding

HSL coding

The HSL Model (Hue, Saturation, Luminance), based on the work of painter Albert H.Munsell (who created the Munsell Atlas), is a representation model known as “natural”, that is, close to the physiological perception of colour by the human eye. Indeed, the RGB model, however well adapted it may be for the computer representation of colours or for showing them on peripheral display devices, does not allow colours to be selected easily.

Indeed, RGB colour adjustment with computer tools is generally done by using three slide blocks or of three cells with the relative values for each primary component, however the lightening of a colour requires the respective values of each component to be increased proportionally. Thus, the HSL model was developed in order to overcome this issue with the RGB model.

The HSL Model consists in breaking up the colour according to physiological criteria:

• Hue, corresponding to the perception of the colour (mauve or orange T-shirt),
• Saturation, describing the purity of the colour, that is, its sharp or dull character (new or washed-out T-shirt),
• Luminance, indicating the amount of light of the colour, that is, its light or dark appearance (T-shirt in the sun or in the shade).

The following is a graphical representation of the HSL model, in which the colour is represented by a chromatic circle and the luminance and saturation are represented by two axes:

The HSL Model was developed with the aim of allowing fast interactive colour selection; however, it is not adapted to the quantitative description of a colour.

There are other natural representation models closely related to the HSL model:

• HSB: Hue, Saturation, Brightness. The brightness describes the perception of the light emitted by a surface.
• HSV: Hue, Saturation, Value.
• HSI: Hue, Saturation, Intensity.
• HCI: Hue, Chrominance, Intensity.