In computer imaging, colours are produced as combinations of the additive primary colours (red, green, and blue). Another means of colour definition is in terms of intensity, hue, and saturation. This is the way people normally think of and manipulate colour.
Intensity is the lightness (or darkness) of a colour. It can be thought of as an axis that is equidistant from the three primary colour axes. The primary colour axes are orthogonal or perpendicular to each other. Zero intensity represents black. As the intensity is increased, the lightness increases and the darkness decreases. Full intensity (for 8-bit data, this is 255) represents white.
Hue refers to the actual colour. It is defined as an angle on a circle that is centred and perpendicular to the intensity axis. For the Hexcone model, zero degrees represents Red and Hue values cycle through Red-Green-Blue-Red, whereas for the Cylinder model, zero degrees represents Blue and Hue values cycle in the opposite direction through Blue-Green-Red-Blue. The following table shows the unscaled and scaled Hue values for both IHS models:
Hue Hexcone Model Cylinder Model Angle Unscaled Scaled Unscaled Scaled Red 0 0 240 170 Yellow 60 42 180 128 Green 120 85 120 85 Cyan 180 127 60 42 Blue 240 170 0 or 360 0 or 255 Magenta 300 212 300 212 Red 360 255 240 170Saturation is the amount of colour present or the radius of the circle described by the hue. Zero saturation represents no colour and would appear as a grey shade depending on the associated intensity. Full saturation (1.0 for unscaled Hexcone model or 208.2066 for 8-bit unscaled Cylinder model) represents full colour for Red, Green, and Blue. Full saturation accentuates the RGB components at the expense of other hues, in that other hues peak in their saturation at less than the maximum possible saturation.
Intensity/Hue/Saturation is an easier means of controlling the output colour than Red/Green/Blue. For example, it is easier to comprehend lightening an image (increasing the intensity) or adding more colour (increasing the saturation) than it is to comprehend the effects of increasing or decreasing the Red/Green/Blue values. One possible application of IHS is to produce Intensity/Hue/Saturation channels from three input channels, then do a stretch on the saturation channel, and then convert the Intensity/Hue/Saturation channel data back to the equivalent Red/Green/Blue channels for video display. The output image would have a better saturation range without changing the colours of the original image.
The following diagram shows the geometric relationship between the RGB and IHS coordinate systems for representing colour. The hexcone or cylinder is oriented so that the angles between the Intensity axis and the primary RGB colour axes are the same. The Saturation axis is perpendicular to the Intensity axis. Hue is the angle between the radius of the circle defined by the RGB value and the Saturation axis.
Red Intensity
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Blue