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A colour in an image or video can most simply be defined by its red, green and blue (RGB) components, but in principle any 3 independant properties can be used. Another way is by specifying its luminance (brightness), its chrominance (the hue of the colour) and its saturation (the pureness of the colour). This is useful because the eye perceives variations in luminance across an image much more sharply than variations of chrominance or saturation. Hence these latter can be presented at lower definition with no visible degradation in the image, so saving storeage or transmission bandwidth.