Luminance chrominance colour spaces consist of 3 components:
- Luminance contains the overall brightness of the image. It is a bit like a greyscale image - if you looked at just the luminance component it would look similar to a black and white photograph.
- Two chrominance components. These indicate the colour once the lightness component has been removed.
There are several colour spaces that work in this way. Pillow supports YCbCr and L*a*b.
The two chrominance components contain similar information to the hue and saturation components of HSL, but it is coded differently. For example, in L*a*b, the *a component represents the position of the colour between magenta and green, the *b component represents the position of the colour between yellow and blue.
Luminance chrominance colour spaces have several uses:
- Analogue television. Black and white television was first broadcast in the mid 1930s, but it was another 30 years before colour TV was introduced. To allow for both systems, colour TV was broadcast as luminance (the original B&W signal) and a separate chrominance signal containing the colour information (that B&W sets would not pick up).
- Image compression. The eye can detect luminance information at greater resolution than chrominance (due to there being more rod cells than cone sells in the retina). JPEG image compression, and some video compression systems, take advantage of that fact. They encode the image data in a luminance chrominance space, but subsample the chrominance data.
- CIELAB is a widely used device independent colour space that uses luminance and chrominance to accurately represent colours as they are perceived by the human eye.
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