Video systems convey colour image data using one component that approximates lightness, and two other components that represent colour, absent lightness. In Colour science for video, on page 287, I will detail how luminance can be formed as a weighted sum of linear RGB values each of which is proportional to optical power. A colour scientist uses the term constant luminance to refer to this sum being constant. Transmitting a single component from which relative luminance can be reconstructed is the principle of constant luminance. Preferably a nonlinear transfer function acts on that component to impose perceptually uniform coding.
Standard video systems do not strictly adhere to that principle; instead, they implement an ...
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