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Old 25th March 2021, 10:28   #6  |  Link
Sharc
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Join Date: May 2006
Posts: 3,997
Quote:
Originally Posted by wswartzendruber View Post
What the devil is going on here...



I took that from Wikipedia. The article it's from specifically states that Y is linear light and that Y' is gamma-corrected, perceptual light. It also states specifically that RGB is linear light.

So how in blue blazes do you get a gamma-corrected Y' value for output when taking linear-light values in as input...using nothing but multiplication?
Perhaps the answer is given here:
http://poynton.ca/papers/SMPTE_98_YYZ_Luma/index.html
Quote:
Since 1953, we have been using the wrong block diagram for color video! The principles of color science dictate that we mix linear RGB to make true luminance, denoted Y. This is known as the Principle of Constant Luminance. But in video we depart from that principle, and implement an engineering approximation: We mix nonlinear ("gamma corrected") R'G'B' to make what I call luma, denoted Y'. (Many video engineers carelessly call this luminance.) To form luma, we use the theoretical coefficients of color science, but we use them in the wrong block diagram: We apply gamma correction before the mixing, instead of after. This alteration in the block diagram is more or less inconsequential in practice, though the departure from theory is apparent in the dark band seen between the green and magenta color bars of the standard video test pattern.
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