Originally Posted by alluringreality
I suppose holding luminance for each of 0%, 25%, 50%, 75% would make sense, but I thought 100% red luminance was 0.213 for 709. I don't get where the gray comes from because that seems close to 0.5 as far as I can tell.
Yes, it looks strange, but it isn't. If you look at real measures, primary colors and gray scale, you will find blue Y near 30% gray Y, red Y around 50% gray Y, and green Y around 80% gray Y. And if you sum those three Y values, you obtain 100% white Y value (a little less if you sum the three gray levels). But if you add 30%+50%+80%, you do not have 100%, you have 160 ! What's wrong ? In reality, it's simple. Between those two scales, percent levels and Y levels, you have a gamma factor. This factor in 0.45 in a sense, 1/0.45 in the other (the famous 2.22 gamma).
Red is 0.213, and 0.213 ^ 0.45 = 0.499, ie 50%. This is it
In the other sense: 30% ^ 2.22 + 50% ^ 2.22 + 80% ^ 2.22 is near 100% (90% more exactly: this is it too ).
Clearly, Y value for Red is 0.213 * YWhite. To obtain white, you send a RGB value (255,255,255). To obtain a gray whose Y measured value will be 0.213 * YWhite, the RGB triplet has three identical values which are 255 * ( 0.213 ^0.45 ), ie (127,127,127) in Rec709 (in Rec601 it is very little different). Pure red is of course (255,0,0), and measured Y value for both colors must be the same (not absolutely perfectly, because we do not have decimals in RGB triplets...). For intermediate saturations, well, it's some little maths
I hope it will make it clear now