Originally Posted by HDTVChallenged
There you go again
. 0.75 is a fixed scaler distance from D65 in the x,y plane ... how will changing gamma change "saturation?"
Because the 75% pattern is a baked on the disc as a YCbCr value.
The reality is that once you change from a target gamma of 2.2 to anything else the RGB triplet for 75% changes. So one of two things happens either you need a new RGB triplet for 75% or you need to recalculate the target based on the saturation change of the encoded triplet.
You can find documentation about the AVS.709 disc for instance. It's RGB values for 75% RGB are red 90.4%, green 26.5%, blue 26.5%.
So the actual RGB triplet of the pattern is 214, 74, 74, or in YCbCr space 104, 112, 198.
First you decode the RGB triplet to percent.
Then you degamma the RGB percent to linear percent <-- gamma formula.
Then you convert the linear RGB to XYZ via a 3x3 matrix.
So any time you change gamma, the target XYZ for a value can change. In fact only White, Red, Green, Blue, Cyan, Magenta, Yellow and Black don't change (100% luminance, 100% saturation). So 75% in x,y never changes, but a pattern labeled 75% will not actually be 75% between White and the Color unless you use the specific gamma the disc was encoded with.