Hello,
1. you go from RGB to XYZ using the ICC profile of the RGB space you
are using, which gives you the coeffs of the 3×3 matrix RGB -> XYZ.
Call that matrix M. Do a matrix dot product [XYZ] = [M] * [RGB]
2. you go from XYZ to Yxy using x = X / (X + Y + Z) and y = Y / (X + Y + Z)
3. you subtract the x and y values by those of the white point you are
using (usually, D50). That's x_D50 = 0.34567 and y_D50 = 0.35850,
using 2° CIE 1931 observer. So
x = X / (X + Y + Z) - x_D50 and y = Y / (X + Y + Z) - y_D50,
4. then, hue = arctan2(y, x), saturation = sqrt(x^2 + y^2)
5. since it's only to produce a boolean mask, you don't need to bother
about the back transform. But in case you need :
1. x = saturation * cos(hue) + x_D50, y = saturation * sin(hue) + y_D50
2. X = xY / y, Y = Y, Z = (1 - x - y) * Y / y
3. and then, again, matrix dot product [RGB] = [M^-1] * [XYZ] where
[M^-1] is the inverse of the matrix M formed with the ICC
profile coefficients.
Cheers,
Aurélien.
Le 11/04/2020 à 18:23, Harold le Clément a écrit :
> Hello,
>
> Thanks a lot for the feedback.
>
> Agree, the problem would be only when trying to use the saturation for
> parametric masking (as the color picker HSL mode is only used for
> that).
>
> Perhaps a cylindrical HSY could be used, but it depends how the
> saturation is computed (most of the information I could find on the
> Internet suppose that the RGB values are bounded).
>
> Anyway, in the mean time I could just disregard the saturation selector.
>
> Thanks,
>
> Harold
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