(Pseudo) Analysis of the Correlation of the Hue and Colourfulness ZCAM Attributes using Dominant Wavelengths
I wanted to quantify how much the hue attribute of ZCAM is affected by the compression of its colourfulness attribute, i.e. quantify the correlation between the two attributes. Ideally, a perceptually uniform model should exhibit no correlation, unfortunately, such model does really not exist!
We can quantify the change of a single hue in term of its Dominant Wavelength shift, i.e. how much the Dominant Wavelength has changed after the colourfulness has been reduced.
Given the Spectral Locus, we can trivially compute all the trajectories its chromaticities follow when the ZCAM colourfulness is reduced:
With the trajectories obtained, we need to find the intersections with the gamut of interest, e.g. sRGB. Raycasting in curved space is not trivial, but fortunately, we have generated a lot of points to draw the trajectories, thus we can find the closest point for any given trajectory to the sRGB gamut segments:
And finally trace a ray between that point and the whitepoint to find the output Dominant Wavelength:
We can then quantify visually and numerically the shifts.
Here, each column is split diagonally like a saw: The upper left triangles are the input Dominant Wavelengths, the bottom right triangles are the output Dominant Wavelengths.
We can confirm that the blues are turning cyan, there is a very noticeable slant. Here are some cherry-picked Dominant Wavelengths in close-up:
Doing so, we have also obtained a function that we could use to potentially correct the hue shifts:
The next steps is to clean-up the code so that I can run it easily on other models!
Cheers,
Thomas