> Okay, sorry, I didn't think it was possible to compare different color
> spaces like sRGB or Adobe RGB without selecting a white point.

The "basis" of most colour spaces is XYZ. XYZ defines a colour exactly 
(in terms of human perception) with just three numbers. The numbers are 
linear in terms of physical power, so if you have two light sources in 
very close proximity, you can simply add the two XYZ values to get the 
resulting colour. What this means though is that doubling XYZ does not 
give a perceived brightness twice as bright. Note there is no dependency 
on any "white point", the three numbers alone are enough.

sRGB in turn defines exactly what "colour" full red, green and blue are 
in terms of XYZ (you can look up the values). I say "colour", because 
obviously you can scale all the values to get a brighter or dimmer 
result, which is still technically sRGB. There is no white point assumed 
or needed in the definitions of sRGB either. The same goes for Adobe 
RGB, that defines a different set of RGB physical colours.

However, given that RGB are defined in sRGB exactly in terms of physical 
colour, this then defines exactly what physical colour "white" in sRGB 
is, ie if you add up equal ratios of the defined R,G and B, you get what 
is "white" in the sRGB colour space. This happens to be very close to 
D65, I assume by design.

> Still, I would like to plot the horseshoe in 3D, anyway, using a
> specific white point. (Or maybe multiple images, each with a different
> white point.)

Sorry this just doesn't make sense. The "horseshoe" shape is totally 
independent of any white point, "using" a white point to plot it is 
meaningless, the shape is the same and fixed by the XYZ values for each 
wavelength of light only.

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