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Joseph Owens
Re: Auto White Balance with a color picker still not in V10??
on Nov 14, 2013 at 5:00:31 pm

[Nat Jencks] " it would be great to push white up and down in temp (along the plackian locus). "
That would be the "Planckian" locus, for those wishing to go google-diving.

Here is where it gets interesting, of course. When we look at the xy CIE-chromaticity diagram, its 2-dimensional thinking. What is really going on though, in practical cinematographic terms, is that we are also running up and down a luminosity hill, which is also being distorted by the real-world toe and shoulder of a sensor's exposure range. Take a look at the MacAdams construction:
http://www.scholarpedia.org/article/File:ColSpFig11.jpg

and you will see how the Planckian locus also gets twisted as you depart from the Z/zero axis. Even in a mathematically perfect world. An attempt to unify this response, especially de-convoluting log transform characteristics seems to be built into the ACES workflow, tailored for camera types, but I would like to hear from others whether that interpretation is on the right track or not.

Something else that occurs to me is the real-world compensations that human-eye colorists do intuitively, when we are dealing with simultaneous dynamic contrast in both hue and luminosity. You know the old optical illusion of surrounding two identical grey patches with brighter/darker, and/or complementary hues -- the grey patch, even though it is mathematically identical in all comparison examples, looks different, relative to its surroundings. A machine would never pick that up, because it is something our brains do.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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