Elle Stone and Nicolas Robidoux, can you please also have a
look on this topic? Thank you in advance.

Wow, this was a very elaborate answer ...

Fred Weinhaus also answered me and he is pointing to some transfer functions that ImageMagick uses, see http://www.imagemagick.org/script/command-line-options.php#intensity.

He also gives some helpful illustrations what effects different transfer functions have on the image:

I understand what Elle says in regard to histogram clipping, i.e. max(R, G, B) being very helpful for finding the white point -- while min (R, G, B) in regard to the black point is missing.

Perhaps one should give the user just a few more options what to display in the histogram and how to transfer R, G, B into a single 'value'. I would be very lucky with 'Rec709Luma' as defined by ImageMagick. I understand that GIMP internally uses the sRGB colour space at the moment (?).

A standalone feature for GIMP would be the possibility to display a cumulated histogram, which would for instance demonstrate the effect of equalisation better than an ordinary histogram. This should be easy to implement, I guess, and would be a simple check box for the user.

BTW: Besides equalisation, one could also try to meet other cumulative histogram distribution, like a Gaussian bell curve, see http://www.fmwconcepts.com/imagemagick/redist.

Wolfgang Hugemann
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