[Digikam-devel] [Bug 128377] Adjust levels: histograms don't match, bad "auto level" function

[Mikolaj Machowski]

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http://bugs.kde.org/show_bug.cgi?id=128377         




------- Additional Comments From mikmach wp pl  2008-03-20 21:36 -------
Returning to auto-level...

I was wrong with Photoshop. It does a-l similarly to digiKam but in less agressive method which IMO gives slightly better results. I uploaded 3 images:

http://www.witkacy.hg.pl/auto-level/Pa.jpg   -> original
http://www.witkacy.hg.pl/auto-level/Pd.jpg   -> a-l in digiKam
http://www.witkacy.hg.pl/auto-level/Pp.jpg   -> a-l in Photoshop *cough* 4.0

Check underexposed areas and they are smaller in PS. Also add auto levelling in levels dialog and you can see how aggressive it is clipping big amounts of channels.

For comparison

Manual correction only of luminosity channel:
http://www.witkacy.hg.pl/auto-level/Pl.jpg

The most subtle correction (that was it I was speaking about, my mistake), using levels tool of Photoshop in RGB "channel": 
http://www.witkacy.hg.pl/auto-level/Pr.jpg

My simulation of that effect in digiKam. This is done by manually detecting lowest level of color and setting this on all channels (apart from luminosity) and doing the same for high level, in case of this image it was 18-249 range. There were small differences with PS, Pr.jpg was done with 18-247:
http://www.witkacy.hg.pl/auto-level/Ps.jpg

So, after more all these tests my conclusion. Auto-level is good - this point is invalid. PS a-l is less aggressive but this may be acquired taste - although PS set standards in this field.

But what would be valuable addition to digiKam is possibility to act on combined channels. At the moment you can only view them in histogram but you have to operate on each one separately which is PITA. Another practical reason. Most new people in digiKam will have some background in Adobe tools, even if not directly at least indirectly because 99% of books about photo correction are describing PS tools. This wish will apply to AdjustLevels and AdjustCurves tools.