Bug 159625

Version:            (using Devel)
Installed from:    Compiled sources
OS:                Linux

De facto standard in photo correction world allows to operate on combined RGB channels. This allows for temperature-neutral operations. For example check this levelling operation. Original:

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

It would be valuable addition to digiKam 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 (and this is reason to file it under general digikamimageplugins section).