[Digikam-users] Gamma correction: many options, different behaviors, I'm confused

Ari El ari.reads at gmail.com
Sun Dec 17 19:38:28 GMT 2006


   I'm using digikam & imageplugins RC2. I'm newbie so pls. forgive me if I
ask obvious questions.

   I've found 3 ways of correcting gamma:
      1 Fix > Colors > Brightness/Contrast/Gamma
      2 Fix > Colors > Levels Adjust
      3 Fix > Colors > Whitebalance

Curiously I seem to be getting different results from all three.

I understand that Gamma can be adjusted either on the Luminance and/or on
each of the RGB channels.

I am not sure what each (1,2,3) option does, as regards to gamma adjusting.
For example to do a 30% gamma increase, in 1 I set gamma=0.3. This gives me
image A. There is no indication as to where this adjustment is done
(Luminance or RGB)

If I do the same in 3, setting gamma=1.3 (usability problem? why not use the
same scale?) I get image B. Image B has noticeably more color saturation
than image A. Again there is no indication of where the channel setting is
being used; apparently this time is in the RGB channels??

If I do the same in 2, the interface clearly shows different gamma settings
for Gamma_luminance, Gamma_R,G and B. Now If I hike gamma 30% in all three
RGB channels, I get image C. Curiously, it looks like image A, and has
little color saturation. If in 2, I just give 30% more gamma to luminance, I
get image D. Image D (luminance gamma adjust) looks exactly the same as
image C (RGB gamma adjust). 

Can someone explain how all this gamma adjustments work? and also why the
"Whitebalance > gamma" adjust gives me much more saturated colors? Any
recommendation on which one to use?


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