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

[Caulier Gilles]

Le dimanche 17 décembre 2006 20:38, Ari El a écrit :
> Hi,
>
>    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?
>
>
> Thanks!
> Ariel

You have 3 different algorithm to adjust gamma. This is why the result can be 
a little bit different. But i'm agree with you all gamma settings must use 
the same excursion values.

Gilles