Histogram adjustment using RGB Curves

(Hermann-Josef) #1

one of my lousy images has the histograms concentrated all only into the central 1/3 of the available range. I tried to adjust the RGB histograms individually with the RGB curves in the colour management tab (to achieve what I can do with CTRL+L in PhotoShop). But if I adjust the R channel, also the G histogram moves. Why is that?

What I want to accomplish is that all three histograms cover the entire available range, i.e. the shadow / highlight points are at the minimum / maximum level.


PS: I use Version: 5.0-r1-gtk3

(Karlheinz Lehmann) #2

What you want to achieve is not done in the RGB curves. It is done with the tone curve in exposure. See the tutorial at https://paulsphotopalace.wordpress.com/2-tone-curve-your-single-point-of-success/

(Pat David) #3

If you attempt to adjust the individual channels, be aware that you are very likely going to introduce color shifts in your image as well.

(Hermann-Josef) #4

Good morning and thanks for your answers.

@ChasingShadows The tone curves adjust the three channels simultaneously, as explained in RawPedia. I indeed want to adjust the histograms individually. Is there a way to accomplish that? According to RawPedia the RGB curves should operate on the individual channels. But as I had mentioned, if I use the RGB curves, the changes in R also affect e.g. the G channel, and I do not understand why this is so. It is not possible for me to set the shadow and highlight points for an individual channel correctly using this tool.
@patdavid: Yes, of course this will introduce colour shifts. But this is what I am after, to correct a colour shift in the original. In the link you gave this is explained for GIMP, but using the same tool in RT poses the problems mentioned above. It works fine in GIMP, but this is only with 8Bits/channel.