RGB curve in darktable



One of the features I had been missing in darktable was the RGB curve. But thanks to the new R- G- B- channel blending modes available in V2.4+, I can now use multiple instances of base curve for different channels. My current problem is that the base curve does not have a color picker so that I cannot see which value I am changing. As far as I know, I cannot use the global color picker module either as it shows the final value of the pixel/area rather than the output of a specific iop (e.g. base curve). Is there any way to overcome this limitation?

P.S. I hope these three blending modes are added to the tone curve in future. I understand that the tone curve works in the Lab space but it might be possible just the way the “automatic in RGB” mode has been added to the “scale chroma” option of this iop.

(Roel) #2

Please see this related feature request and judge for yourself if it is likely to happen :wink:
Also, the color picker idea was recently proposed for RawTherapee, but is apparently hard to implement. I guess this would be of similar difficulty for darktable.


Thanks for your reply. I have already seen the feature request. Actually I am not asking to add a new RGB curve. I am just asking for a tip to have an insight when I am creating a base curve in R- G- B- channel blending mode :slight_smile:

darktable already has a powerful color picker :wink:

(Roger) #4

I do not guess what effect you try to get by the RGB curve but the Color Zones has worked for most of my needs and it has a colour picker.


There are a lot of color grading effects that need RGB curves. A simple example is Instagram’s Nashville: https://smidis.deviantart.com/art/Instagram-Nashville-Effect-Action-274856742
You can achieve a relatively similar result using Color Balance in combination with parametric masks but I don’t think Color Zones can help here.

(Roger) #6

I see, you want to make an effect, not a colour correction.
To achieve the results that you provide I use the Channel Mixer.


Although my original question still stands, I am curious to see how you achieve a similar result using Channel Mixer. So in the link above, please use the left photos (marked before) and share your xmp file. Thanks :slight_smile:

(Roger) #8

I am sorry for not being able to help you because I do not understand your question neither I have the time to go further.
I tough you need a colour picker, then an image effect,. I neither found the images you mentioned but a single one that I can not process currently.
However, under my pour understanding, if you want to make an effect, which does not need a colour picker accordingly to my work flow, the Channel Mixer, works as the RGB curves because in the “destination” option you can choose the channel, so, the example you provided uses the “destination” set to “red” and increments of red and decrements of green and blue in the controls.