Hello,
I’m not sure I’m posting in the right topic but I need help with the channel mixer in RawTherapee. When I change the values, absolutely nothing happens to my picture. I can’t understand why.
I hope someone here will be able to help me.
Thanks.
Good evening, @alexis & welcome to the forum!
This may be a silly suggestion, but have you turned the little on/off indicator on? You know, the little ring immediately to the left of the text Channel Mixer:
Have fun!
Claes in Lund, Sweden
2 Likes
Hi Claes.
It was as simple as that… 
I had no idea you had to click on this tiny circle to activate the mixer. I just started working on RAW format.
Thank you for your help !
1 Like
To be on the safe side: this on/off thing can be found in (almost) every module!
I just have to say that I’m very happy that this this thread got necromanced. I could never figure out how the channel mixer in RT worked.
Now I understand what it’s doing, thanks to Pat’s explanation. But I couldn’t figure out the practical application, until I got to @stefan.chirila’s post about making greens look more blue, and it was funny, because last night I was trying to do exactly that, make green plant foliage more blue.
I was messing around with a photo, and only know about the LAB HH adjustment, and the HSV equalizer. I just tried the “channel mixer” technique, and still prefer my results with HSV in this particular picture, but it’s a good tool to know about.
3 Likes
Indeed. Thanks for the quick answer. 
thanks for tagging me! I love the RGB channels too; though I dislike the fact that you have to be careful when adding, that you have to also subtract an equal amount …or the balance is shifted. I wish we had a checkbox that did that for you. It is relevant to keep in mind that results will be different from HSV or HSL because those only work on the greens, while channel mixer will blueify all the green there is in the image, even in pixels that have 20% green 70% red and 10% blue.
Hi’ @MStraeten
Thank you for your answer. It makes good sense.
I used the color zones module to get close to the blue color I wanted and used the channel mixer wth a mask to do the last fine tuning.
I have tried to read the thread Please help me to understand "Channel Mixer" - #4 by patdavid several times and noted the following:
…………but adjusting the blue slider in the red channel from 0 to 2 makes changes the G-value from 87 to 78? And the B-value changes as well. See screen shots below. This is not according to the theory?
When using darktable you must habe in mind that there are colorspace transformations in the colorout module. If you didn’t change the operating colorspace it is usually rec2020. So if you add something to one channel that results in a value not within rgb colorspace then the values can change when the larger colorspace is forced into a narrower (IMO colorout doesn’t just clip channels)
1 Like
Could I kindly ask for some help with an issue over here 
Hi @stefan.chirila, with my limited knowledge of the module, I don’t think this is entirely unexpected. You’re putting in negative values, so the module subtracts those. If there was very little green to begin with, and you subtract -20.8 (%?) coming from a strong red channel, you can ultimately end up with negative pixel values.
Something like that?