Just trying to understand channel mixer and how to use it: I set this up:
with a color wheel:
No change from the original color wheel. If I set this up:
the whole image goes black. The same thing happens with a similar setup using blue. Why does red not go black the same way? Is there a bug?
Using lightness as I have seen it done:
does this to the color wheel:
Seems easy to black out a large part of your gamut.
Blue does it too:
But not red, which seems to be logical considering what I showed above:
Again, is this a bug?
Finally, at least knowing what happens when I use a negative value for green and blue means I will look very carefully at the results of applying that combination to an image.
Something weird is going on for me too - looks like a bug in the red channel of ‘lightness’ category. To illustrate, I have below a series of gradients, black > colour > white, comparing ‘grey’ category with blend mode of ‘lightness’ to ‘lightness’ category with blend mode of ‘normal’. We see green gives same result in both, as does blue, but red has an error.
The current channel mixer module in darktable is rather buggy and you need to take real care to not mess up your edit using it.
If learning about the channel mixer is your main goal I would suggest using GIMP to do so.
One tip when you do use the current channel mixer: Place the module as far up the pipeline as possible (at least after the last linear module you used, maybe even further up).
I mention current a few times: Aurelien is working on a new one (reboot channel mixer [github]). I’ve played with it and it is rather nice! It will probably be part of the upcoming 3.4 release.
