I’m trying to improve my B&W processing skills with darktable 3.2.1 and have a question for everyone here.
While there is more than one way to convert a color image to B&W in darktable, I decided to focus on two for my use: 1) Channel Mixer, and 2) Lut 3D. For the record, Lut 3D is my preference - I’ve found some B&W Luts that - in my opinion - produce some good looking images. However, if I’m not going to use a Lut, I’ll use Channel Mixer.
Here’s my issue: Back when I was shooting B&W on film, I would expose a frame of film with a (for example) yellow filter over the lens. This would result in an exposed negative which would print with a darkened sky making the clouds pop more.
Now with darktable I don’t need to attach a yellow filter to my lens, I should be able to get the same effect digitally. To do this I convert my image to B&W using one of the two modules I described above, and then (ideally) use the Color Zones module to darken blues and greens (mimicking what a yellow filter would do)… However when I tried doing just that with Color Zones it didn’t work right, Color Zones having almost no affect. I think that is because the Color Zones module occurs in the pixel pipe AFTER both Channel Mixer and Lut 3D, so by the time pixels get to the Color Zones module, the pixels in the pixel pipe are just shades of gray - so there would be no blue and green pixels for the Color Zones module to work on.
Yes, I could do the B&W conversion AND darken blues in greens in one step - albeit for the entire image - by using just the Channel Mixer. However sometimes I might want to change the tonality of just a portion of the image (i.e., in addition to changing the sky, I also want to change the B&W tone of a red flower to separate it from green foliage), and I am not sure if the Channel Mixer is the right tool for that level of work.
I think the right tool for playing with color levels is the Color Zones module - and please correct me if I’m wrong here. (Yes, Color Balance is potentially an alternative to Color Zones, but it also falls after Channel Mixer/Lut 3D in the pixel pipeline, so I would have the same issues with it as I do Color Zones)
I know I can change the order of modules in the pixel pipe for a particular image, and when I tried moving Color Zones BEFORE Channel Mixer/Lut 3D, that made Color Zones work the way I would want. However, I recall an earlier admonition in the 3.x release to the effect of “only change the order of modules in the pixel pipe if you really know what you’re doing” - and I’m pretty sure that my knowledge of darktable is not at the “really know” level.
Part of my question is how to ‘best’ change the order of modules in the pixel pipe. I could either move Color Zones up in the pixel pipe so it occurs BEFORE Channel Mixer/Lut 3D (I’d use one of the two modules not both), or move Channel Mixer/Lut 3D down in the pixel pipe so it occurs AFTER Color Zones. To complicate matters a bit, I suspect that there is a possibility that I might have multiple instances of Color Zones if I needed to tweak a number of colors…
Hope my explanation is clear - please don’t hesitate to ask for clarifications.