- Create a preview of an RGB image.
- Switch to it.
- Select luminance as display mode.
- Open ACDNR and enable the luminance mask. Disable the crominance noise reduction entirely.
- Apply to the preview. The display mode remais in Luminance of course.
- In the "Luminance mask" section of the ACDNR interface, enable the Preview checkbox.
- Apply the process to the preview again. The result will be the luminance mask that will be used during the ACDNR.
- Disable the Preview checkbox.
- Apply again. Now the display mode switches to RGB. I expect it to remain in Luminance.
This could happen with entire images (not just previews), and with crominance too... but I'm feeling kinda lazy to try it now. Maybe later, when I defeat this ACDNR monster :^P.