#Darktable - parametric masking
#TILT - Things I Learned Today
Almost all DT modules can have a #parametricMask that limits the area of the image that will be effected by the module. The parameter is a #dataChannel in the chosen #colorSpace.

choose a channel and hover the cursor over the value slider then press "c".

This will switch the display to show just the chosen data channel. The goal is to find the channel that best isolates the elements you want the module to change.