In essence we are heading towards layer~channel territory here - but this might be an intermediate step…
Being able to lock-in the settings of a local adjustment from the effects of global correction(s) or other local corrections.
e.g. You have the foreground subject (local) lit as You wish - but now want to darken the background (global) without affecting the foreground… or the background is a local correction that You wish to blur and have the foreground local adjustment stay sharp.
The inverted mask could do this if there was just one local adjustment. But suppose You had several local corrections, each one needing its own correction not based on any of the others.
At its core this is a time saver type of feature where changing the global correction can leave the local adjustments unaffected - so one does not have to go back and re-adjust all the local corrections to what they were originally intended to be.
One nightmare scenario would be lots of French windows where all the little panes~squares (many of which might be a local adjustment) in every window would have to be recorrected again to compensate for a changed global adjustment.
My own personal nightmare - this house (please see Fig.1 below) was full of these French windows & doors - for which many panes had to be individually corrected. In the image below we are looking from the kitchen towards the living room.
Now imagine the client wants the room “a bit lighter” - under the current regime many of the little panes (each one a local adjustment) in every window (and all other windows) would have to be recorrected again to compensate for the changed global adjustment.
Fig. 1This spherical projection was used for QuickTime VR, reduced 85% from the original:
I understand this. But I also like to have options for different needs - i.e. this type of set-up does not allow for granular control of a complex lighting environment - either in-camera or synthesized later.
An example - done with Photoshop (years ago) - but illustrates the point regarding isolating masks in order to control~create complex light. Obviously adding the black dress & wings is not in PhotoLab’s purview - but the lighting effects would be.
This image would have been incredibly time consuming to create with out individual (“isolating”) masks.
Original shot:Under dull grey skies - perfect for manipulating the light in post…