Experimented with a patch of solid colour, which of course is not the normal use case, but …
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The normal Control Line ‘covered’ the complete patch and LA darkened it to a certain amount.
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The negative Control Line only covered part of the patch and lightened that part by lets say 50%.
→ Moving the opactity slider towards 0 lightened both parts – keeping their brightness relation. -
Instead of the negative Control Line, I duplicated the normal Control Line, moved it again to only cover a part of the patch and changed LA to counter the reduced luminance.
→ Now I had 2 independent opacitiy sliders available.
also see → Editing high dynamic range images in PhotoLab 5 - #328 by Wolfgang
with the duplicated Control Line in VC3 (using the same Mask selectivity settings, but picking up a different colours to broaden the colour range)