I have this footage that I need to remove a cellphone screen. I created a mask and did a roto, animated the path (not in a solid layer, the mask in on the actually layer of the footage)

Now I come with this situation, to create a content-placeholder-composition for this screen, i need track the four anchor points, but how do that? 

If i created a solid layer on top of the mask-footage-layer and put the blending mode on Exclusion, i be able to see a Black-White screen, like a view of a alpha channel, it's possible take view and create a alpha channel for after that, tracking down the archer points to match the position for screen placeholder?

Best regards! 
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