[Gio Nad]"I can see the vertical line between masks; one side is darker or lighter than the other. How can I fix this?"
I can't visualize what you mean. Can you post a screenshot?
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I agree with Walter. I don't exactly understand what you mean, a screenshot or something would help a lot.
I am going to guess though that you have a shot of the same person on opposite sides of the shot and the mask that you drew between them has a hard line because the light changed too much between takes. If this is the case adjust the brightness of the shots with either levels or curves to get them as close as you can. It probably still wont be prefect, but after you do that you will have to draw your mask strategically. When you are drawing your mask try to draw along natural lines in the background. If there is a corner of the room in the shot or around furniture or things like that. Hide the lines of the mask in places where someone would expect to see a line. Also exorbitant amounts of feathering... if the shot allows. lol
Without seeing it that is the only advice I could give. Hope that helps