Similar to when Dorothy leaves her house and enters the Land of Oz, I would like to create the effect of leaving a B&W world and entering a color one.
In my shot I have a hallway that I want in B&W, and a doorway where everything beyond it will be in color.
Currently I have my footage as one layer, and then an adjustment layer where I placed the B&W effect, and then masked and inverted the mask to make the doorway be in color. That creates the effect, but then as the camera moves through the doorway, the mask moves too.
How can I make a mask track to the four points of the doorway?
If there isn't anything blocking or crossing the doorway, you can track the opening of the door with Mocha (included with After Effects). The Mocha help file is a tutorial and should get you through what seems to be a relatively easy shot.