Tracking a paint layer in AE

I need to know how to track a paint layer. I know how to animate a paint layer, I know how to track to a null object, but I don’t know how to apply the motion settings to a paint layer using a track point. I know there’s a way to do it in After Effects.

Basically, the subject is standing in a white environment like in the Mac Vs PC commercials and needs to write on the background.

I know what you’re thinking. A.) Put a white posterboard there and have the actor write with a sharpie on it. When you key the green and increase the contrast, the white will disappear into the backdrop leaving only the black. That doesn’t work. When you’re shooting the white blows out in the lighting setup leaving the black invisible. So then you think B.) get a chroma green poster board and THEN write on it with the sharpie. When you key the green, the entire poster board will disappear leaving just the sharpie. Wrong again. Though the black shows up on camera this time, the keyer isn’t precise enough to leave the sharpie strokes clean and strong, they always end up looking aliased and stringy.

So after running those two screen tests I have now devised option C.) Using a chroma-key posterboard and a green highlighter. The poster board will be there to simulate the weight, and the highlighter will be used as a prop, as well as a method of simulating the actual writing so the actor can be sure they are making the proper movements. I would then key out all the green, leaving them waving their hand in the air, then track the motion of the tip of the marker, to which I’d marry a paint layer to follow the movements. Right? Isn’t that nice and simple and clean and dandy? Sure! Except it’s not. Why? Because as far as I can see, there IS no option to track the paint layer. Sure, I can make a single dot and track that motion, but it won’t move in a continuous stroke. It’ll simply be a dot moving around.

My question is: How do I actually simulate writing? I know they do it because you see it in commercials all the time. Like a commercial when a man is writing on the background and then pushing the illustrations away and making new ones. After Effects is known for mogra (motion graphics), so there must be a way to simulate that effect. If anyone knows that would be very helpful. Thanks!

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