In some 'making of' green screening videos, I saw these black tapes on the actors. What are these for?
I assume these are home made motion markers in order to track the motion of the head in the shot. This is useful for adding other objects (or images) to the object, therefore the technique is called object tracking.
Note: The example given is only useful for 2d tracking, because most 3d solvers needs a minumium of 8 tracking markers around the object (in each frame).