The method and apparatus determine a specific position of a user-selectable region at a particular point in time within a sequence of display frames. The method and apparatus first interpolate between a first matrix representing the user-selectable region at a first point in time and a second matrix representing the user-selectable region at a second point in time. In one embodiment, the matrix generated by this interpolation is then applied to a region definition for the user-selectable region to determine the specific area occupied by the region at the particular point in time. According to one embodiment, the first matrix and the second matrix are both stored within the same sample of a container track. This sample corresponds to one or more frames of the sequence of display frames. In an alternate embodiment, the first matrix and the second matrix are stored in two separate samples of the container track.