1. A computerized method for dynamically placing an advertisement on top of video content in a media file based on movement of an object therein, the method comprising: performing an offline authoring process for generating a trajectory, the offline authoring process comprising: (a) targeting a patch within the object appearing in the video content of the media file; (b) tracking the movement of the patch over a sequence of frames within the media file; and (c) based on the tracked movement of the patch, writing locations of the patch within the sequence of frames to the trajectory; and performing an online rendering process upon initiating play of the media file, the online rendering process comprising: (a) automatically selecting the advertisement; and (b) while the media file is playing, dynamically placing the selected advertisement on top of the video content as a function of the locations within the trajectory.