1. A method for specifying the trajectory of an object in an image sequence, comprising the steps of: a) taking a subsampled image sequence from said image sequence in a memory, b) displaying said subsampled image sequence in two orthogonal directions on an interactive screen that is attached to a computer, c) marking two orthogonal trajectories of said object using a spatial input device on said subsampled image sequence displayed in said two orthogonal directions on said interactive screen, d) computing the intersection of said two orthogonal trajectories in each image of said subsampled image sequence, thereby defining a coordinate in each said image, and thereby defining a neo-trajectory of said object in said subsampled image sequence comprising said coordinate in each said image in said subsampled image sequence, whereby said neo-trajectory is defined by two orthogonal trajectories as input achieved by two quick strokes of said spatial input device, and e) computing said trajectory in said image sequence using said two orthogonal trajectories on said subsampled image sequence using the computer, whereby said spatial input device can be a digital pen, a computer mouse, a stylus, or any device that can be used to specify a location on said interactive screen.