1. A device for obtaining a vital sign of a subject, comprising: an interface configured to receive a set of image frames of a subject, one or more processors configured to: analyze a subset of image frames of a region of interest in a subject comprising a plurality of contiguous image frames to determine direction changes indicating changes of a direction of motion appearing within said region of interest and at least one of a number of directions per unit time and times between contiguous direction changes, determine two projection profiles in two predetermined orthogonal directions from said contiguous subset of image frames and determine the number of direction changes per unit time from said projection profiles, decide whether said region of interest of said contiguous subset of image frames comprises vital sign information in response to physical motion direction changes appearing at substantially the same moment in time in both projection profiles, extract the vital sign of the subject from said region of interest within said contiguous subset of image frames.