A device for obtaining a vital sign of a subject despite motion of the subject, in particular for discriminating a vital sign such as a respiratory information signal from noise in a projection based vital signs registration, comprises an interface that receives a set of image frames of a subject. An analysis unit determines the amount of direction changes and/or the time distances between direction changes within a region of interest in a subset of image frames comprising a number of image frames of said set. An evaluation unit determines if said region of interest within said subset of image frames comprises a vital sign information and/or noise by use of the determined amount of direction changes and/or the time distances for said subset of image frames. A processor determines the desired vital sign of the subject from said region of interest within said subset of image frames.
