1. A method of detecting a watermark in a multimedia signal being rendered by an application executed by a computer system in a window of a display screen connectable to said computer system, said window covering a part of the image area of said display screen, the method comprising the steps of: examining the video signal being generated by said computer system and applied to said display screen to locate image areas in which the video signal changes from frame to frame; defining a bounding box around said image areas to provide an area of interest; and detecting the watermark in said area of interest.