In one embodiment, a system includes a processor to retrieve, from a memory, data of a video frame, the data including measurements of luminance, each of the measurements of luminance being associated with a different entry in a matrix, calculate a weighted average luminance value Ly for the measurements of luminance weighted according to a row position of the one measurement of luminance in the matrix, calculate a weighted average luminance value Lx for the measurements of luminance weighted according to a column position of the one measurement of luminance in the matrix, and create a video signature, S.sub.0, of the video frame, the video signature, S.sub.0, including the weighted average luminance value Ly and the weighted average luminance value Lx. Related apparatus and methods are also described.