1. A playback unit, comprising: an input for receiving an encoded data stream bearing a video image; a decoder for decoding the encoded data stream; and means for imparting a prescribed transformation to the video image for warping the video image in a manner, and by an amount, not readily visible to a viewer such that a composite video image produced by multiple video playback units will be distorted and the distortion of the composite video image can be seen by the viewer, wherein said warping changes with time during playback of the video image.