1. A method of processing a stereoscopic image sequence to generate a warning that the stereoscopic image sequence has been derived from a 2D image sequence, the method comprising the steps in a video processor of: receiving left-eye image data and right-eye image data from a sequence of stereoscopic images; performing a correlation process upon said left-eye image data and right-eye image data to identify any temporal offset where left-eye image data is substantially equal to delayed right-eye image data or right-eye image data is substantially equal to delayed left-eye image data; and performing an analysis of said temporal offset to generate a warning signal that the said stereoscopic image sequence has been derived from a 2D image sequence where said temporal offset is constant at one value for a first set of said images and constant at a different value for a second set of said images.