1. A method of detecting the presence of the left and right constituent images of a stereoscopic image packed within an image frame or within a sequence of image frames, according to an unknown one of a plurality known formats, the method comprising the steps in a processor of unpacking images according to each one of said known formats; forming a candidate measure according to each unpacking; and comparing said candidate measures to identify the presence of left and right images packed according to an identified format, wherein said candidate measure is a measure of the difference between the left and right images and wherein said measure of the difference between the left and right images is low pass filtered.