To detect 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, images are unpacked according to each one of said known formats; a candidate measure is formed according to each unpacking and the candidate measures are compared to identify the presence of left and right images packed according to an identified format. The candidate measure may be a low pass filtered measure of the difference between the left and right images and may be a high pass filtered measure of the activity in either the left or the right image.