1. A method for generating a composite image of a stereoscopic video stream comprising a pair of a right image (R) and a left image (L) of a scene, said right image (R) and left image (L) being such that, when viewed by a spectator's right eye and left eye, respectively, they cause the spectator to perceive the scene as being three-dimensional, said method comprising the steps of: generating a composite image (C) comprising all the pixels of the pair of right (R) and left (L) images, defining a grid of macroblocks of the composite image (C), each macroblock of said grid comprising a plurality of adjacent pixels, decomposing one image of said pair of right image and left image into a plurality of component regions (Ri) comprising a plurality of contiguous pixels, processing said component regions (Ri) in a manner such as to generate corresponding derived regions (Rider), said derived regions (Rider) comprising at least all the pixels of a corresponding component region and being such that they can be decomposed into an integer number of macroblocks, arranging the non-decomposed image of said pair and said plurality of derived regions (Rider) in said composite image (C) in a manner such that all the edges of the non-decomposed image and of the derived regions coincide with edges of macroblocks of said grid.