1. A method of decoding a stereoscopic image data stream, the method comprising: determining a plurality of monoscopic image data regions from among image data regions of the stereoscopic image data stream; extracting composition time stamps of the plurality of monoscopic image data regions; and if, among the plurality of monoscopic image data regions, there are two or more colliding image data regions which are image data regions having the same composition time stamps, extracting monoscopic image data from one of the two or more colliding image data regions which is of a main stream, wherein the stereoscopic image data stream comprises the main stream and at least one auxiliary stream.