True stereoscopic 3D cannot be simulated by shifting 2D content off the screen plane

Generating stereoscopic 3D (S3D) content is expensive, so industry producers sometimes attempt to save money by including brief sections of 2D content displayed with a uniform disparity, i.e. the 2D image is geometrically shifted behind the screen plane. This manipulation is believed to produce an illusion of depth which, while not as powerful as true S3D, is nevertheless more compelling than simple 2D. Our study examined whether this belief is correct. 30 s clips from a nature documentary were shown in the original S3D, in ordinary 2D and in shifted versions of S3D and 2D. Participants were asked to determine the impression of depth on a 7 point Likert scale. There was a clear and highly significant difference between the S3D depth perception (mean 6.03) and the shifted 2D depth perception (mean 4.13) (P = 0.002, ANOVA). There was no difference between ordinary 2D presented on the screen plane, and the shifted 2D. We conclude that the shifted 2D method not only fails to mimic the depth effect of true S3D, it in fact has no benefit over ordinary 2D in terms of the depth illusion created. This could impact viewing habits of people who notice the difference in depth quality.