Mirror-reflecting a picture of an object: what happens to the shape percept?

Although a two-dimensional picture never fully specifies the actual layout of the depicted three-dimensional scene, one is still able to make a three-dimensional interpretation. When a picture is mirror-reflected, the range of plausible scenes possibly corresponding with the depicted scene has not changed with respect to the original depiction. We were curious to find out whether the inherent picture ambiguities would be solved the same way or differently. Participants performed local attitude settings on three sets of pictures: (1) original pictures, (2) left-right mirrored pictures, and (3) up-down mirrored pictures. Pairwise comparison of the pictorial reliefs of the depicted object, reconstructed from the raw settings, revealed dissimilarities. The differences, however, could be drastically diminished by conducting an affine transformation correction taking into account not only the depths, but also the picture plane coordinates. The inherent ambiguities seemed thus to be solved differently between conditions. By factoring out different solutions to the ambiguities, the pictorial reliefs were found to be equivalent.