A new view through Alberti's window.

In his famous treatise on perspective, Alberti compared picture perception with looking through a window. Although Alberti himself was more concerned with picture production than perception, the window metaphor is still widely used to describe picture perception. By performing depth perception experiments, we investigated whether Alberti's hypothesis makes sense in a geometrical fashion. If pictures are regarded as windows, the locus of objects with equal depth should be similar for pictorial and real space-ideally, spherical. Furthermore, if the loci of equidistance are indeed similar for real and pictorial space, their difference should be flat. We designed two experiments to investigate this claim. In the first experiment, a pairwise depth comparison task was used to compute the global perceived depth structure of a complex scene. We found that perception of the real space is more accurate and less ambiguous than pictorial space. More interestingly, we found that the relative differences between these two spaces (locus of relative equidistance) are curved, which contradicts the window hypothesis. In the second experiment, we wanted to measure the absolute locus of equidistance that we believed was diagnostic for the difference between real and pictorial space perception. We found that under normal circumstances, the distribution of equally perceived depths is curved in real space, and relatively flat in pictorial space. However, we also found exceptions. For example, viewing real space with one eye yielded similar results as normal pictorial space perception. We conclude that Alberti's hypothesis needs a revision.