While binocular viewing of 2D pictures generates an impression of 3D objects and space, viewing a picture monocularly through an aperture produces a more compelling impression of depth and the feeling that the objects are “out there”, almost touchable. Here, we asked observers to actually reach into pictorial space under both binocular- and monocular-aperture viewing. Images of natural scenes were presented at different physical distances via a mirror-system and their retinal size was kept constant. Targets that observers had to reach for in physical space were marked on the image plane, but at different pictorial depths. We measured the 3D position of the index finger at the end of each reach-to-point movement.
Observers found the task intuitive. Reaching responses varied as a function of both pictorial depth and physical distance. Under binocular viewing, responses were mainly modulated by the different physical distances. Instead, under monocular viewing, responses were modulated by the different pictorial depths. Importantly, individual variations over time were minor, that is, observers conformed to a consistent pictorial space. Monocular viewing of 2D pictures thus produces a compelling experience of an immersive space and tangible solid objects that can be easily explored through motor actions.
The fundamental visual property that drives 3D stereoscopic technology is the compelling qualitative experience of tangible solid objects, immersive space and realism that is lacking in conventional 2D displays. This qualitative visual phenomenon, referred to as ‘stereopsis’, is widely assumed to be a by-product of binocular vision. However, its underlying cause, variation and functional role remain largely unexplained. In this theoretical paper I briefly present an alternative hypothesis that stereopsis is not a phenomenon restricted to binocular vision, but a more basic qualitative visual property related to the perception of egocentric distance and scale. I review recent empirical evidence showing that stereopsis is not simply a product of binocular disparities or the mere perception of “more depth”. The theory and results imply critical distinctions between the qualitative experience of stereopsis and the quantitative perception of 3D structure. I describe how this alternative view has important implications for the perception of scale and realism in both conventional and stereoscopic display systems; e.g., perception of miniaturization (puppet-theater effect) and gigantism.