SOLVING OCCLUSION INDETERMINANCY IN CHROMATICALLY HOMOGENEOUS PATTERNS

Overlapping figures can produce consistent depth stratification even w hen chromatically homogeneous. Since neither T-junctions nor X-junctio ns are present in these patterns, the problem arises of what rules det ermine the direction of depth stratification, ie which surfaces appear in front and which behind. In a series of demonstrations and formal e xperiments involving perception of stereopsis, motion, transparency, m otion in depth, and reversible figures, the validity of the principle that the visual system tends to minimise the formation of interpolated modal contours was tested. The reason why larger surfaces tend to be seen modally in front, rather than behind, would reflect the geometric al property that when, in overlapping objects, larger surfaces are clo ser there will be shorter occluding boundaries than when smaller surfa ces are closer. It is shown that this constraint is independent of the empirical depth cue of relative size. An example is also given of a s imple computational strategy that extracts, from chromatically homogen eous patterns, occluding subjective contours corresponding to those pe rceived by human observers.