Volume completion

The visual system completes image fragments into larger regions when those fragments are taken to be the visible portions of an occluded object. Kellm an and Shipley (1991) argued that this "amodal" completion is based on the way that the contours of image fragments "relate." Contours relate when the ir imaginary extensions intersect at an obtuse or right angle. However, it is shown here that contour relatability is neither necessary nor sufficient for completion to take place. Demonstrations that go beyond traditional ex amples of overlapping flat surfaces reveal that "mergeable" volumes, rather than relatable contours, are the critical elements in completion phenomena . A volume is defined as a 3-D enclosure. Typically, this refers to a surfa ce plus the inside that it encloses. Two volumes are mergeable when their u nbounded visible surfaces are relatable or the insides enclosed by those su rfaces can completely merge. Two surfaces are relatable when their visible portions can be extended into occluded space along the trajectories defined by their respective curvatures so that they merge into a common surface. A volume-based account of amodal completion subsumes surface completion as a special case and explains examples that neither a contour- nor a surface-b ased account can explain, (C) 1999 Academic Press.