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Roland Fleming, Manish Singh; Three dimensional shape and the perception of physical stability. Journal of Vision 2009;9(8):47. doi: 10.1167/9.8.47.
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© ARVO (1962-2015); The Authors (2016-present)
Most work on 3D shape perception focuses on the estimation of local geometrical properties such as depth, slant or curvatures. However, our phenomenal experience of shape is also influenced by global properties such as part structure, center of mass, symmetry, etc. Here we study how global shape properties determine our impression of whether an object is physically stable. Physical stability is behaviourally important as it affects our expectations about object behaviour, and the planning of motor actions. Objects topple over when the gravity-projected centre-of-mass (COM) falls outside the support area (the convex hull of points of contact with the groundplane). We can use this to define a continuous measure of geometrical stability as the angle through which an object must be rotated before it will fall over. This measure depends on support area and the mass distribution that defines the shape's COM. We measure perceived stability in a series of adjustment and YES/NO tasks. Subjects were presented with computer rendered images of objects with different 3D mass distributions that were placed close to the edge of a precipitous table edge. Their task was to identify the orientation of the object relative to the groundplane at which it appeared to be critically stable (i.e., was equally likely to right itself, or fall off the table). The results show a consistently strong correlation between physical stability and perceived stability, although different subjects make different classes of systematic error (e.g., judgments of stability consistent with perceiving the COM to be lower than ground truth). We argue that perceived stability is an important, holistic property of shape that the visual system readily estimates, and analyse the contribution of mid-level shape properties (such as symmetry, curvatures and part structure) to the perception and misperception of stability across a wide range of shape classes.
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