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Abstract

A new approach to surface and volume formation is introduced in response to the question:
``Why do some silhouettes look 3-D and others look 2-D?'' The central idea is that form information
can propagate away from a `propagable segment' (PS) of occluding contour that could have
projected onto the image from the visible portion of a cross-section of a surface. A key property
of a PS is that it exhibits abrupt curvature changes where it meets the rest of the occluding contour.
Evidence is provided that the visual system is highly sensitive to the existence of contour curvature
discontinuities in a scene. An algorithm is described for filling in curved surfaces from a PS.
When copies of a PS are propagated into the interior, they act as cross-sectional surface contours
that also exhibit abrupt curvature changes with the rest of the occluding contour. The result is
a non-metric coding of 3-D shape in terms of local ordinal surface curvature and orientation relationships that is invariant to scale, translation, and rotation.