Constraints for interpretation of line drawings under perspective projection

Abstract

Problems of surface orientation recovery from line drawings in a single image obtained under perspective projection are studied. Two constraints, the shared boundary constraint and the orthogonality constraint, previously used in orthographic projection, are extended to perspective projection. New constraints are derived from observations of parallelism. We define a new symmetry, called convergent symmetry. Convergent symmetry results from perspective projection of a symmetric object. Unlike skew symmetry, convergent symmetry provides sufficient constraints to recover unique surface orientations. The set of techniques given should allow extension of all previous orthographic analysis and provide new tools for additional, more constrained analysis. An example illustrating the use of our techniques is provided. Finally, extension of the constraints for some class of curved surfaces is discussed.