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<p>The variation, with respect to view, of 2D features defined for projections of 3D point sets and line segments is studied. It is established that general-case view-invariants do not exist for any number of points, given true perspective, weak perspective, or orthographic projection models. Feature variation under the weak perspective approximation is then addressed. Though there are no general-case weak-perspective invariants, there are special-case invariants of practical importance. Those cited in the literature are derived from linear dependence relations and the invariance of this type of relation to linear transformation. The variation with respect to view is studied for an important set of 2D line segment features: the relative orientation, size, and position of one line segment with respect to another. The analysis includes an evaluation criterion for feature utility in terms of view-variation. This relationship is a function of both the feature and the particular configuration of 3D line segments. The use of this information in objection recognition is demonstrated for difficult discrimination tasks.</p>
feature variation; image recognition; view variation; 3D point sets; weak perspective approximation; 2D line segment features; feature extraction; image recognition

J. B. Burns, E. Riseman and R. Weiss, "View Variation of Point-Set and Line-Segment Features," in IEEE Transactions on Pattern Analysis & Machine Intelligence, vol. 15, no. , pp. 51-68, 1993.
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