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Analysis of Affine Invariants as Approximate Perspective Invariants

In this paper we first introduce the concept of the orthoscaled approximation for the perspective projection of a point on an object, and then, through the further analysis of the maximum relative error between the perspective projection of a line segment on the object and its orthoscaled approximation, obtain a practically valuable orthographic triangle error decision rule (OTEDR). Starting from the rule, we investigate the possibility of using some affine invariants, e.g., on a plane of a given object, the ratio of lengths of two parallel line segments or the ratio of areas with a common base etc. as approximate perspective invariants. Experimental results have verified the reliability of our analysis.