Object-Image Correspondence for Algebraic Curves under Projections

TL;DR

This paper introduces an efficient algorithm to determine if a planar curve is an image of a spatial curve under unknown projections, reducing parameter complexity by leveraging algebraic invariants and curve equivalence criteria.

Contribution

The paper presents a novel, computationally efficient algorithm that simplifies the projection problem using algebraic invariants and curve equivalence under affine and projective transformations.

Findings

Reduces the number of parameters needed to verify projections.

Provides explicit formulas for rational differential invariants.

Demonstrates effectiveness in establishing object-image correspondences.

Abstract

We present a novel algorithm for deciding whether a given planar curve is an image of a given spatial curve, obtained by a central or a parallel projection with unknown parameters. The motivation comes from the problem of establishing a correspondence between an object and an image, taken by a camera with unknown position and parameters. A straightforward approach to this problem consists of setting up a system of conditions on the projection parameters and then checking whether or not this system has a solution. The computational advantage of the algorithm presented here, in comparison to algorithms based on the straightforward approach, lies in a significant reduction of a number of real parameters that need to be eliminated in order to establish existence or non-existence of a projection that maps a given spatial curve to a given planar curve. Our algorithm is based on projection criteria that reduce the projection problem to a certain modification of the equivalence problem of planar curves under affine and projective transformations. To solve the latter problem we make an algebraic adaptation of signature construction that has been used to solve the equivalence problems for smooth curves. We introduce a notion of a classifying set of rational differential invariants and produce explicit formulas for such invariants for the actions of the projective and the affine groups on the plane.