  The commonly used radial distortion model for camera calibration is in fact
an assumption or a restriction. In practice, camera distortion could happen in
a general geometrical manner that is not limited to the radial sense. This
paper proposes a simplified geometrical distortion modeling method by using two
different radial distortion functions in the two image axes. A family of
simplified geometric distortion models is proposed, which are either simple
polynomials or the rational functions of polynomials. Analytical geometric
undistortion is possible using two of the distortion functions discussed in
this paper and their performance can be improved by applying a piecewise
fitting idea. Our experimental results show that the geometrical distortion
models always perform better than their radial distortion counterparts.
Furthermore, the proposed geometric modeling method is more appropriate for
cameras whose distortion is not perfectly radially symmetric around the center
of distortion.
