  Preparing appropriate images for camera calibration is crucial to obtain
accurate results. In this paper, new suggestions for preparing such data to
alleviate the adverse effect of radial distortion for a calibration procedure
using principal lines are developed through the investigations of: (i)
identifying directions of checkerboard movements in an image which will result
in maximum (and minimum) influence on the calibration results, and (ii)
inspecting symmetry and monotonicity of such effect in (i) using the above
principal lines. Accordingly, it is suggested that the estimation of principal
point should based on linearly independent pairs of nearly parallel principal
lines, with a member in each pair corresponds to a near 180-degree rotation (in
the image plane) of the other. Experimental results show that more robust and
consistent calibration results for the foregoing estimation can actually be
obtained, compared with the renowned algebraic methods which estimate
distortion parameters explicitly.
