An Easy Technique for Fisheye Camera Calibration

We present an easy fisheye camera calibration technique based on a square in the scene plane in this paper. Firstly, aiming at a generic quadrangle including two parallel line segments with the known length ratio, the planar homography between the scene planar including the generic quadrangle and its image is estimated by images of two parallel line segments, then constraints on fisheye camera basic internal parameter can be obtained from fisheye images. Secondly, when the generic quadrangle becomes a square in planar scene, two linear constraint equations can be determined from one image of the square. Under the assumption of zero skew, we may obtain four constraint equations on the basic intrinsic parameters (the focal length, the aspect ratio and the principal point) from two fisheye images. Lastly, because the projected line of the scene straight line on the unit sphere is a great circle, the angle between the normal vector of the great circle and the projected point of fisheye image point is estimated by the initial value. An objective function may be established from these angles. The fisheye lens distortion parameters may further be refined on minimizing the objective function from all fisheye image points corresponding on the same scene straight line. Experimental results demonstrate the validity of our method. n