On-line calibration method for stereovision systems used in vehicle applications

This paper presents an on-line calibration method of the absolute extrinsic parameters of a stereovision system suited for vision based vehicle applications. The method uses as prior knowledge the intrinsic parameters and the relative extrinsic parameters (relative position and orientation) of the two cameras, which are calibrated using off-line procedures. These parameters are remaining unchanged if the two cameras are mounted on a rigid frame (stereo-rig). The absolute extrinsic parameters are defining the position and orientation of the stereo system relative to a world coordinate system. They must be calibrated every time after mounting the stereo-rig in the vehicle and are subject to changes due to static (variable load) and dynamic (acceleration, bumpy road) factors. The proposed method is able to perform on-line the estimation of the absolute extrinsic parameters by driving the car on a flat and straight road, parallel with the longitudinal lane markers. The edge points of the longitudinal lane markers are extracted after a 2D image classification process and reconstructed by stereovision in the stereo-rig coordinate system. After filtering out the noisy 3D points the normal vectors of the world coordinate system axes are estimated in the stereo-rig coordinate system by 3D data fitting. The output of the method is the height and the orientation of the stereo rig relative to the world coordinate system