Flexible Calibration Method for a Quad-Directional Stereo Vision Sensor Based on Unconstraint 3-D Target

Multidirectional stereo vision sensors (MVSs) have been paid increasing attention in the fields of environment perception and 3-D reconstruction attributed to their advantage of wide field of view (FOV). Vision sensor setup requires the parameters to be calibrated accurately, which remains a challenging problem because of the discontinuity in the camera field. This article proposes a flexible calibration method using an unconstrained 3-D target for a quad-directional stereo vision sensor (QSVS) composed of a single camera and a dual mirror pyramid. We build the measurement models of the local virtual binocular structures and the global virtual multicamera based on the optical properties of the catadioptric elements to reduce the calibration steps of the mirror poses. On this basis, this article resolves the following three problems. First, the unique intrinsic parameters of the virtual multicamera are obtained through the intrinsic calibration of the actual camera. Second, based on the proposed sorting algorithm of the feature points, we can determine the correspondence between the spatial feature points and the mirrored 2-D points in the global calibration image. Third, we can determine the global structure parameters of the QSVS from a single calibration image based on the designed unconstrained 3-D target with six degrees of freedom (DOFs). Also, we optimize the calibration parameters by flexibly adjusting the target pose. Simulation and physical experiments are conducted to demonstrate the performance of the proposed method. This method also provides a new way to calibrate the miniaturized multicamera vision systems and omnidirectional vision sensors without an overlapping FOV.