Direct calibration methodology for stereo cameras

This paper describes a new technique for stereoscopic three-dimensional (3D) position measurement. By defining stereo cameras as a system for image-to-world mapping, the mapping function is determined. The direct representation of 3D coordinates of a world point with corresponding stereo image coordinates is derived using the pin-hole model. One camera frame is related to the other before being related to the world frame so that the stereo itself rather than each camera can be directly related with the 3D world. The equations obtained are simple, straightforward, and closed-form. However, since the nonlinearity of actual imaging system is not considered, high accuracy is difficult to be expected when the equation is employed for 3D measurements. For tackling this problem a multilayer feedforward neural network trained by back-propagation (BP) algorithm is used. The network played the role of fine correction satisfactorily using its function learning capability after rough mapping by the linear equations in our experiment.