Calibration of an Active Binocular Head

Sheng-Wen Shih, Yi-Ping Hung and Wei-Song Lin

TR-IIS-96-007 (Fulltext)


Keywords:
Active Vision, Binocular Head, Motorized Lens Calibration, Head/Eye Calibration, Head Calibration.

Abstract

In this paper, we show how an active binocular head, the IIS head, can be easily calibrated with very high accuracy. Our calibration method can also be applied to many other binocular heads. In addition to the proposal and demonstration of a four-stage calibration process, there are three major contributions in this paper. First, we propose a MFL (Motorized-Focus Lens) camera model which assumes constant nominal extrinsic parameters. The advantage of having constant extrinsic parameters is to have a simple head/eye relation. Second, a calibration method for the MFL camera model is proposed in this paper, which separates the estimation of the image center and effective focal length from the estimation of the camera orientation and position. This separation has been proved to be crucial; otherwise, the estimates of camera parameters would be very noise-sensitive. Thirdly, we show that, once the parameters of the MFL camera model is calibrated, a nonlinear recursive least-square estimator can be used to refine all the 39 {\em kinematic} parameters. Real experiments have shown that the proposed method can achieve accuracy of one pixel prediction error and 0.2 pixel epipolar error, even when all the joints, including the left and right focus motors, are moved simultaneously. This accuracy is good enough for many 3D vision applications, such as 3D navigation, 3D object tracking, and even 3D reconstruction.