Maintaining stereo calibration by tracking image points

An important problem in active 3-D vision is updating the camera calibration matrix as the focus, aperture, zoom or vergence angle of the cameras changes dynamically. Techniques are presented to compute the projection matrix from five-and-a-half points in a scene without matrix inversion, and to correct the projective transformation matrix by tracking reference points. The authors´ experiments show that a change of focus can be corrected by an affine transform obtained by tracking three points. For a change in camera vergence, a projective correction, based on tracking four image points, is slightly more precise than an affine correction matrix. It is shown how stereo reconstruction makes it possible to `hop´ a reference frame from one object to another. Any set of four non-coplanar points in the scene may define such a reference frame. It is shown how to keep the reference frame locked onto a set of four points as a stereo head is translated or rotated. These techniques make it possible to reconstruct the shape of an object in in its intrinsic coordinates without having to match new observations to a partially reconstructed description