This paper describes a method for visual control of the zoom setting of an active camera during tracking. The method assumes an affine projection, and tracking is achieved using affine transfer, a process which is fundamentally invariant to zoom. However, the form of the projection matrices determined to achieve transfer allows the relative scale between the affine bases in different views to be determined, and hence controlled at unity. Unlike its projective equivalent, the method requires no self-calibration, the zooming camera may move quite generally, and no restriction need be applied to the nature of the scene being viewed. The performances of 3D and 2D versions of the algorithms are examined using synthetic data under varying levels of noise and under varying degrees of degeneracy in motion and structure. Real-time results are presented from imagery of laboratory scenes and off-line results obtained from an outdoor surveillance video sequence.
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