Many different approaches have been suggested for the measurement of structure in space from spatially separated cameras. In this report we critically examine some of these techniques. Through a series of examples we show that none of the current mechanisms of disparity measurement are particularly robust. By considering some of the implications of disparity in the frequency domain, we present a new definition of disparity that is tied to the interocular phase difference in bandpass versions of

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... the monocular images. Finally, we present a new technique for measuring disparity as the local phase difference between bandpass versions of the two images, and we show how this technique surmounts some of the difficulties encountered by current disparity detection mechanisms.