Trinocular active devices have the advantage of freedom from mechanical scanning and rapid image capture compared with more conventional active designs based on scanning laser stripes. Their efficient operation relies, however, on a good solution to the correspondence problem. This requires careful geometric design to take account of epipolar geometry and thorough modeling of image-measurement error. A design that, involves setting up the projector-camera geometry to be degenerate-so that depth computation is ill-conditioned-and then backing off a little is presented. This is called near-degenerate epipolar alignment. The result is that unambiguous stereo matching can, in principle, be guaranteed within a given working volume. This is in marked contrast with passive stereo in which ambiguity cannot be guaranteed, merely minimized statistically. The principles have proved to work well in laboratory tests, achieving unambiguous operation over a working volume of a 50-mm cube with a depth resolution of around 0.2 mm.
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