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An analysis of the properties of the centroid method for subpixel accuracy image feature location is presented. This method is free of systematic error if the maximum spatial frequency of the image incident on the image sensor is less than the sensor's sampling frequency. This can be achieved by using a lens aperture setting such that the modulation transfer function cut-off frequency due to diffraction is appropriately small. Both simulation and experimental tests of this prediction are presented for the case of the location of the center lines of the images of projected light stripes in a triangulation-based three-dimensional shape measurement system.

A 3-D shape measurement system using multiple light stripe active triangulation is described. The system employs a liquid crystal light valve mounted in a conventional projector to code, or label, an array of 64 stripes of light projected onto the scene to be measured. The scene is viewed by a camera displaced from the projector. Firstly the stripes are located to an accuracy of approximately 0.1 pixel in an image digitized with all the stripes turned on. The stripes are then coded by the projection of a sequence of six patterns of stripes using the light valve. In each pattern the intensity of each stripe, on or off, indicates one bit in a six bit code assigned to the stripe. Each located stripe can therefore be identified by determining the six bit number defined by its intensity in the images of the coding patterns. Once the stripes have been identified triangulation can be performed.