Measuring the shape of time-varying objects.
University of Southampton, Electronics and Computer Science,
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This thesis describes the first implementation of a colour encoded structured light (CESL) range-finder capable of measuring the shape of time-varying or moving surfaces. The system is shown to have mean square accuracies of better than 0.5mm when measuring the shape of the human mouth during continuous speech sampled at 50Hz, and better than 0.2mm when measuring static objects of similar dimensions. In order to sample range at video-rates, the images of the scene to be digitised are stored on video tape. This allows the image processing to be performed off-line so the sampling rate of the system only depends on the frame-rate of the video equipment used. The work was motivated by the need to acquire information on mouth shape for acoustics of speech experiments so that the results presented are for measurements of mouth shape and objects relating to the field. The reasons for producing a new system, and the choice of CESL are discussed. The work covers the entire implementation of the range-finder, including code and slide design, feature extraction, feature interpretation, calibration and 3D reconstruction, and performance evaluation. A modification on the Blackman and Tukey classical power density spectrum (PDS) estimator was used for feature extraction which was shown to perform better than other techniques evaluated. The accuracy of detected features and the probability that they were not spurious was determined, based on the feature confidence output by the PDS estimator. Detected features were tracked to produce segments, and encoded as a directed acyclic graph (DAG) which was then matched with the original code sequence using a fast but sub-optimal technique. An existing camera calibration technique was used which was extended to include a second step for projector calibration. The projector model used was linear, had the correct number of degrees of freedom, and was particular to our stripe system.
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