Browsing by Author "Forbes, Keith"

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    Calibration, recognition, and shape from silhouettes of stones
    (2007) Forbes, Keith; Nicolls, Fred; De Jager, Gerhard
    Multi-view shape-from-silhouette systems are increasingly used for analysing stones. This thesis presents methods to estimate stone shape and to recognise individual stones from their silhouettes. Calibration of two image capture setups is investigated. First, a setup consisting of two mirrors and a camera is introduced. Pose and camera internal parameters are inferred from silhouettes alone. Second. the configuration and calibration of a high throughput multi-camera setup is covered. Multiple silhouette sets of a stone are merged into a single set by inferring relative poses between sets. This is achieved by adjusting pose parameters to maximise geometrical consistency specified by the epipolar tangency constraint. Shape properties (such as volume, flatness, and eiongation) are inferred more accurately from the merged silhouette sets than from the original silhouette sets. Merging is used to recognise individual stones from pairs of silhouette sets captured on different occasions. Merged sets with sufficient geometrical consistency are classified as matches (produced by the same stone), whereas inconsistent sets are classified as mismatches. Batch matching is determining the one-to-one correspondence between two unordered batches of silhouette sets of the same batch of stones. A probabilistic framework is used to combine recognition by merging (which is slow, but accurate) with the efficiency of computing shape distribution-based dissimilarity values. Two unordered batches of 1200 six-view silhouette sets of uncut gemstones are correctly matched in approximately 68 seconds (using a 3.2 GHz Pentium 4 machine]. An experiment that compares silhouette-based shape estimates with mechanical sieving demonstrates an application using the developed methods. A batch of 494 garnets is sieved 15 times. After each sieving, silhouette sets are captured for sub-batches in each bin. Batch matching is used to determine the IS sieve bins per stone. Better estimates of repeatability, and better understanding of the variability of the sieving process is obtained than if only histograms (the natural output of sieving) were considered. Silhouette-based sieve emulation is found to be more repeatable than mechanical sieving.
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    Open Access
    Visual hulls from single uncalibrated snapshots using two planar mirrors
    (IAPR, 2004) Forbes, Keith; Voigt, Anthon; Bodika, Ndimi
    Two mirrors are used to create five views of an object: a view onto the object, two reflections and two reflections of reflections. The five views are captured in a single snapshot. Epipolar geometry of the object’s five silhouettes is determined directly from the image without knowing the poses of the camera or the mirrors. The epipolar geometry provides constraints on the pose of each silhouette, allowing the pose of each silhouette to be computed in a common reference frame using only the silhouette outlines. Once the pose associated with each silhouette has been computed, a five-view visual hull of the object can be computed from the five silhouettes. By capturing several images of a rigid object in different poses, sets of five silhouettes can be combined into a single silhouette set in which the pose of each silhouette is known in a common reference frame. This allows visual hulls of an arbitrary number of views to be computed if more than one image is used. The method is applied to an ornamental cat, and experimental results are shown.
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    Volume estimation of fruit from digital profile images
    (2000) Forbes, Keith; Tattersfield, George
    This dissertation investigates the feasibility of using the same digital profile images of fruit that are used in commercial packing houses for colour sorting and blemish detection purposes to estimate the volumes of the corresponding individual pieces of fruit, Data sets of actual fruit volumes and digitial images of the fruit that simulate both single and multiple camera set-ups are obtained.