A more difficult situation is encountered if the

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Unformatted text preview: border of the hole. A more difficult situation is encountered if the borders are traced in gray-level images where regions have not yet been defined. Therefore, the border is represented by a simple path of high gradient pixels in the image. Border tracing should be started in a pixel with a high probability of being a border element, and then border construction is based on the idea of adding the next elements which are in the most probable direction. To find the following border elements, edge gradient magnitudes and directions are usually computed in pixels of probable border continuation. Border detection as graph searching • Whenever additional knowledge is available for boundary detection, it should be used e.g., known approximate starting point and ending point of the border, even some relatively weak additional requirements such as smoothness, low curvature, etc. may be included as prior knowledge. • A graph is a general structure consisting of a set of nodes n_i and arcs between the nodes [n_i,n_j]. We consider oriented and numerically weighted arcs, these weights being called costs. The border detection process is transformed into a search for the optimal path in the weighted graph, the aim being to find the best path that converts two specified nodes, the starting and ending nodes. While cost minimization is considered throughout this section, the approach works equally well if a maximum cost path is sought. Hough Transform: If an image consists of objects with known shape and size, segmentation can be viewed as a problem of finding this object within an image. The Hough transform is a method for detecting straight lines and curves in gray-tone (or color) images. The method is given the family of curves being sought and produces the set of curves from that family that appear on the image. The Hough transform requires an accumulator array whose dimension corresponds to the number of unknown parameters in the equation of the family of curves being sought. For example, finding line segments using equation y= mx + b requires finding two parameters for ea...
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This document was uploaded on 03/12/2014 for the course MECHANICAL 214 at University of Manchester.

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