Fast detection of ellipses using chord bisectors

The horizontal and vertical chord bisectors of objects are used to detect elliptic objects in a binary image. In 3-D applications, a circular plane viewed from different viewing angles will always be projected as a pseudoellipse. Hence, the detection of ellipses, which are parameterized with five parameters (namely x0, y0, a, b and theta ), is much more general and useful for both 2-D and 3-D image recognition tasks. For any general ellipse, the loci of its chord bisectors and chord lengths possess various properties which can be utilized to extract the center (0, y0) and to facilitate discrimination of elliptic objects from others. Different pairs of parallel strips are used to calculate the hypothesized values of two newly defined parameters of an ellipse. The statistical modes for these two parameters are extracted for the computation of the remaining parameters theta , a, and b. This two-stage algorithm is fast due to the fact that locating bisecting points solves only very simple additions and shifting operations, and the second stage involves very few computations for each object.<
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