Extracting Region with Holes with Edge Expanding Method

In this paper, a method of region expanding evenly along edge is presented, meanwhile holes can be detected as byproducts. The complexity of the algorithm is O(n), where n is the quantity of pixels in the region. The source region is selected by 5 contiguous pixels with shape "+", of which the surrounding 4 pixels constitute the original edge. For every edge segment, a pixel out of the edge is checked whether it is similar to region pixels. The region is expanded by one pixel when a positive answer is given. During expanding period, if the pixel to be checked is already an edge pixel, the edge circle is divided into two parts - the one with more pixels remains as the surrounding edge, the other is the inner edge of a hole. Expanding process will stop if no change takes place in a repetition period.