说明：  三步搜索法的步长从等于或者大于最大搜索范围的一半开始，在每一步中，比较九个搜索点，它们包括搜索正方形中心点和八个位于搜索区边界上的搜索点。每一步以后搜索步长减小一半，至搜索步长为一个像素时搜索结束。
(Three-step search method is equal to or greater than the step from the largest range of half of the search starting at each step, compare nine search points are the center square, including the search in the search area and eight search points on the boundary. After each step of the search step size reduced by half, to search for a pixel step search ended.)