中文说明：光斑的中心提取：重心法和圆拟合。先用多次迭代求阈值并且对图像二值化，再用重心法和圆拟合提取光斑中心。
English Description：
The center extraction of facula: barycenter method and circle fitting. Firstly, the threshold value is obtained by multiple iterations and the image is binarized. Then the center of spot is extracted by center of gravity method and circle fitting.