说明：  利用Canny算子实现图像的边缘检测,本文的算法步骤，第一步：高斯平滑处理；第二步：计算图像梯度；第三步：非极大抑制；第四步：双阈值检测
(Using Canny operator to achieve image edge detection, the first step: Gaussian smoothing; the second step: calculating the image gradient; the third step: non maximum suppression; the fourth step: double threshold detection)