canny边缘检测的步骤： 1，生成高斯核，与图像做卷积 2，计算梯度图像 3，非极大值抑制 4，双阈值法和连接边缘(Canny edge detection steps: 1, Generate Gaussian kernel, convolution with the image 2, calculate the gradient image 3, non-maximal inhibition 4, double threshold method and connect th