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 the edge)