说明：  canny检测器是很有效的边缘检测器，该函数可以实现对目标图像的边缘提取。该方法总结如下：1.图像使用带有指定标准差的高斯滤波器来平滑，以此减少噪声；2.在每一点计算局部梯度和边缘方向；3.第二步中确定的边缘点会导致梯度幅度图像中出现脊，然后追踪所有脊的顶部，并将所有不再脊顶部的像素设置为0；4.执行边缘链接
(canny detector is very effective edge detector, this function can be achieved on the target image edge detection. This method is summarized as follows: 1. Image used with the specified standard deviation of the Gaussian filter to smooth out, thereby reducing the noise 2. In the calculation of each point of local gradient and edge direction 3. The second step in determining the edge points will led to the image gradient magnitude ridge appears, and then keep track of all of the top of the ridge, and ridge at the top of all the pixels are no longer set to 0 4. the implementation of the edge link)