利用图象边缘轮廓呈现的一种沟壑性质，开发了一种通过求象素亮度的极值来进行边缘检测的方法.首先，在3×3象素的窗口中，如果中心值为最小值，则以该点为基础，以±45°、0°和90°方向作为近似法线带，然后判断该法线上的象素最小值是否在切点上（即是否在法线的中心点上），再通过检验切点与轮廓法线上亮度极大值的差值是否超过阈值，来判断切点是否为边缘轮廓上的点.由于最小值随着图象局部亮度的变化，就像山谷一样，决不会在一个水平面上，而是会随局部地势变化而变化的，所以该方法是根据局部轮廓法线上亮度的最小值来判别边缘轮廓的，其特点是可以自适应地利用周围象素亮度值来判断物体边缘.

This paper introduces a method of edge detection. By comparison with topography, it is found that the character of edge of image shows the same valley' s character as relief map. Firstly, according to this character, primacy central point is detected by the exploring extremum method in the windows of 3×3 pixels. Then, basing on this central point, approximate normal line at±45°、0°and 90°angle are drew.if the minimums of normal line is at central point and the difference between the minimums and the maximum in the normal line is more than the threshold, this point is considered to the edge point of image. Because the brightness of this point varies with local image like valley—the valley geographic location is not always at the same level and it varies with physiognomy. So this method of edge detection is according to local minimum point in the normal line. the character of this method has self-adjusting situation of local image on detecting edge of image.