Non-maximum Suppression
After getting gradient magnitude and direction, a full scan of image is done to remove any unwanted pixels which may not constitute the edge. For this, at every pixel, pixel is checked if it is a local maximum in its neighborhood in the direction of gradient. Check the image below:
在得到梯度大小和方向後，對圖像進行全掃描，去除可能不構成邊緣的任何不想要的像素。為此，在每個像素處，像素在梯度方向上是否為其鄰域内的局部最大值。檢查下面的圖像:
Point A is on the edge ( in vertical direction). Gradient direction is normal to the edge. Point B and C are in gradient directions. So point A is checked with point B and C to see if it forms a local maximum. If so, it is considered for next stage, otherwise, it is suppressed ( put to zero).
In short, the result you get is a binary image with "thin edges".
上圖中左邊的為邊緣，右邊的因為A和C之間的極大值約等于A和B的極大值，是以右圖中垂直A的線就不是，會被抑制。