Hi crackwitz, You are right, tks. I stop using Canny, the result is better. But the result is not very stable. The total number of red dots changed sometimes, but the precise final result is very import to me. Right now I put the camera and laser lights on the same side and the camera position is higher than laser lights, so it is kind of a top view, making the laser lights in the bottom really close to each other in the picture from the camera. I think that is the reason why the final result is not stable. Do you know how to solve it?