How to recognize movement in webcam?

I used webcamtexture.GetPixel(x,y). But it always returned different values every time, even though I covered webcam with paper. I just want to return true if there is any movement in webcam, and return false vice versa. I don't have any other devices such as Leapmotion controller or Kinect. I can't buy them. Then, how can i recognize any movement in Unity webcam? It doesn't matter whether it would be Unity 2D or 3D. Thank you in advance!