Calculating optical flow with opencv

Question

I am using the function:

cvCalcOpticalFlowBM

To calculate the optical flow. The code is running, but the velocity matrices are zero (both x and y), although the input video clearly changes. Does anyone have an idea why would the velocity matrices be zero (black)?

Solution

This tutorial shows how to use optical flow in opencv from scratch. It explains how to draw velocity vectors etc.

http://robots.stanford.edu/cs223b05/notes/CS%20223-B%20T1%20stavens_opencv_optical_flow.pdf