Give your gizmo the gift of sight
The reason you see just one optical flow measurement displayed in the center of the image is that there is only one optical flow value being computed. If you want to see more vectors, you'll have to modify the code so that the image is divided into smaller sub-imges, with an optical flow value computed for each sub-image.
To answer your previous question about how to use optical flow to avoid objects- there are many possibilities. With a single optical flow measurement you can only detect an obstacle on one side, for example if you are approaching something from an angle. More optical flow measurements will allow more possibilities. for example, suppose you are computing two optical flows, one diagonally forward to the left and one diagonally forward to the right. You can do this with two sensors, or with just one sensor programmed to compute the two optical flows. You can then compute the divergence of the optical flow, or the difference between the two optical flows. This divergence will increase as you get close to an obstacle.
For more info, look here: http://centeye.com/technology/optical-flow/
And attached is a paper (there is an error in the paper though- equation 1 should have a sin function not cos).