How i can determine the direction of the pixel??

I am using lucas & kanade method to calculate the optical flow, I need that to classify the pixels into right,left,up and down.

Answer 1

[Walter Roberson]

Look at whether abs(delta_x) > abs(delta_y) to determine whether you are dealing with left/right or with up/down . Then within each group, look at the sign of the controlling change to determine which of the two directions it is.

[sassi nizar]

I have the two derivatives matrix fx and fy, i can use the tan inverse (fy/fx), and this give me a value in 0-360... my problem, how i can classify this value to right,left,up or down. exactly now what i need it is the inetrval how i can i define it.