Write a functionin opencv,my_non_maximal_suppression(magnitude, orientation, tl), to perform non-maximal suppression along the gradient direction:

1. Create a zero matrix called mag_thin

2.For each pixel magnitude[i][j] whose value is smaller than t_low, ignore it

3. For each pixel whose value value is bigger than t_low, if it is bigger than its two neighbors in the gradient direction, let mag_thin[i][j] = mag[i][j].

4. Plot and return mag_thin.