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I'm not sure I understand the question, but here is a an example which might help you. Usually when I have an nonsense numerical result as you describe, it is due to a data type error. For example, if you are somehow casting to an integer before dividing or passing to arctan the fractional part would be discarded resulting in a zero angle.

#!/usr/bin/env python
import cv2, cv                                                                   
import numpy as np                                                               

# draw a rectangle                                                               
image = np.zeros((100,100), dtype='f4')                                          
image[25:75,30:70] = 50                                                          

# take the x and y sobel derivative                                              
dx = cv2.Sobel(image, -1, 1, 0)                                                  
dy = cv2.Sobel(image, -1, 0, 1)                                                  

# display the results with nice scaling                                          
cv2.imshow("original", image)                                                    
cv2.imshow("dx", cv2.convertScaleAbs(dx, None, 1./2, 100))                       
cv2.imshow("dy", cv2.convertScaleAbs(dy, None, 1./2, 100))                       

# convert dx, dy to magnitude and angle                                          
# angle is in radians                                                            
mag, theta = cv2.cartToPolar(dx, dy)                                             

# display in HSV so angle has a simple mapping                                   
theta_hsv = np.zeros((100,100,3), dtype='f4')                                    
# Hue is angle in degrees                                                        
theta_hsv[...,0] = np.degrees(theta)                                             
# S and V are 1.0                                                                
theta_hsv[...,1:] = 1.0                                                          
# Perform the colorspace conversion                                              
# Note that you need to import the old                                           
# cv modlue to get the conversion constants                                      
theta_bgr = cv2.cvtColor(theta_hsv, cv.CV_HSV2BGR)                               

# and show the angles                                                            
cv2.imshow("theta", theta_bgr)                                                   

# press 'q' to exit                                                              
while cv2.waitKey(10) != ord('q'):