title: How to convert pixel to angle
body: <p>i'm new to image processing..
  I have a grace-scale pic in size 1024*768 represent in a numpy array.
  I want to do some calculation form the middle of the pic (pix 511*383) with angles, how do i convert the pix position from x,y to angle?</p>
  
  <p>for example: i want to sum all the pixels in 12 degree and divide them with all the pixels in 60 degree  </p>
  
  <p>BTW i'm using python and opencv </p>