title: OpenCV: Calculate angle between camera and pixel
body: <p>I'd like to know how I can go about calculating the angle of some pixel in a photo relative to the webcam that I'm using.  I'm new to this sort of thing and I'm using a webcam.  Essentially, I take a photo, process it, and I end up with a pixel value in the image that is what I'm looking for.  I then need to somehow turn that pixel value into some meaningful quantity---I need to find a line/vector that passes through the pixel and the camera.  I don't need magnitude, just phase.</p>
  
  <p>How does one go about doing this?  Is camera calibration necessary?  I've been reading a bit about it but am unsure.</p>
  
  <p>Thanks</p>