title: Camera projection for lines orthogonal to camera z-axis
body: <p>I'm working on an object tracking application using openCV.  I want to convert my pixel coordinates to world coordinates to get more meaningful information.  I have read a lot about computing the perspective transform matrix, and I know about cv2.solvePnP.  But I feel like my case should be special, because I'm tracking a runner on a track and field runway with the runway orthogonal to the camera's z-axis.  I will set up the camera to ensure this.  </p>
  
  <p>If I just pick two points on the runway edge, I can calculate a linear conversion from pixels to world coords at that specific height (ground level) and distance from the camera (i.e. along that line). Then I reason that the runner will run on a line parallel to the runway at a different height and slightly different distance from the camera, but the lines should still be parallel in the image, because they will both be orthogonal to the camera z-axis.  With all those constraints, I feel like I shouldn't need the normal number of points to track the runner on that particular axis.  My gut says that 2-3 should be enough.  Can anyone help me nail down the method here?  Am I completely off track?  With both height and distance from camera essentially fixed, shouldn't I be able to work with a much smaller set of correspondences?</p>
  
  <p>Thanks, Bill</p>