title: OpenCV: Confusion about solvePnP
body: <p>I have a small confusion regarding the use of the solvePnP function in OpenCV.</p>
  
  <p>I have the matrix for the intrinsic parameters of the camera and I have identified some keypoints in image and I am trying to estimate the extrinsic parameters for the calibration.</p>
  
  <p>The documentation for solvePnP says:</p>
  
  <pre><code>cv2.solvePnP(objectPoints, imagePoints, cameraMatrix, 
  distCoeffs[, rvec[, tvec[, useExtrinsicGuess[, flags]]]]) → retval, rvec, tvec
  </code></pre>
  
  <p>I am guesisng my <code>imagePoints</code> parameters are the keypoints I have detected.  and these arte specified in pixels as <code>(x1, y1), (x2, y2), (x3, y3)</code>.</p>
  
  <p>I am totally confused about the objectPoints. So, the documentation says:</p>
  
  <pre><code>objectPoints – Array of object points in the object coordinate space,
  3xN/Nx3 1-channel or 1xN/Nx1 3-channel, where N is the number of points.
  vector&lt;Point3f&gt; can be also passed here.
  </code></pre>
  
  <p>How can I generate these object points from my image points? What does it mean when they say the object coordinate space in here?</p>