title: How to verify that the camera calibration is correct? (or how to estimate the error of reprojection)
body: <p>The quality of calibration is measured by the reprojection error (is there an alternative?), which requires a knowledge world coordinates of some 3d point(s).</p>
  
  <p>Is there a simple way to produce such known points?  Is there a way to verify the calibration in some other way (for example, Zhang's calibration method only requires that the calibration object be planar and the geometry of the system need not to be known)</p>