title: Measurement unit for image of a spherical object
body: <p>What is the unit to measure distence between any 2 points in an image of a spherical object.</p>
  
  <p>For example the distance between the 2 red points on the tennis ball.</p>
  
  <p><img src="https://i.stack.imgur.com/U6zi4.jpg" alt="enter image description here"> </p>
  
  <p>NOTE: As a matter of fact, the "Euclidean distance" cannot be used, since ball is non-Euclidean. Ball is almost spherical, the imaging system projects the ball surface on a plane, where the image elements are not equispaced, neither represent equal areas. The true Euclidean distance depends on the actual position of the points with respect to the camera. </p>