title: Surface normal on depth image
body: <p>How to estimate the surface normal of point <em>I(i,j)</em> on a depth image (pixel value in mm) without using Point Cloud Library(PCL)? I've gone through <a href="https://stackoverflow.com/questions/14877375/integral-image-normal-estimation-from-kinect-depth-image">(1)</a>, <a href="https://stackoverflow.com/questions/23894389/how-to-estimate-determine-surface-normals-and-tangent-planes-at-points-of-a-dept">(2)</a>, and <a href="https://stackoverflow.com/questions/14877375/integral-image-normal-estimation-from-kinect-depth-image?rq=1">(3)</a> but I'm looking for a simple estimation of surface normal on each pixel with C++ standard library or openCV.</p>
  
  <p><img src="https://i.stack.imgur.com/uesC9.png" alt="enter image description here">   </p>