title: Equation to reproject 2D image point to 3D coordinate
body: <p>I had a similar situation like described in <a href="https://stackoverflow.com/questions/12299870/computing-x-y-coordinate-3d-from-image-point">this question</a>. Within the question, the user who asked wrote the following equation:</p>
  
  <p><a href="https://i.stack.imgur.com/5R4QI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5R4QI.png" alt="enter image description here"></a></p>
  
  <p>It seems to work *, but does somebody know, how to get this equation? I was not able to find anything like this anywhere else... </p>
  
  <p>Thanks in advance!</p>
  
  <p>*) I also guess I kinda understand the equation:
  the left part is the ray coming from the camera and going through the image point, therefore on the right side the real-world coordinates have to be translated and rotated as if the camera would be placed in (0,0,0)... sort of? 
  I just can't find a paper or anything to confirm the equation </p>