title: Finding the 4th point of quadrangle. (After perspective transform)
body: <p>There is an image of square, and to that image the perspective transform is applied.  </p>
  
  <p>So the perspective image of a square will be not a square but a quadrangle. 
  The question:  </p>
  
  <p>If I know three corner points of the perspective image (quadrangle)
  then what will be the fourth corner point of the quadrangle?</p>
  
  <p>The problem also arises because the corner points are given in the image space,
  not in the world coordinate system. So they have only two coordinates, 
  for example: N1 = (x1, y1), N2 = (x2, y2), N3 = (x3, y3)  </p>
  
  <p>And I need to find N4 = (x4, y4) using information that the real image in world coordinates was a square before the perspective transformation was made.  </p>
  
  <p>Is it possible to find that 4th point? I think there will not be only one solution of this problem, but I want to know how many solutions we have and I want to get that solutions. </p>
  
  <p>I have read almost the same problem here, but the author didn't say that it was a square before the transformation. </p>