title: how to find slope of a 3D plane
body: <p>I have a 3D terrain in my scene, I have the location of all the vertices of the terrain.</p>
  
  <p>I want to place any geometry on this terrain procedurally. At the moment I can place the 3D models only on the vertices, as seen in the image below. </p>
  
  <p>If I were to render grass for example in the plane which is highlighted, I need to calculate the slope of the plane, hence the question. </p>
  
  <p>How do one go about calculating slope of a plane in 3D in order to get the coordinates inside the planes which is made by the 4 points? As stated earlier I have all the 4 points which make the plane. </p>
  
  <p>Also Is there any other more efficient way to achieve what I am trying to do [collision detection with the plane may be?]</p>
  
  <p><img src="https://i.stack.imgur.com/nQKLv.jpg" alt="enter image description here"></p>