//Set new camera position and redraw scene
   //===================================================================
   function setCamera(x,y,z) {
    camera.x = x;
    camera.y = y;
    camera.z = z;
    init(); 
   }
  
   function setViewport(x,y) {
    viewport.a.x = x;
    viewport.a.y = x;
    viewport.b.x = y;
    viewport.b.y = y;
    document.getElementById("back").style.width = y-x;
    document.getElementById("back").style.height = y-x;
   }