[SOLVED] How to find intersection between two Rays?

Questions
6

Thanks @prisoner849 and @hofk !!
The missing piece was to use the dot product (which I still have to get my head around) to get the result I was looking for. the method name could use a better name if anyone uses it.

function isPositive( start, end, intersection ){ // all parameters are THREE.Vector3()
  let v1 = new THREE.Vector3().copy( end ).sub( start );
  let v2 = new THREE.Vector3().copy( intersection ).sub( start );
  return v1.dot( v2 ) >= 0;
}

// checks if two line segments will intersect on a point 
// when traveling in one specific direction from start to end
// but not in the direction from end to start
// params (THREE.Line3, THREE.Line3)
function getIntersectionOnAPoint(line1, line2)
{ 
  var intersection = null;
  var A = line1.start;
  var B = line1.end;
  var C = line2.start;
  var D = line2.end;

  // Line AB represented as a1x + b1y = c1 
  var a1 = B.y - A.y; 
  var b1 = A.x - B.x; 
  var c1 = a1*(A.x) + b1*(A.y); 

  // Line CD represented as a2x + b2y = c2 
  var a2 = D.y - C.y; 
  var b2 = C.x - D.x; 
  var c2 = a2*(C.x)+ b2*(C.y); 

  var determinant = a1*b2 - a2*b1; 

  if (determinant == 0) { 
    // The lines are parallel.
  } else { 
      var x = (b2*c1 - b1*c2)/determinant; 
    var y = (a1*c2 - a2*c1)/determinant; 
    intersection = new THREE.Vector3(x, y); 
  }
  
  // if there is an intersection. verify intersection occurs on the two line segments
  // when calculating from start to end
  if (intersection) {
  	var line1result = isPositive( line1.start, line1.end, intersection );
    var line2result = isPositive( line2.start, line2.end, intersection );
    if ( line1result && line2result ) {
      // do nothing when the intersection is not "false" as both results are "true"
    }
    else { // 
      // set intersection to null when the intersection is "false" as one of results is "false"
      intersection = null;
    }
  }
  return intersection;
}

more tweaks would be needed but do you guys think such a function deserves to be on the Ray class?
so someone can do

var ray1 = new THREE.Ray(); // set the origin and direction
var ray2 = new THREE.Ray(); // set the origin and direction
var intersection = ray1.intersectRay(ray2); // returns null if no intersection