blackslate · November 27, 2015 15:20 · Nov 27, 2015
diff --git a/gistfile1.txt b/gistfile1.txt
@@ -0,0 +1,87 @@
+THREE.Ray.prototype.closestPointToRay = function (that, details) {
+  // that: THREE.Ray() 
+  // details: (optional) object
+  // { pointOnThisRay: <THREE.Vector3>
+  // , pointOnThatRay: <THREE.Vector3>
+  // , midPoint: <THREE.Vector3>
+  // , distanceBetweenClosestPoints: <float>
+  // }
+  // For an explanation of the vector mathematics, see: 
+  // http://morroworks.com/Content/Docs/Rays%20closest%20point.pdf
+
+  // @return undefined if rays are invalid or parallel
+  //         or THREE.Vector3() point on this ray which is closest
+  //         to that ray.
+
+  if (!(that instanceof THREE.Ray)) {
+    return
+  }
+
+  var thisDirection = this.direction
+  var thatDirection = that.direction
+
+  if (!thisDirection.clone().cross(thatDirection).length()) {
+    // Rays are parallel
+    return 
+  }
+
+  if ( !thisDirection.dot(thisDirection)
+    || !thatDirection.dot(thatDirection)) {
+    // At least one of the rays is just a point with no direction
+    return
+  }
+
+  var closestPoint = new THREE.Vector3()
+  var thisOrigin = this.origin
+  var thatOrigin = that.origin
+  var sameOrigin = thisOrigin.equals(thatOrigin)
+
+  if (sameOrigin) {
+    // Simple case
+    closestPoint.copy(thisOrigin)
+  } else {
+    var a = thisDirection.clone().normalize()
+    var b = thatDirection.clone().normalize()
+    var c = thatOrigin.clone().sub(thisOrigin)
+
+    var p = a.dot(b)
+    var q = a.dot(c)
+    var r = b.dot(c)
+    var s = a.dot(a) // already known to be non-zero
+    var t = b.dot(b) // already known to be non-zero
+
+    var divisor = (s * t - p * p)
+
+    if (!divisor) {
+      // The two rays are colinear. They are "closest" at all points
+      // This case should already have been excluded by the .cross()
+      // check made at the start.
+      return
+    }
+
+    var d = (q * t - p * r) / divisor
+    closestPoint.copy(thisOrigin).add(a.multiplyScalar(d))
+  }
+
+  if ( typeof details === "object" ) {
+    details.pointOnThisRay = closestPoint
+
+    if (sameOrigin) {
+      // Should all points be the same object or clones?
+      details.pointOnThatRay = closestPoint
+      details.midPoint = closestPoint
+      details.distanceBetweenClosestPoints = 0
+    } else {
+      // TODO: Add other details
+      d = (p * q - r * s) / divisor
+      var thatPoint = new THREE.Vector3().copy(thatOrigin).add(b.multiplyScalar(d))
+      details.pointOnThatRay = thatPoint
+      details.midPoint = closestPoint.clone()
+                                     .add(thatPoint)
+                                     .divideScalar(2)
+      details.distanceBetweenClosestPoints = closestPoint.distanceTo(thatPoint)
+    }
+  }
+
+  return closestPoint
+}