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+/*
+ * Copyright 2009, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+// A shout out to Terrance J. Grant at tatewake.com for his tutorial on arcball
+// implementations.
+
+/**
+ * @fileoverview This file contains functions for implementing an arcball
+ * calculation.  It puts them in the "arcball" module on the o3djs object.
+ *
+ *     Note: This library is only a sample. It is not meant to be some official
+ *     library. It is provided only as example code.
+ *
+ */
+
+o3djs.provide('o3djs.arcball');
+
+o3djs.require('o3djs.math');
+o3djs.require('o3djs.quaternions');
+
+/**
+ * A Module for arcball manipulation.
+ *
+ * This is useful for rotating a model with the mouse.
+ *
+ * @namespace
+ */
+o3djs.arcball = o3djs.arcball || {};
+
+/**
+ * Creates a new arcball.
+ * @param {number} areaWidth width of area arcball should cover.
+ * @param {number} areaHeight height of area arcball should cover.
+ * @return {!o3djs.arcball.ArcBall} The created arcball.
+ * @see o3djs.arcball
+ */
+o3djs.arcball.create = function(areaWidth, areaHeight) {
+  return new o3djs.arcball.ArcBall(areaWidth, areaHeight);
+};
+
+/**
+ * A class that implements an arcball.
+ * @constructor
+ * @param {number} areaWidth width of area arcball should cover.
+ * @param {number} areaHeight height of area arcball should cover.
+ * @see o3djs.arcball
+ */
+o3djs.arcball.ArcBall = function(areaWidth, areaHeight) {
+  this.startVector = [0, 0, 0];
+  this.endVector = [0, 0, 0];
+  this.areaWidth = areaWidth;
+  this.areaHeight = areaHeight;
+};
+
+
+/**
+ * Sets the size of the arcball.
+ * @param {number} areaWidth width of area arcball should cover.
+ * @param {number} areaHeight height of area arcball should cover.
+ */
+o3djs.arcball.ArcBall.prototype.setAreaSize = function(areaWidth, areaHeight) {
+  this.areaWidth = areaWidth;
+  this.areaHeight = areaHeight;
+};
+
+/**
+ * Converts a 2d point to a point on the sphere of radius 1 sphere.
+ * @param {!o3djs.math.Vector2} newPoint A point in 2d.
+ * @return {!o3djs.math.Vector3} A point on the sphere of radius 1.
+ */
+o3djs.arcball.ArcBall.prototype.mapToSphere = function(newPoint) {
+  // Copy parameter into temp
+  var tempPoint = o3djs.math.copyVector(newPoint);
+
+  // Scale to -1.0 <-> 1.0
+  tempPoint[0] = tempPoint[0] / this.areaWidth * 2.0 - 1.0;
+  tempPoint[1] = 1.0 - tempPoint[1] / this.areaHeight * 2.0;
+
+  // Compute square of length from center
+  var lengthSquared = o3djs.math.lengthSquared(tempPoint);
+
+  // If the point is mapped outside of the sphere... (length > radius squared)
+  if (lengthSquared > 1.0) {
+    return o3djs.math.normalize(tempPoint).concat(0);
+  } else {
+    // Otherwise it's on the inside.
+    return tempPoint.concat(Math.sqrt(1.0 - lengthSquared));
+  }
+};
+
+/**
+ * Records the starting point on the sphere.
+ * @param {!o3djs.math.Vector2} newPoint point in 2d.
+ */
+o3djs.arcball.ArcBall.prototype.click = function(newPoint) {
+  this.startVector = this.mapToSphere(newPoint);
+};
+
+/**
+ * Computes the rotation of the sphere based  on the initial point clicked as
+ * set through Arcball.click and the current point passed in as newPoint
+ * @param {!o3djs.math.Vector2} newPoint point in 2d.
+ * @return {!o3djs.quaternions.Quaternion} A quaternion representing the new
+ *     orientation.
+ */
+o3djs.arcball.ArcBall.prototype.drag = function(newPoint) {
+  this.endVector = this.mapToSphere(newPoint);
+
+  return o3djs.math.cross(this.startVector, this.endVector).concat(
+      o3djs.math.dot(this.startVector, this.endVector));
+};