単位四元数を回転軸と回転角に変換する

単位四元数を回転軸と回転角に変換する

導出に関しては簡単なので、説明はせずに該当するglMatrix.jsのメソッドを貼り付けておきます。

/**
 * Gets the rotation axis and angle for a given
 *  quaternion. If a quaternion is created with
 *  setAxisAngle, this method will return the same
 *  values as providied in the original parameter list
 *  OR functionally equivalent values.
 * Example: The quaternion formed by axis [0, 0, 1] and
 *  angle -90 is the same as the quaternion formed by
 *  [0, 0, 1] and 270. This method favors the latter.
 * @param  {vec3} out_axis  Vector receiving the axis of rotation
 * @param  {ReadonlyQuat} q     Quaternion to be decomposed
 * @return {Number}     Angle, in radians, of the rotation
 */

function getAxisAngle(out_axis, q) {
  var rad = Math.acos(q[3]) * 2.0;
  var s = Math.sin(rad / 2.0);

  if (s > EPSILON) {
    out_axis[0] = q[0] / s;
    out_axis[1] = q[1] / s;
    out_axis[2] = q[2] / s;
  } else {
    // If s is zero, return any axis (no rotation - axis does not matter)
    out_axis[0] = 1;
    out_axis[1] = 0;
    out_axis[2] = 0;
  }

  return rad;
}