function slerp(out, a, b, t) {
  // benchmarks:
  //    http://jsperf.com/quaternion-slerp-implementations
  var ax = a[0],
      ay = a[1],
      az = a[2],
      aw = a[3];
  var bx = b[0],
      by = b[1],
      bz = b[2],
      bw = b[3];

  var omega = void 0,
      cosom = void 0,
      sinom = void 0,
      scale0 = void 0,
      scale1 = void 0;

  // calc cosine
  cosom = ax * bx + ay * by + az * bz + aw * bw;
  // adjust signs (if necessary)
  if (cosom < 0.0) {
    cosom = -cosom;
    bx = -bx;
    by = -by;
    bz = -bz;
    bw = -bw;
  }
  // calculate coefficients
  if (1.0 - cosom > 0.000001) {
    // standard case (slerp)
    omega = Math.acos(cosom);
    sinom = Math.sin(omega);
    scale0 = Math.sin((1.0 - t) * omega) / sinom;
    scale1 = Math.sin(t * omega) / sinom;
  } else {
    // "from" and "to" quaternions are very close
    //  ... so we can do a linear interpolation
    scale0 = 1.0 - t;
    scale1 = t;
  }
  // calculate final values
  out[0] = scale0 * ax + scale1 * bx;
  out[1] = scale0 * ay + scale1 * by;
  out[2] = scale0 * az + scale1 * bz;
  out[3] = scale0 * aw + scale1 * bw;

  return out;
}
Performs a spherical linear interpolation between two quat

@param {quat} out the receiving quaternion
@param {quat} a the first operand
@param {quat} b the second operand
@param {Number} t interpolation amount between the two inputs
@returns {quat} out