public Quaternion opposite(Quaternion store) {
        if (store == null) {
            store = new Quaternion();
        }

        Vector3f axis = new Vector3f();
        float angle = toAngleAxis(axis);

        store.fromAngleAxis(FastMath.PI + angle, axis);
        return store;
    }
Returns a rotation with the same axis and the angle increased by 180
degrees. If the quaternion isn't normalized, or if the rotation angle is
very small, the result is undefined.

<p>The current instance is unaffected, unless {@code store} is
{@code this}.

@param store storage for the result, or null for a new Quaternion
@return either {@code store} or a new Quaternion
jme3-core/src/main/java/com/jme3/math/Quaternion.java