private void fixRefreshFlags(){
        // force transforms to update below this node
        spatial.updateGeometricState();

        // force world bound to update
        Spatial rootNode = spatial;
        while (rootNode.getParent() != null){
            rootNode = rootNode.getParent();
        }
        rootNode.getWorldBound();
    }
Aligns this Billboard to the screen, but keeps the Z axis fixed.
jme3-core/src/main/java/com/jme3/scene/control/BillboardControl.java