@Override
    public void write(JmeExporter ex) throws IOException {
        super.write(ex);
        OutputCapsule oc = ex.getCapsule(this);
        oc.write(duration, "duration", 1);
    }
fades the scene out (scene to black)
jme3-effects/src/main/java/com/jme3/post/filters/FadeFilter.java