@Override
    public void simpleUpdate(float tpf){
        Quaternion q = new Quaternion();
        angle += tpf;
        angle %= FastMath.TWO_PI;
        q.fromAngles(angle, 0, angle);

        offBox.setLocalRotation(q);
        offBox.updateLogicalState(tpf);
        offBox.updateGeometricState();
    }
This test renders a scene to an offscreen framebuffer, then copies
the contents to a Swing JFrame. Note that some parts are done inefficiently,
this is done to make the code more readable.
jme3-examples/src/main/java/jme3test/post/TestRenderToMemory.java