@Override
    public void simpleUpdate(float timePerFrame) {
        /*
         * Since SimpleApplication is unaware of the background viewport,
         * the application must explicitly update its scenes.
         */
        List<Spatial> scenes = backgroundViewport.getScenes();
        for (Spatial scene : scenes) {
            scene.updateLogicalState(timePerFrame);
            scene.updateGeometricState();
        }
    }
Demonstrates how to render a non-moving background image using a pre
ViewPort.

@author Stephen Gold sgold@sonic.net
jme3-examples/src/main/java/jme3test/renderer/TestBackgroundImage.java