@Override
    public void simpleInitApp() {
        this.flyCam.setMoveSpeed(10);
        cam.setLocation(new Vector3f(0.028406568f, 2.015769f, 7.386517f));
        cam.setRotation(new Quaternion(-1.0729783E-5f, 0.9999721f, -0.0073241726f, -0.0014647911f));


        makeScene();

        //Creating the main view port post processor
        FilterPostProcessor fpp = new FilterPostProcessor(assetManager);
        fpp.addFilter(new ColorOverlayFilter(ColorRGBA.Blue));
        viewPort.addProcessor(fpp);

        //creating a frame buffer for the main viewport
        FrameBuffer mainVPFrameBuffer = new FrameBuffer(cam.getWidth(), cam.getHeight(), 1);
        Texture2D mainVPTexture = new Texture2D(cam.getWidth(), cam.getHeight(), Image.Format.RGBA8);
        mainVPFrameBuffer.setDepthTarget(FrameBufferTarget.newTarget(Format.Depth));
        mainVPFrameBuffer.addColorTarget(FrameBufferTarget.newTarget(mainVPTexture));

        viewPort.setOutputFrameBuffer(mainVPFrameBuffer);

        // Create the post processor for the GUI viewport.
        final FilterPostProcessor guiFpp = new FilterPostProcessor(assetManager);
        guiFpp.setFrameBufferFormat(Image.Format.RGBA8);
        guiFpp.addFilter(new ColorOverlayFilter(ColorRGBA.Red));
        // This will compose the main viewport texture with the GUI-viewport back buffer.
        // Note that you can switch the order of the filters so that GUI-viewport filters are applied or not to the main viewport texture
        guiFpp.addFilter(new ComposeFilter(mainVPTexture));

        guiViewPort.addProcessor(guiFpp);
        
        // Compositing is done by mixing texture depending on the alpha channel, so
        // it's important that the GUI-viewport clear-color alpha value is set to 0.
        guiViewPort.setBackgroundColor(ColorRGBA.BlackNoAlpha);
        guiViewPort.setClearColor(true);


    }
This test showcases the possibility to compose the post filtered outputs of several viewports.
The usual use case is when you want to apply some post process to the main viewport and then other post process to the gui viewport
@author Nehon
jme3-examples/src/main/java/jme3test/post/TestPostFiltersCompositing.java