Generate transparent icon from model

Troubleshooting graphics
10

I also need to generate transparent icon from model.

This is my test code, but the generated png file has no transparency

public class TestIconGenerator extends SimpleApplication {

    public static final FunctionId F_GENERATE_ICON = new FunctionId("Generate Icon");

    private FrameBuffer fb;
    
    public TestIconGenerator() {

    }

    public static void main(String[] args) {
        TestIconGenerator main = new TestIconGenerator();
        AppSettings settings = new AppSettings(true);
        settings.setRenderer(AppSettings.LWJGL_OPENGL3);
        settings.setGammaCorrection(true);
        settings.setVSync(true);
        main.setSettings(settings);
        //main.setShowSettings(false);
        main.start();
    }

    @Override
    public void simpleInitApp() {
        GuiGlobals.initialize(this);
             
        viewPort.setBackgroundColor(ColorRGBA.BlackNoAlpha);
        
        //Camera camera = app.getCamera().clone();
        //camera.resize(256, 256, true);
        //camera.resize(1024, 256, false);

        fb = new FrameBuffer(getCamera().getWidth(), getCamera().getHeight(), 1);
        viewPort.setOutputFrameBuffer(fb);

        //Texture2D fbTex = new Texture2D(getCamera().getWidth(), getCamera().getHeight(), Format.RGBA8);
        fb.setDepthBuffer(Image.Format.Depth);
        fb.setColorBuffer(Format.BGRA8);
        //fb.setColorTexture(fbTex);

        initLight();
        //initFilters();
        initInput();

        Spatial model = getAssetManager().loadModel("tree/model-1/green-tree.gltf");
        rootNode.attachChild(model);
    }

    private void initLight() {
        Node probeNode = (Node) assetManager.loadModel("lightprobe/test-scene.j3o");
        LightProbe probe = (LightProbe) probeNode.getLocalLightList().iterator().next();
        rootNode.addLight(probe);

        /**
         * A white ambient light source.
         */
        AmbientLight ambient = new AmbientLight();
        ambient.setColor(ColorRGBA.White);
        rootNode.addLight(ambient);
    }

    private void initInput() {
        InputMapper inputMapper = GuiGlobals.getInstance().getInputMapper();
        inputMapper.addDelegate(F_GENERATE_ICON, this, "generateIcon");
        inputMapper.map(F_GENERATE_ICON, KeyInput.KEY_SPACE);
    }

    private void initFilters() {
        FilterPostProcessor fpp = new FilterPostProcessor(assetManager);
        fpp.addFilter(new FXAAFilter());
        fpp.addFilter(new ToneMapFilter(Vector3f.UNIT_XYZ.mult(11.0f)));//4.0f
        fpp.addFilter(new SSAOFilter(0.5f, 3, 0.2f, 0.2f));
        viewPort.addProcessor(fpp);
    }

    public void savePng(File f, Image img) throws IOException {
        OutputStream out = new FileOutputStream(f);
        try {
            JmeSystem.writeImageFile(out, "png", img.getData(0), img.getWidth(), img.getHeight());
        } finally {
            out.close();
        }
    }

    protected Image createFrameBufferImage(FrameBuffer fb) {
        int width = fb.getWidth();
        int height = fb.getHeight();
        int size = width * height * 4;
        ByteBuffer buffer = BufferUtils.createByteBuffer(size);
        //Image.Format format = fb.getColorBuffer().getFormat();

        // I guess readFrameBuffer always writes in the same
        // format regardless of the frame buffer format
        Image.Format format = Image.Format.BGRA8;
        return new Image(format, width, height, buffer);
    }

    public void generateIcon() {

        Renderer renderer = getRenderer();
        Image image = createFrameBufferImage(fb);
        //Texture2D texture = new Texture2D(image);

        renderer.readFrameBuffer(fb, image.getData(0));
        //image.setUpdateNeeded();

        try {
            savePng(new File("/home/ali/Desktop/icon.png"), image);
        } catch (IOException ex) {
            Logger.getLogger(IconGeneratorState.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

this is how my model looks at runtime :

and this png file i get after generating icon :

It looks darker and there is no transparency.

Something should be wrong in my code, I have no deep understanding of how this FrameBuffer stuff works.

Will appreciate any help.