Estoy usando LibGDX 0.9.9. Estoy tratando de renderizar cubemap y niebla. Entonces mi fragmento de código a continuación:

public void show() {
    modelBatch = new ModelBatch();
    environment = new Environment();
    environment.set(new ColorAttribute(ColorAttribute.AmbientLight, 1.0f, 0.4f, 0.4f, 1f));

    environment.set(new ColorAttribute(ColorAttribute.Fog, 0.9f, 1f, 0f, 1f));
    environment.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f, -0.2f));

    cubemap = new Cubemap(Gdx.files.internal("cubemap/pos-x.png"), 
                Gdx.files.internal("cubemap/neg-x.png"), 
                Gdx.files.internal("cubemap/pos-y.png"), 
                Gdx.files.internal("cubemap/neg-y.png"), 
                Gdx.files.internal("cubemap/pos-z.png"), 
                Gdx.files.internal("cubemap/neg-z.png"));
    environment.set(new CubemapAttribute(CubemapAttribute.EnvironmentMap, cubemap));

    cam = new PerspectiveCamera(67, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    cam.position.set(1f, 1f, 1f);
    cam.lookAt(0,0,0);
    cam.near = 0.1f;
    cam.far = 300f;
    cam.update();

    ModelLoader loader = new ObjLoader();

    model = loader.loadModel(Gdx.files.internal("earth/earth.obj"));


    instance = new ModelInstance(model);



    NodePart blockPart = model.nodes.get(0).parts.get(0);

    renderable = new Renderable();
    blockPart.setRenderable(renderable);
    renderable.environment = environment;
    renderable.worldTransform.idt();        

    renderContext = new RenderContext(new DefaultTextureBinder(DefaultTextureBinder.WEIGHTED, 1));

    shader = new DefaultShader(renderable);
    shader.init();

    camController = new CameraInputController(cam);
    Gdx.input.setInputProcessor(camController);
}

@Override
public void render(float delta) {
camController.update();

    Gdx.gl.glViewport(0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT | GL20.GL_DEPTH_BUFFER_BIT);

    renderContext.begin();
    shader.begin(cam, renderContext);

    shader.render(renderable);
    shader.end();
    renderContext.end();
}

Pero nada pasa. Solo veo objeto. ¿Qué estoy haciendo mal?