@Override
    public void simpleInitApp() {
        cam.setLocation(new Vector3f(3, 3, 3));
        cam.lookAt(Vector3f.ZERO, Vector3f.UNIT_Y);

        //setup main scene
        Geometry quad = new Geometry("box", new Box(1, 1, 1));

        Texture offTex = setupOffscreenView();

        Material mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
        mat.setTexture("ColorMap", offTex);
        quad.setMaterial(mat);
        rootNode.attachChild(quad);
        inputManager.addMapping(TOGGLE_UPDATE, new KeyTrigger(KeyInput.KEY_SPACE));
        inputManager.addListener(this, TOGGLE_UPDATE);
    }
This test renders a scene to a texture, then displays the texture on a cube.
jme3-examples/src/main/java/jme3test/post/TestRenderToTexture.java