@Override
    protected void initFilter(AssetManager manager, RenderManager renderManager, ViewPort vp, int w, int h) {
        material = new Material(manager, "Common/MatDefs/Post/Compose.j3md");
    }
sets the compositeTexture

@param compositeTexture the desired texture (alias created)
jme3-effects/src/main/java/com/jme3/post/filters/ComposeFilter.java