How to modify the post_processing.rs example to apply to UI as well in bevy 0.13?

[bilowik]

I modified the post processing example to fit my needs, but noticed it wasn't applying to the UI, and neither does the base example, which is likely intentional of course but for my use case I would like it to apply to UI element as well. I have tried the following:

    render_app
            .add_render_graph_node::<ViewNodeRunner<PostProcessNode>>(
                SubGraphUi, 
                PostProcessLabel, 
            ) 
            .add_render_graph_edge(
                SubGraphUi, 
                NodeUi::UiPass,
                PostProcessLabel, 
            );

Thinking this would ensure that it is run after the UiPass node, but I probably have some fundamental misunderstanding of exactly how the edges interact or if the SubGraphUi graph is handled differently than Core2d/3d that make it not work as intended with the PostProcessPlugin as it is.

[bilowik]

After diving in some more on the structure of the bevy's render graph I added different edges that worked as expected, though I'm still unsure why the above does not also work. The following change has the post processing apply to UI as well which works for my specific use case:

            .add_render_graph_node::<ViewNodeRunner<PostProcessNode>>(
                // Specify the label of the graph, in this case we want the graph for 3d
                Core2d,
                // It also needs the label of the node
                PostProcessLabel,
            )
            .add_render_graph_edges(
                Core2d,
                // Specify the node ordering.
                // This will automatically create all required node edges to enforce the given ordering.
                (
                    NodeUi::UiPass,
                    PostProcessLabel,
                    Node2d::Upscaling,
                ),
            );