Serialise connections to children of ScenePlug/ImagePlug

[johnhaddon]

Summary

Some advanced users would benefit from being able to manage connections to the individual children of ScenePlug and ImagePlug. This is trivially achieved when writing a Python or C++ subclass of SceneProcessor/ImageProcessor as the connections can be made in the constructor. But if the connections are made manually in a regular node graph, they are not serialised because the Serialisable flag has been turned off for the children.

User story

A common use is to use an expression to drive the globals or metadata, while passing everything else through via pass-through connections.

Implementation notes

This is trivially achieved by modifying the ScenePlug and ImagePlug constructors to keep the Serialisable flag for the children. But doing this leads to test failures because we end up serialising internal connections for regular SceneProcessor/ImageProcessor subclasses. I think ideally we should make SceneProcessor.out and ImageProcessor.out non-serialisable by default as it has continually tripped us up, but this is a breaking change and we don't know the consequences for custom nodes in the wild. Perhaps it is sufficient just to document the change and say "turn serialisation back on in your constructor if you need it"?

[johnhaddon]

The more I think about it, the more I think we should disable serialisation of inputs for *Processor.out. Serialising them leaks internal implementation details and can cause problems when the implementation needs to change. We've disabled it manually on a node-by-node basis as we discover the problems caused, but haven't been brave enough just to make the change across the board. I think now is the time.

[danieldresser-ie]

Disabling serialization of *Processor.out definitely sounds good. It would be an extremely special case where you would actually want to serialize that.

I don't entirely understand why it relates to this though ... I would hope that we would only be serializing the child connections in the case where they aren't already implied by the parent connection, like what happens currently with shader color connections? And I would think that the internal connections to "out" would be whole plug connections anyway, so I don't understand why this would affect them?

But standardizing no serialization for the out plug sounds good anyway.

[johnhaddon]

I would think that the internal connections to "out" would be whole plug connections anyway, so I don't understand why this would affect them?

It's really common to pass through some children directly via connections and then implement compute for the remaining unconnected outputs.