@cpinter is this issue still relevant?

VR display nodes have the particularity to reference displayable node. Therefore they don't really need the displayable node to reference them. But it would be misleading to have display nodes without displayable node.

It is true that vol.ren. display nodes explicitly reference the volume node unlike other display nodes including the volume displayable node, which do an exhaustive search in the scene (but it has a one-item cache, see LastFoundDisplayableNode). And this reference is set in the logic by the UpdateDisplayNodeFromVolumeNode function (and I think this is part of the reason volume rendering doesn't work without having to call these update functions). It seems that this explicit reference has performance reasons, because it is faster to get the volume node from the display node in the displayable manager in functions like UpdateDisplayNodePipeline.

It may make sense to try and remove this extra complexity because it seems unlikely that performance would noticeably decrease.

For this reason, it would be better if the VR displayable manager hides VR display nodes if they are not referenced by at least one volume displayable node.

I'm not sure if this could be a problem or whether it is handled by the current code. However, doing the simplification would solve it for sure.

Thanks for the analysis. I'll take care of this then.