IECoreUSD : Add SceneAlgo::sceneFromStage

[andrewkaufman]

This utility can be used to wrap a live in-memory pxr::UsdStage into an IECoreScene::SceneInterface (as though the stage was already written to a file).

I'm putting this up as a draft for now. It follows some discussion @johnhaddon and I had already, but I'm not convinced I like the utility method... As a client of SceneInterfaces, its confusing to not just call SceneInterface.create or SharedSceneInterfaces.get as normal...

I wonder if it'd be worth the extra effort of adding SceneInterface.create( const CompoundData *args ) (as we've discussed for other uses) and having USDScene check args for a stage, so we can replace the python access from

scene = IECoreUSD.SceneAlgo.sceneFromStage( stage )

to

scene = IECoreScene.SceneInterface.create( { "stage" : stage } )

Note also I've currently hard-coded to Read mode only when providing a stage. I'm not sure if that's a good idea, but it was easier to get the draft PR up today without testing write mode...

[johnhaddon]

I do think it's worth adding SceneInterface::create(), as we have other potential use cases for it. But you can't put a UsdStageRefPtr directly in a CompoundData, so we would need an alternative way of passing it. This snippet from the USD docs hints at one possible mechanism :

Caches provide a mechanism that associates a lightweight key, UsdStageCache::Id, with a cached stage. A UsdStageCache::Id can be converted to and from long int and string. This can be useful for communicating within a third party application that cannot transmit arbitrary C++ objects.

Worth investigating?

[andrewkaufman]

I put up a WIP commit demoing the UsdStageCache idea. I'm not sure if its any better than the sceneFromStage function, the client still needs to know to do something special with the stage... I guess one advantage is it may be easier to shoe-horn a live stage into Gaffer by passing ids through a context.

But I also realized I have a pretty easy workaround for my immediate production need, generating a light-weight tmp usda file, so I'm going to close this PR for now.

We should take some time in the near future to spec out the SceneInterface::create( args ) mechanism (#1124).