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I am struggling trying to traverse an MDLAsset instance created by loading an SCNScene file (.scn).

I want to identify and extract the MDLMeshs as well as camera(s) and lights. I see no direct way to do that.

For example I see this instance method on MDLAsset:

func childObjects(of objectClass: Swift.AnyClass) -> [MDLObject]

Is this what I use?

I have carefully labeled things in the SceneKit modeler. Can I not refer to those which would be ideal. Surely, there is a dictionary of ids/labels that I can get access to. What am I missing here?

UPDATE 0

I had to resort to pouring over the scene graph in the Xcode debugger due to the complete lack of Apple documentation. Sigh ...

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A few things. I see the MDLMesh and MDLSubmesh that is what I am after. What is the traversal approach to get it? Similarly for lights, and camera.

I also need to know the layout of the vertex descriptors so I can sync with my shaders. Can I force a specifc vertex layout on the parsed SCNScene?

dugla
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2 Answers2

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MDLObject has a name (because of its conformance to the MDLNamed protocol), and also a path, which is the slash-separated concatenation of the names of its ancestors, but unfortunately, these don't contain the names of their SceneKit counterparts.

If you know you need to iterate through the entire hierarchy of an asset, you may be better off explicitly recursing through it yourself (by first iterating over the top-level objects of the asset, then recursively enumerating their children), since using childObjects(of:) repeatedly will wind up internally iterating over the entire hierarchy to collect all the objects of the specified type.

Beware that even though MDLAsset and MDLObjectContainerComponent conform to NSFastEnumeration, enumerating over them in Swift can be a little painful, and you might want to manually extend them to conform to Sequence to make your work a little easier.

warrenm
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To get all cameras,

[asset childObjectsOfClass:[MDLCamera class]]

Similarly, to get all MDLObjects,

[asset childObjectsOfClass:[MDLObjects class]]

Etc.

MDLSubmeshes aren't MDLObjects, so you traverse those on the MDLMesh.

There presently isn't a way to impose a vertex descriptor on MDL objects created from SCN objects, but that would be useful.

One thing you can do is to impose a new vertex descriptor on an existing MDL object by setting a mesh's vertexDescriptor property. See the MDLMesh.h header for some discussion.

Nick Porcino
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