GLTF: Use scene root nodes for root nodes, don't include orphan nodes

[aaronfranke]

This fixes a bug noticed by the Blender Project DogWalk team where Godot was including orphan nodes as root nodes. This is incorrect, only the nodes actually indicated as scene root nodes should be used as root nodes. https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#scenes

Luckily the fix was pretty simple, just remove the code that re-assigns top orphan nodes as root nodes. Test file:

{
	"asset": {
		"version": "2.0"
	},
	"nodes": [
		{ "name": "A" },
		{ "name": "B" }
	],
	"scenes": [
		{ "nodes": [0] },
		{ "nodes": [1] }
	],
	"scene": 0
}

In master: Imports both A and B as glTF root nodes as children of the Godot root node (incorrect).
With this PR: Imports only A as the glTF root node as a child of the Godot root node as indicated by the scene (correct).

I also looked into the general root node handling logic. Currently Godot does not correctly handle the case of a file not specifying scenes or scene root nodes, and will error when trying to parse anything that doesn't look like a scene. This will need to be improved for godotengine/godot-proposals#7494, but for this PR I kept things limited to just this bug fix.

[fire]

Did you link any sample wrong and correct gltf files?

[aaronfranke]

@fire The PR contains an example file already. Did you want me to also include a file that is unaffected by this PR?

I’ll check with the creator of this par.

[yankscally]

in blender, you can have multiple scenes, and it is respected in the gltf file:

here is the export:

{
	"asset":{
		"generator":"Khronos glTF Blender I/O v4.3.47",
		"version":"2.0"
	},
	"scene":0,
	"scenes":[
		{
			"name":"Scene",
			"nodes":[
				0
			]
		},
		{
			"name":"Scene.2",
			"nodes":[
				1
			]
		}
	],
	"nodes":[
		{
			"mesh":0,
			"name":"Cube"
		},
		{
			"mesh":1,
			"name":"Sphere"
		}
	],
	"meshes":[
		{
			"name":"Cube",

		},
		{
			"name":"Sphere",

                }

with blender being able to export to multiple scenes, it seems like a shame to not be using this in godot, and discarding extra scenes for the "selected" one. The selected scene from blender is the one you are currently looking at in that drop down, and still exports the other scenes data. Discarding it in godot feels like a waste of a possibly useful feature.

I experimented with using scenes in a blender project to section reusable mesh objects into a space I don't have to look at.

I thought about this:

But that doesn't feel like a particularly great solution when we are adding KHR_node_visibility soon, but in some ways thats what blender is doing under the hood. wish I had a better idea for importing multiple scenes.

Here is blender "hiding" objects that are not in the scene:

Edit*
I read over this again and actually think hiding the not selected "scene root" nodes under the godot root if there are multiple scenes might work well and is not a terrible idea, a godot scene does not equate to a glTF scene, we can easily use a node in godot for this purpose and hide it.

It's just a lot of work exposing all of this to godot users for no anticipated benefits.

[fire]

Needs discussion in a Godot Engine pipeline meeting

[lyuma]

I like this...
What do you think about adding this to the Godot 4.5 or later compatibility mode from #104184? (So older compatibility setting will still include all the orphan nodes)

[aaronfranke]

@lyuma Sure, we can do that.

[lyuma]

Approved contingent upon adding the compatibility mode check merged in this pr

• GLTF: Don't collapse non-joint leaf nodes when importing skeletons #104184

[lyuma]

Looks good! Nice fix

In the future, we may want to figure out how to expose the other scenes, perhaps as sub-resources, but this fix is necessary to allow us to further develop gltf scene functionality in the future.

[akien-mga]

Thanks!