Broken mesh and absent skeleton if not modeled under a common parent

[charlesfleche]

Describe the bug

If a Mesh is skinned but its Skeleton is not placed under a common parent:

• the Skeleton is not exported
• the export Mesh is broken (as it does not have any properties beyond its transform

Steps to reproduce

The scene attached to this issues contains three elements:

• SkinnedMeshRoot contains a skeleton and a skinned mesh
• JustASkeletonRoot contains a skeleton, without a mesh
• BoxC is skinned by the Skeleton BottomC/MiddleC/TopC, but they don't have a common parent.

When exported with these options:

Then:

• OK: SkinnedMeshRoot is exported as expected: a Mesh and Skel nested under a SkelRoot
• OK: JustASkeletonRoot is exported as expected: a Skel under a Skelroot
• PROBLEM: BoxC is only exported as a transform and does not contain other properties
• PROBLEM: BottomC/MiddleC/TopC is not exported at all

Expected behavior

First, the mesh should not be broken.

Second, I think that we should export both the Mesh and the Skeleton without a SkelRoot. The SkelRoot itself does not have properties that are authored to represent the skinning of the mesh. SkelRoot are "only" needed when the USD is consumed down the line by something that needs the skinning operation to be computed, but not represented. By requiring a top level 3ds Max node, we force an uneeded scene structure convention: we might want to export just a Skeleton from a scene, without a SkelRoot, to be assembled later by another process down the pipeline.

It's actually a very common workflow for our gaming pipeline, where the assembly is not even done in USD but by the game engine itself: we just need to transfer a Skeleton and a skinned Mesh.

For new scenes, it would be possible to always create this common toplevel node. The problem we're facing is with current production scenes: it forces us to adapt our export scripts to temporarily reparent the objects to be exported under a common node, export, then de-parent the exported objects and deal with an USD structure that does not match the 3ds Max original scene structure.

I think the right way would be for 3ds Max to:

• export the most top-level node as a SkelRoot to indicate that this stage subtree is Skinned
• just not have a SkelRoot if there is no top level node

In the example below:

• b will become a SkelRoot, because the subtree below has skinned prims
• Mesh003 and Skel003 do not have a SkelRoot ancestor, and that's exactly the intent

.
├── a/
│   └── b/
│       ├── Mesh001 (skinned by Skel001)
│       ├── d/
│       │   └── Skel001
│       └── e/
│           ├── Mesh002 (skinned by Skel002)
│           └── Skel002
├── Mesh003 (Skinned by Skel003)
└── Skel003

Attachments

test-no-skelroot.zip

Contains:

• test-no-skelroot.max, the sample scene above
• test-no-skelroot.usda, the full exported usda

Specs (if applicable):

• 3ds Max version: 2024.2.11
• 3ds USD version: 0.15.0.14

[antonkhelou]

Hi again @charlesfleche, thanks for the ticket, we're going to have a look at it internally and get back to you soon.

[barbalt]

Hi @charlesfleche , just to give an update on this.
For the second point. Our dcc has to accommodate usd skel working by default (for the users that just want to export the scene and doesn't have a fully fledge pipeline working). For that it needs the skel root wrapping the whole scene.
We do agree that there should exist an option to also work for scenarios like yours. So we are working to figure what would be the best approach here.

[charlesfleche]

With what I suggest, the current workflow in 3ds Max USD would not change a bit: "you need an USD SkelRoot ? Have a 3ds Max node parenting the skinned section of your scene". That's already a requirement.

But it would allow the simple, explicit, pipeline friendly way of just exporting top level Skeleton + Mesh.

[charlesfleche]

For the record I'm having the same discussion with the Blender devs at the moment : https://projects.blender.org/blender/blender/issues/161702

[barbalt]

Hello @charlesfleche , we've been trying to repro your issue with boxC with no success. We tried using the exporter UI with the same options selected like you have shown here and it exports fine. We also noticed from your usd file results example that you changed the name of the skel prim, which currently can only be done with via script, but even then we still got the box with the correct information.
Is there any chance that could be something else affecting the results?

[charlesfleche]

Hum, I'm going to check again and report back here.

[charlesfleche]

I think I understood where the problem was: in our export scripts, we set the usd export option to RootPrimPath to /. Try to export with this script, it mimicks our exporter script settings:

import tempfile

from pymxs import runtime as rt

export_options = rt.USDExporter.createOptions()

export_options.RootPrimPath = "/"   # <-- THE IMPORTANT SETTING

export_options.Meshes = True
export_options.Shapes = False
export_options.Lights = False
export_options.Cameras = False
export_options.Skin = True
export_options.Morpher = True
export_options.Materials = True
export_options.ShadingMode = rt.name("useRegistry")  # none, useRegistry
export_options.AllMaterialTargets = rt.name("UsdPreviewSurface")

export_options.UsdStagesAsReferences = False
export_options.HiddenObjects = True
export_options.UseUSDVisibility = False
export_options.AllowNestedGprims = False

export_options.FileFormat = rt.name("ascii")  # ascii, binary
export_options.Normals = rt.name("asAttribute")  # none, asAttribute, asPrimvar
export_options.MeshFormat = rt.name("polyMesh")  # fromScene, polyMesh, triMesh
export_options.TimeMode = rt.name("current")

export_options.SetChannelPrimvarMapping(-2, "displayOpacity")  # Vertex Alpha
export_options.SetChannelPrimvarMapping(-1, "LayerWeights")  # Vertex Illum
export_options.SetChannelPrimvarMapping(0, "displayColor")  # Vertex Color

export_options.StartFrame = float(rt.animationRange.start)
export_options.EndFrame = float(rt.animationRange.end)
export_options.SamplesPerFrame = 1.0

export_options.UpAxis = rt.name("z")  # y, z
export_options.BakeObjectOffsetTransform = True
export_options.PreserveEdgeOrientation = True

export_options.BonesPrimName = "MyDefaultBonesPrimName"
export_options.AnimationsPrimName = "MyDefaultBonesPrimName"

export_options.IncludeAllBones = True
export_options.PreserveBoneMeshes = False
export_options.SimplifyBonePaths = True

export_options.UseWorldspaceRoot = False

export_options.UseProgressBar = True

export_options.LogLevel = rt.name("info")  # off, info, warn, error

export_options.OpenInUsdview = False

export_options.ChaserNames = rt.Array()
export_options.AllChaserArgs = rt.Dictionary()

with tempfile.NamedTemporaryFile(prefix="maxusd-", suffix=".usda", delete=False) as fp:
    path = fp.name

print(path)

rt.USDExporter.ExportFile(
	path,
	exportOptions=export_options,
	contentSource=rt.name("all"),  # all, selected, nodeList,
)

On Max 2027 / MaxUSD v0.16.2.31, the bug is still present, see the attached exported USD.

maxusd-2027-v0.16.2.31.zip