UglTFRuntimeAsset will not load static mesh when nodes contain just "lines"

[matthew-rimmer]

Hello!

There appears to be an issue in loading models that have only "lines", i.e. vertices/edges with no faces. I have an example model here (GitHub will only let me upload it as zip, apologies. But you can recreate it manually in blender by just making a line):

plantModel.zip

This model has nodes which are just "lines" around some of the leaves. When I load it into an asset and load the static mesh via asset->LoadStaticMeshRecursive, it returns a nullptr and just refuses to load the model. I believe this is due to this function returning false when bSkipLine is true:

if (Primitive.Mode >= 1 && Primitive.Mode <= 3 && MaterialsConfig.bSkipLines)
{
	return false;
}

Which in turn causes LoadMeshIntoMeshLOD to return false, and then LoadStaticMeshRecursive returns a nullptr here:

FglTFRuntimeMeshLOD* LOD = nullptr;
if (!LoadMeshIntoMeshLOD(JsonMeshObject.ToSharedRef(), LOD, StaticMeshConfig.MaterialsConfig))
{
	AddError("LoadStaticMeshRecursive()", FString::Printf(TEXT("Unable to load Mesh \"%s\""), *ChildNode.Name));
	return nullptr;
}

This behaviour seems inconsistent with AglTFRuntimeAssetActor which successfully loads the model, I imagine because it skips over the weird nodes. I would have thought it would be desired for it to skip for static meshes too and render what nodes work? It could be worked around already by working out what nodes are lines and adding them to the excludeNodes param, but this seems like it could be a parameter to the function anyway.

[rdeioris]

Hi, lines in Unreal are basically not supported (you have a bunch of slow apis in the editor and some tools for debugging), what I have exposed is a bunch of algorithms for triangulation but given how slow they are they are turned off by default. Probably the safest (works with raytracing too) is this setup (open triangular prism):

other algorithms are half baked or do not work with raytracing (both are definitely fixable, but as you can immagine there are no many requests for lines support)

[rdeioris]

Btw, regarding the recursive, probably it is worthy to have a "robust" one that simply do not exit on broken meshes (the node is what defines the transform so it should be safe)

[matthew-rimmer]

If I update LoadStaticMeshRecursive here to do this instead of returning a nullptr:

FglTFRuntimeMeshLOD* LOD = nullptr;
if (!LoadMeshIntoMeshLOD(JsonMeshObject.ToSharedRef(), LOD, StaticMeshConfig.MaterialsConfig))
{
	continue;
}

I seem to get the same result as the AssetActor which skips the bad nodes but still maintains all the transforms:

An option could be added to FglTFRuntimeStaticMeshConfig to allow skipping of nodes? Or another param? I am happy to make a PR for this

[matthew-rimmer]

To be clear, I don't mind too much about rendering the lines or not. As you say, it would be ideal just to have a function or update the existing that skips over the bad nodes which I'm happy to look at

[rdeioris]

I would add a parameter to LoadStaticMeshRecursive (something like bContinueOnMeshError default to false) or maybe you could create a new config structure FStaticMeshRecursiveConfig where you can put configuration specific to the recursive process and pass it to a new LoadStaticMeshRecursiveAdvanced function.