[FBX][UV] Problem exporting multiple UVs (>2)

[zwrtron]

I've ran into a problem with exporting more than two UV channels.
If any object has three or more UV channels, the exported object has no UVs at all.
Forcefully setting hasUV[n] to false for n >= 2 on all ImportedMesh resolves the problem. (only correctly exports first 2 channels)

Simple reproductive example:

• works for any asset
• just copy UV0 (if exists) to UV1 (work) and UV2 (not work)
• no UVs are exported (UV0 is lost)

var converter = new ModelConverter(root, ImageFormat.Png);

foreach (var mesh in converter.MeshList) {
    if (!mesh.hasUV[0]) {
        continue; // no UVs
    }

    mesh.hasUV[1] = true;
    mesh.hasUV[2] = true;

    foreach (var vertex in mesh.VertexList) {
        if (vertex.UV[1] == null) {
            vertex.UV[1] = new float[2];
            Array.Copy(vertex.UV[0], vertex.UV[1], 2);
        }

        if (vertex.UV[2] == null) {
            vertex.UV[2] = new float[2];
            Array.Copy(vertex.UV[0], vertex.UV[2], 2);
        }
        
    }
}

ModelExporter.ExportFbx(
    file, 
    converter,
    true,  //eulerFilter
    0.25f, //filterPrecision
    true,  //allNodes
    true,  //skins
    true,  //animation
    true,  //blendShape
    true, //castToBone
    1, //boneSize
    false, //exportAllUvsAsDiffuseMaps 
    1, //scaleFactor
    3, //versionIndex
    false //isAscii
);

If the UV[2] parts are commented out, it works - UV1 is successfully copy of UV0 (if UV1 not exist)

Does anyone know how to fix it?

[aelurum]

Your issue is probably also related to this one Perfare#1047

From what I tested earlier, the problem seems to be in UVs of the same type.
In the current AS code uv0 = diffuse map, uv1 = normal map, uv2+ = diffuse map.
Would be more correct if we change uv2 to some other type, such as displacement map.

You would need to add something like this to AssetStudioFBXNative\api.cpp

AS_API(void) AsFbxMeshCreateDisplacementUV(FbxMesh* pMesh, int32_t uv)
{
	if (pMesh == nullptr)
	{
		return;
	}

	auto pUV = pMesh->CreateElementUV(FbxString("UV") + FbxString(uv), FbxLayerElement::eTextureDisplacement);
	pUV->SetMappingMode(FbxGeometryElement::eByControlPoint);
	pUV->SetReferenceMode(FbxGeometryElement::eDirect);
}

to AssetStudioFBXNative\api.h

AS_API(void) AsFbxMeshCreateDisplacementUV(fbxsdk::FbxMesh* pMesh, int32_t uv);

to AssetStudioFBXWrapper\FbxExporterContext.PInvoke.cs

 [DllImport(FbxDll.DllName, CallingConvention = CallingConvention.Winapi)]
 private static extern void AsFbxMeshCreateDisplacementUV(IntPtr mesh, int uv);

And then replace this part

AssetStudio/AssetStudioFBXWrapper/FbxExporterContext.cs

Lines 256 to 268 in f7e6d23

	for (int i = 0; i < importedMesh.hasUV.Length; i++)
	{
	if (!importedMesh.hasUV[i]) { continue; }
	if (i == 1 && !exportAllUvsAsDiffuseMaps)
	{
	AsFbxMeshCreateNormalMapUV(mesh, 1);
	}
	else
	{
	AsFbxMeshCreateDiffuseUV(mesh, i);
	}
	}

with something like this

for (var i = 0; i < importedMesh.hasUV.Length; i++)
{
    if (!importedMesh.hasUV[i]) { continue; }

    switch (i)
    {
        case 1 when !exportAllUvsAsDiffuseMaps:
            AsFbxMeshCreateNormalMapUV(mesh, i);
            break;
        case 2 when !exportAllUvsAsDiffuseMaps:
            AsFbxMeshCreateDisplacementUV(mesh, i);
            break;
        default:
            AsFbxMeshCreateDiffuseUV(mesh, i);
            break;
    }
}

Still not sure how correct this workaround is, but it may solve some issues with multiple UVs 👀

[zwrtron]

Honestly I have no idea how "Diffuse" and "Normal Map" UVs are different, but I can confirm that using different type, as you suggested, works as expected and UV2 is correctly exported.
Thank you!