Transform matrices in scene hierarchy not interpreted correctly.

[nwessing]

I have a stripped down example here where the GTLFUtility does not generate the correct transform for a particular node. Here is the file

gltf_files.zip

Here is how the model should be rendered (screenshot from Windows 3D viewer):

Here is how the model is rendered after imported into Unity with GLTFUtility:

I have tested this in several other GLTF viewers and they all match behavior with Windows 3D Viewer. Babylon.js

Cesium

Filament

Threejs

Khronos's GLTF importer interprets the matrix a bit differently than this importer does. https://github.com/KhronosGroup/UnityGLTF/blob/1e003333698be2d6026211c2ee85ddbd25d07b72/UnityGLTF/Assets/UnityGLTF/Runtime/Scripts/Extensions/SchemaExtensions.cs#L171

https://github.com/KhronosGroup/UnityGLTF/blob/1e003333698be2d6026211c2ee85ddbd25d07b72/UnityGLTF/Assets/UnityGLTF/Runtime/Scripts/Extensions/SchemaExtensions.cs#L42

[code-monkey-101]

I'm surprised, this bug hasn't been fixed yet since it makes the tool basically unusable.

This is how you fix it based on the code that @nwessing posted: Change the ApplyTRS method in GLTFNode.cs to this:

/// <summary> Set local position, rotation and scale </summary>
public void ApplyTRS(Transform transform) {
	/// glTF is a right-handed coordinate system, where the 'right' direction is -X relative to Unity's coordinate system.
	/// glTF matrix: column vectors, column-major storage, +Y up, +Z forward, -X right, right-handed
	/// Unity matrix: column vectors, column-major storage, +Y up, +Z forward, +X right, left-handed
	/// Multiply by a negative X scale to convert handedness
	Vector3 coordinateSpaceConversionScale = new Vector3(-1, 1, 1);
	if (matrix != Matrix4x4.identity)
	{
		Matrix4x4 convert = Matrix4x4.Scale(coordinateSpaceConversionScale);
		Matrix4x4 matrixLH = convert * matrix * convert;

		transform.localPosition = matrixLH.GetColumn(3);

		Vector3 x = matrixLH.GetColumn(0);
		Vector3 y = matrixLH.GetColumn(1);
		Vector3 z = matrixLH.GetColumn(2);
		Vector3 calculatedZ = Vector3.Cross(x, y);
		bool mirrored = Vector3.Dot(calculatedZ, z) < 0.0f;
		transform.localScale = new Vector3(x.magnitude * (mirrored ? -1.0f : 1.0f), y.magnitude, z.magnitude);

		transform.localRotation = Quaternion.LookRotation(matrixLH.GetColumn(2), matrixLH.GetColumn(1));
	}
	else
	{
		transform.localPosition = Vector3.Scale(translation, coordinateSpaceConversionScale);

		Vector3 fromAxisOfRotation = new Vector3(rotation.x, rotation.y, rotation.z);
		Vector3 toAxisOfRotation = -1.0f * Vector3.Scale(fromAxisOfRotation, coordinateSpaceConversionScale);
		transform.localRotation = new Quaternion(toAxisOfRotation.x, toAxisOfRotation.y, toAxisOfRotation.z, rotation.w);

		transform.localScale = scale;
	}
}