do it properly

irr/include/SkinnedMesh.h

@@ -319,8 +319,10 @@ public:
 				return transform;
 
 			if (std::holds_alternative<core::matrix4>(transform)) {
-				// TODO raise a warning: Attempt to animate a static joint.
-				return transform;
+				// .x lets animations override matrix transforms entirely.
+				core::Transform trs;
+				keys.updateTransform(frame, trs);
+				return {trs};
 			}
 
 			auto trs = std::get<core::Transform>(transform);

irr/src/CAnimatedMeshSceneNode.cpp

@@ -6,7 +6,6 @@
 #include "CBoneSceneNode.h"
 #include "IVideoDriver.h"
 #include "ISceneManager.h"
-#include "MatrixBoneSceneNode.h"
 #include "S3DVertex.h"
 #include "Transform.h"
 #include "matrix4.h"
@@ -22,6 +21,7 @@
 #include "quaternion.h"
 #include <algorithm>
 #include <cstddef>
+#include <optional>
 
 namespace irr
 {
@@ -546,14 +546,11 @@ void CAnimatedMeshSceneNode::addJoints()
 		if (joint->ParentJointID)
 			parent = JointChildSceneNodes.at(*joint->ParentJointID); // exists because of topo. order
 		assert(parent);
-		if (const auto *matrix = std::get_if<core::matrix4>(&joint->transform)) {
-			JointChildSceneNodes.push_back(new MatrixBoneSceneNode(
-					parent, SceneManager, 0, i, joint->Name, *matrix));
-		} else {
-			JointChildSceneNodes.push_back(new CBoneSceneNode(
-					parent, SceneManager, 0, i, joint->Name,
-					std::get<core::Transform>(joint->transform)));
-		}
+		const auto *matrix = std::get_if<core::matrix4>(&joint->transform);
+		JointChildSceneNodes.push_back(new CBoneSceneNode(
+				parent, SceneManager, 0, i, joint->Name,
+				matrix ? core::Transform{} : std::get<core::Transform>(joint->transform),
+				matrix ? *matrix : std::optional<core::matrix4>{}));
 	}
 }
 
@@ -562,11 +559,13 @@ void CAnimatedMeshSceneNode::updateJointSceneNodes(
 {
 	for (size_t i = 0; i < transforms.size(); ++i) {
 		const auto &transform = transforms[i];
-		IBoneSceneNode *node = JointChildSceneNodes[i];
+		auto *node = static_cast<CBoneSceneNode*>(JointChildSceneNodes[i]);
 		if (const auto *trs = std::get_if<core::Transform>(&transform)) {
-			dynamic_cast<CBoneSceneNode*>(node)->setTransform(*trs);
+			node->setTransform(*trs);
+			// .x lets animations override matrix transforms entirely.
+			node->Matrix = std::nullopt;
 		} else {
-			assert(dynamic_cast<MatrixBoneSceneNode*>(node));
+			node->Matrix = std::get<core::matrix4>(transform);
 		}
 	}
 }

irr/src/CBoneSceneNode.h

@@ -8,6 +8,7 @@
 
 #include "IBoneSceneNode.h"
 #include "Transform.h"
+#include "matrix4.h"
 
 #include <optional>
 
@@ -23,8 +24,10 @@ public:
 	CBoneSceneNode(ISceneNode *parent, ISceneManager *mgr,
 			s32 id = -1, u32 boneIndex = 0,
 			const std::optional<std::string> &boneName = std::nullopt,
-			const core::Transform &transform = {}) :
-		IBoneSceneNode(parent, mgr, id, boneIndex, boneName)
+			const core::Transform &transform = {},
+			const std::optional<core::matrix4> &matrix = std::nullopt) :
+		IBoneSceneNode(parent, mgr, id, boneIndex, boneName),
+		Matrix(matrix)
 	{
 		setTransform(transform);
 	}
@@ -42,6 +45,17 @@ public:
 		}
 		setScale(transform.scale);
 	}
+
+	core::matrix4 getRelativeTransformation() const override
+	{
+		if (Matrix)
+			return *Matrix;
+		return IBoneSceneNode::getRelativeTransformation();
+	}
+
+	//! Some file formats alternatively let bones specify a transformation matrix.
+	//! If this is set, it overrides the TRS properties.
+	std::optional<core::matrix4> Matrix;
 };
 
 } // end namespace scene

irr/src/MatrixBoneSceneNode.h

@@ -1,56 +0,0 @@
-// Copyright (C) 2025 Joe Mama
-// SPDX-License-Identifier: LGPL-2.1-or-later
-
-#pragma once
-
-#include "IBoneSceneNode.h"
-#include "matrix4.h"
-#include "vector3d.h"
-#include <iostream>
-
-#include <optional>
-
-// We must represent some transforms differently:
-// Bones can have static non-TRS matrix transforms,
-// for example shearing (can not be decomposed at all)
-// or negatively scaling an axis (can not be decomposed uniquely).
-// Hence well-defined animation is not possible for such nodes
-// (and in fact glTF even guarantees that they will never be animated).
-
-namespace irr
-{
-namespace scene
-{
-
-class MatrixBoneSceneNode : public IBoneSceneNode
-{
-public:
-	//! constructor
-	MatrixBoneSceneNode(ISceneNode *parent, ISceneManager *mgr,
-			s32 id = -1, u32 boneIndex = 0,
-			const std::optional<std::string> &boneName = std::nullopt,
-			const core::matrix4 &matrix = core::IdentityMatrix) :
-		IBoneSceneNode(parent, mgr, id, boneIndex, boneName),
-		matrix(matrix)
-	{}
-
-	const core::matrix4 matrix;
-
-	// Matrix nodes should not be fake decomposed.
-	const core::vector3df &getPosition() const override { assert(false); }
-	const core::vector3df &getRotation() const override { assert(false); }
-	const core::vector3df &getScale() const override { assert(false); }
-
-	// This node should be static.
-	void setPosition(const core::vector3df &pos) override { assert(false); }
-	void setRotation(const core::vector3df &euler_deg) override { assert(false); }
-	void setScale(const core::vector3df &scale) override { assert(false); }
-
-	core::matrix4 getRelativeTransformation() const override
-	{
-		return matrix;
-	}
-};
-
-} // end namespace scene
-} // end namespace irr

irr/src/SkinnedMesh.cpp

@@ -6,7 +6,6 @@
 #include "IBoneSceneNode.h"
 #include "CBoneSceneNode.h"
 #include "IAnimatedMeshSceneNode.h"
-#include "MatrixBoneSceneNode.h"
 #include "SSkinMeshBuffer.h"
 #include "Transform.h"
 #include "aabbox3d.h"