Add validation for rotation axis to SpringBoneSimulator3D

doc/classes/SpringBoneCollision3D.xml

@@ -7,6 +7,7 @@
 		A collision can be a child of [SpringBoneSimulator3D]. If it is not a child of [SpringBoneSimulator3D], it has no effect.
 		The colliding and sliding are done in the [SpringBoneSimulator3D]'s modification process in order of its collision list which is set by [method SpringBoneSimulator3D.set_collision_path]. If [method SpringBoneSimulator3D.are_all_child_collisions_enabled] is [code]true[/code], the order matches [SceneTree].
 		If [member bone] is set, it synchronizes with the bone pose of the ancestor [Skeleton3D], which is done in before the [SpringBoneSimulator3D]'s modification process as the pre-process.
+		[b]Warning:[/b] A scaled [SpringBoneCollision3D] will likely not behave as expected. Make sure that the parent [Skeleton3D] and its bones are not scaled.
 	</description>
 	<tutorials>
 	</tutorials>

doc/classes/SpringBoneSimulator3D.xml

@@ -10,6 +10,7 @@
 		Several properties can be applied to each joint, such as [method set_joint_stiffness], [method set_joint_drag], and [method set_joint_gravity].
 		For simplicity, you can set values to all joints at the same time by using a [Curve]. If you want to specify detailed values individually, set [method set_individual_config] to [code]true[/code].
 		For physical simulation, [SpringBoneSimulator3D] can have children as self-standing collisions that are not related to [PhysicsServer3D], see also [SpringBoneCollision3D].
+		[b]Warning:[/b] A scaled [SpringBoneSimulator3D] will likely not behave as expected. Make sure that the parent [Skeleton3D] and its bones are not scaled.
 	</description>
 	<tutorials>
 	</tutorials>
@@ -583,6 +584,7 @@
 			<description>
 				Sets the rotation axis of the bone chain. If sets a specific axis, it acts like a hinge joint.
 				The value is cached in each joint setting in the joint list.
+				[b]Note:[/b] The rotation axis and the forward vector shouldn't be colinear to avoid unintended rotation since [SpringBoneSimulator3D] does not factor in twisting forces.
 			</description>
 		</method>
 		<method name="set_stiffness">

scene/3d/spring_bone_simulator_3d.cpp

@@ -939,6 +939,10 @@ void SpringBoneSimulator3D::set_joint_rotation_axis(int p_index, int p_joint, Ro
 	Vector<SpringBone3DJointSetting *> &joints = settings[p_index]->joints;
 	ERR_FAIL_INDEX(p_joint, joints.size());
 	joints[p_joint]->rotation_axis = p_axis;
+	Skeleton3D *sk = get_skeleton();
+	if (sk) {
+		_validate_rotation_axis(sk, p_index, p_joint);
+	}
 }
 
 SpringBoneSimulator3D::RotationAxis SpringBoneSimulator3D::get_joint_rotation_axis(int p_index, int p_joint) const {
@@ -1252,6 +1256,35 @@ void SpringBoneSimulator3D::remove_child_notify(Node *p_child) {
 	}
 }
 
+void SpringBoneSimulator3D::_validate_rotation_axes(Skeleton3D *p_skeleton) const {
+	for (int i = 0; i < settings.size(); i++) {
+		for (int j = 0; j < settings[i]->joints.size(); j++) {
+			_validate_rotation_axis(p_skeleton, i, j);
+		}
+	}
+}
+
+void SpringBoneSimulator3D::_validate_rotation_axis(Skeleton3D *p_skeleton, int p_index, int p_joint) const {
+	RotationAxis axis = settings[p_index]->joints[p_joint]->rotation_axis;
+	if (axis == ROTATION_AXIS_ALL) {
+		return;
+	}
+	Vector3 rot = get_vector_from_axis(static_cast<Vector3::Axis>((int)axis));
+	Vector3 fwd;
+	if (p_joint < settings[p_index]->joints.size() - 1) {
+		fwd = p_skeleton->get_bone_rest(settings[p_index]->joints[p_joint + 1]->bone).origin;
+	} else if (settings[p_index]->extend_end_bone) {
+		fwd = get_end_bone_axis(settings[p_index]->end_bone, settings[p_index]->end_bone_direction);
+		if (fwd.is_zero_approx()) {
+			return;
+		}
+	}
+	fwd.normalize();
+	if (Math::is_equal_approx(Math::absf(rot.dot(fwd)), 1.0f)) {
+		WARN_PRINT_ED("Setting: " + itos(p_index) + " Joint: " + itos(p_joint) + ": Rotation axis and forward vectors are colinear. This is not advised as it may cause unwanted rotation.");
+	}
+}
+
 void SpringBoneSimulator3D::_find_collisions() {
 	if (!collisions_dirty) {
 		return;
@@ -1422,6 +1455,10 @@ void SpringBoneSimulator3D::_update_joints() {
 		settings[i]->joints_dirty = false;
 	}
 	joints_dirty = false;
+	Skeleton3D *sk = get_skeleton();
+	if (sk) {
+		_validate_rotation_axes(sk);
+	}
 #ifdef TOOLS_ENABLED
 	update_gizmos();
 #endif // TOOLS_ENABLED

scene/3d/spring_bone_simulator_3d.h

@@ -163,6 +163,9 @@ protected:
 	virtual void move_child_notify(Node *p_child) override;
 	virtual void remove_child_notify(Node *p_child) override;
 
+	void _validate_rotation_axes(Skeleton3D *p_skeleton) const;
+	void _validate_rotation_axis(Skeleton3D *p_skeleton, int p_index, int p_joint) const;
+
 public:
 	// Setting.
 	void set_root_bone_name(int p_index, const String &p_bone_name);