Fix for SkeletonIK not working correctly with 0 interpolation and incorrectly rotating with animation. Now the root bone rotates differently to ensure it always rotates correctly and works with BoneAttachment3D nodes.

scene/3d/skeleton_ik_3d.cpp

@@ -255,9 +255,22 @@ void FabrikInverseKinematic::make_goal(Task *p_task, const Transform &p_inverse_
 
 void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool override_tip_basis, bool p_use_magnet, const Vector3 &p_magnet_position) {
 	if (blending_delta <= 0.01f) {
+		// Before skipping, make sure we undo the global pose overrides
+		ChainItem *ci(&p_task->chain.chain_root);
+		while (ci) {
+			p_task->skeleton->set_bone_global_pose_override(ci->bone, ci->initial_transform, 0.0, false);
+
+			if (!ci->children.is_empty()) {
+				ci = &ci->children.write[0];
+			} else {
+				ci = nullptr;
+			}
+		}
+
 		return; // Skip solving
 	}
 
+	// This line below is part of the problem - removing it fixes the issue with BoneAttachment nodes...
 	p_task->skeleton->set_bone_global_pose_override(p_task->chain.chain_root.bone, Transform(), 0.0, true);
 
 	if (p_task->chain.middle_chain_item) {
@@ -268,9 +281,9 @@ void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool ove
 		p_task->skeleton->set_bone_global_pose_override(p_task->chain.tips[i].chain_item->bone, Transform(), 0.0, true);
 	}
 
-	// Update the initial root transform
-	p_task->chain.chain_root.initial_transform = p_task->skeleton->get_bone_global_pose(p_task->chain.chain_root.bone);
-	p_task->chain.chain_root.current_pos = p_task->chain.chain_root.initial_transform.origin;
+	// Update the transforms to their global poses
+	// (Needed to sync IK with animation)
+	_update_chain(p_task->skeleton, &p_task->chain.chain_root);
 
 	make_goal(p_task, p_task->skeleton->get_global_transform().affine_inverse(), blending_delta);
 
@@ -286,22 +299,48 @@ void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool ove
 		Transform new_bone_pose(ci->initial_transform);
 		new_bone_pose.origin = ci->current_pos;
 
-		if (!ci->children.is_empty()) {
-			/// Rotate basis
-			const Vector3 initial_ori((ci->children[0].initial_transform.origin - ci->initial_transform.origin).normalized());
-			const Vector3 rot_axis(initial_ori.cross(ci->current_ori).normalized());
-
-			if (rot_axis[0] != 0 && rot_axis[1] != 0 && rot_axis[2] != 0) {
-				const real_t rot_angle(Math::acos(CLAMP(initial_ori.dot(ci->current_ori), -1, 1)));
-				new_bone_pose.basis.rotate(rot_axis, rot_angle);
+		// The root bone needs to be rotated differently so it isn't frozen in place.
+		if (ci == &p_task->chain.chain_root && !ci->children.is_empty()) {
+			new_bone_pose = new_bone_pose.looking_at(ci->children[0].current_pos);
+			const Vector3 bone_rest_dir = p_task->skeleton->get_bone_rest(ci->children[0].bone).origin.normalized().abs();
+			const Vector3 bone_rest_dir_abs = bone_rest_dir.abs();
+			if (bone_rest_dir_abs.x > bone_rest_dir_abs.y && bone_rest_dir_abs.x > bone_rest_dir_abs.z) {
+				if (bone_rest_dir.x < 0) {
+					new_bone_pose.basis.rotate_local(Vector3(0, 1, 0), -Math_PI / 2.0f);
+				} else {
+					new_bone_pose.basis.rotate_local(Vector3(0, 1, 0), Math_PI / 2.0f);
+				}
+			} else if (bone_rest_dir_abs.y > bone_rest_dir_abs.x && bone_rest_dir_abs.y > bone_rest_dir_abs.z) {
+				if (bone_rest_dir.y < 0) {
+					new_bone_pose.basis.rotate_local(Vector3(1, 0, 0), Math_PI / 2.0f);
+				} else {
+					new_bone_pose.basis.rotate_local(Vector3(1, 0, 0), -Math_PI / 2.0f);
+				}
+			} else {
+				if (bone_rest_dir.z < 0) {
+					// Do nothing!
+				} else {
+					new_bone_pose.basis.rotate_local(Vector3(0, 0, 1), Math_PI);
+				}
 			}
-
 		} else {
-			// Set target orientation to tip
-			if (override_tip_basis) {
-				new_bone_pose.basis = p_task->chain.tips[0].end_effector->goal_transform.basis;
+			if (!ci->children.is_empty()) {
+				/// Rotate basis
+				const Vector3 initial_ori((ci->children[0].initial_transform.origin - ci->initial_transform.origin).normalized());
+				const Vector3 rot_axis(initial_ori.cross(ci->current_ori).normalized());
+
+				if (rot_axis[0] != 0 && rot_axis[1] != 0 && rot_axis[2] != 0) {
+					const real_t rot_angle(Math::acos(CLAMP(initial_ori.dot(ci->current_ori), -1, 1)));
+					new_bone_pose.basis.rotate(rot_axis, rot_angle);
+				}
+
 			} else {
-				new_bone_pose.basis = new_bone_pose.basis * p_task->chain.tips[0].end_effector->goal_transform.basis;
+				// Set target orientation to tip
+				if (override_tip_basis) {
+					new_bone_pose.basis = p_task->chain.tips[0].end_effector->goal_transform.basis;
+				} else {
+					new_bone_pose.basis = new_bone_pose.basis * p_task->chain.tips[0].end_effector->goal_transform.basis;
+				}
 			}
 		}
 
@@ -319,6 +358,20 @@ void FabrikInverseKinematic::solve(Task *p_task, real_t blending_delta, bool ove
 	}
 }
 
+void FabrikInverseKinematic::_update_chain(const Skeleton3D *p_sk, ChainItem *p_chain_item) {
+	if (!p_chain_item) {
+		return;
+	}
+
+	p_chain_item->initial_transform = p_sk->get_bone_global_pose(p_chain_item->bone);
+	p_chain_item->current_pos = p_chain_item->initial_transform.origin;
+
+	ChainItem *items = p_chain_item->children.ptrw();
+	for (int i = 0; i < p_chain_item->children.size(); i += 1) {
+		_update_chain(p_sk, items + i);
+	}
+}
+
 void SkeletonIK3D::_validate_property(PropertyInfo &property) const {
 	if (property.name == "root_bone" || property.name == "tip_bone") {
 		if (skeleton) {

scene/3d/skeleton_ik_3d.h

@@ -118,6 +118,8 @@ public:
 	static void set_goal(Task *p_task, const Transform &p_goal);
 	static void make_goal(Task *p_task, const Transform &p_inverse_transf, real_t blending_delta);
 	static void solve(Task *p_task, real_t blending_delta, bool override_tip_basis, bool p_use_magnet, const Vector3 &p_magnet_position);
+
+	static void _update_chain(const Skeleton3D *p_skeleton, ChainItem *p_chain_item);
 };
 
 class SkeletonIK3D : public Node {