Bug Flip bug with inverse kinematics (Limb Solver 2D)

Hello! I'm setting up an arm with inverse kinematics in my Unity 2D project but I'm running into a problem that I think is due to a bug in the LimbSolver2D component.

The problem I am encountering is that the X axis rotation of the first bone changes from 0 to -180 when my target is positioned in certain specific positions causing a flip glith of the arm.



Some of the target positions in which this flip occurs:

Vector3(-0.832000017,8.32999992,0)
Vector3(-0.587000012,5.82000017,0)
Vector3(-0.615999997,6.08500004,0)

Has anyone encountered this problem before?

 

Hi!
I am using version 9.0.4 of the 2D Animation package.

I think I have already found the problem, in the DoUpdateIK function of limbSolver it uses Quaternion.FromToRotation(), this function sometimes returns a value of 180º for the euleraxis X, which causes my gameobject to flip.

I have created a function that calculates the rotation for the vector direction but only for the Z axis and it seems that the problem is solved with a customization of limbSolver.

Code (CSharp):

1.  
2. /// <summary>
3. /// Updates the IK and sets the chain's transform positions.
4. /// </summary>
5. /// <param name="targetPositions">List of target positions.</param>
6. protected override void DoUpdateIK(List<Vector3> targetPositions)
7. {
8.     var targetPosition = targetPositions[0];
9.     var upperLimb = m_Chain.transforms[0];
10.     var lowerLimb = m_Chain.transforms[1];
11.     var effector = m_Chain.effector;
12.  
13.     var targetLocalPosition2D = (Vector2)upperLimb.InverseTransformPoint(targetPosition);
14.     targetPosition = upperLimb.TransformPoint(targetLocalPosition2D);
15.     Debug.Log(Quaternion.FromToRotation(Vector3.right, new Vector2(-2f, -4f)).eulerAngles);
16.  
17.     if (targetLocalPosition2D.sqrMagnitude > 0f && Limb.Solve(targetPosition, m_Lengths, m_Positions, ref m_Angles)) {
18.         var upperLimbRotation = FromToZRotatiton(targetLocalPosition2D) * Quaternion.FromToRotation((Vector2)lowerLimb.localPosition, Vector3.right);
19.         upperLimbRotation *= Quaternion.AngleAxis((flip ? -1f : 1f) * m_Angles[0], Vector3.forward);
20.         upperLimb.localRotation *= upperLimbRotation;
21.  
22.         var loweLimbRotation = FromToZRotatiton(lowerLimb.InverseTransformPoint(targetPosition)) * Quaternion.FromToRotation((Vector2)effector.localPosition, Vector3.right);
23.         m_Chain.transforms[1].localRotation *= loweLimbRotation;
24.     }
25. }
26.  
27. /// <summary>
28. /// Calculates the rotation around the Z axis based on the direction of the provided vector
29. /// </summary>
30. /// <param name="direction"></param>
31. /// <returns></returns>
32. protected Quaternion FromToZRotatiton(Vector3 direction)
33. {
34.     return Quaternion.Euler(0f, 0f, Mathf.Atan2(direction.y, direction.x) * Mathf.Rad2Deg);
35. }
36.  

What surprises me a lot is that no one has noticed this problem before. To verify it, it is as simple as running these lines with some example values.

Debug.Log(Quaternion.FromToRotation(Vector3.right, new Vector2(-30f, -23f)).eulerAngles);

In this first case you can check how to obtain an appropriate rotation value (0.00, 0.00, 243.43)

Debug.Log(Quaternion.FromToRotation(Vector3.right, new Vector2(-0.5f, -0f)).eulerAngles);

However, in this other case you can see how Quaternion.FromToRotation returns a value for the other axis that causes a flip

Thanks for your interest

 

You're welcome and thanks for registering the Bug. It's a small detail but it can be easily solved!