Inverse Kinematics

Most character animation is created by rotating the angles of joints in a skeleton to predetermined values. The position of a child joint changes according to the rotation of its parent. The end point of a chain of joints is determined by the angles and relative positions of the individual joints along the chain. This method of posing a skeleton is known as forward kinematics.

However, it is often useful to pose joints from the opposite point of view. Start from a chosen position in space, or a goal, and work backwards to find a way to orient the joints so that the end point reaches the goal. This can be useful if you want a character to grab an object or stand on an uneven surface. This approach is known as inverse kinematics (IK). It is supported in Mecanim for a humanoid character with a correctly configured Avatar.

To set up IK for a character, you typically have objects around the scene that a character interacts with. You can use these objects and your character to set up IK within a script. You can use the following Animator functions:

• SetIKPositionWeight
• SetIKRotationWeight
• SetIKPosition
• SetIKRotation
• SetLookAtPosition
• bodyPosition
• bodyRotation

For example, the image above shows a character touching a cylindrical object. To do this with IK and through scripting, follow these steps:

1. Create a valid Avatar for your character.

2. Create an Animator Controller with at least one animation for this character.

3. In the Layers pane of the Animator window, click the Cog icon in the Base Layer and enable IK Pass from the context menu. When enabled, IK Pass sends an OnAnimatorIK callback. In a later step, you will use this callback to implement inverse kinematics in a script.

   

4. Make sure the Animator Controller is assigned to the character’s Animator Controller.

   

5. Next, add a script to your character. In this example, the script is named IKControl. This script sets the IK target for the right hand of the character. This script also changes the look at position so that the character looks towards the cylinder object when grabbed. The full script is listed below:

   using UnityEngine;
   using System;
   using System.Collections;
   
   [RequireComponent(typeof(Animator))]
   public class IKControl : MonoBehaviour {
   
       protected Animator animator;
   
       public bool ikActive = false;
       public Transform rightHandObj = null;
       public Transform lookObj = null;
   
       void Start ()
       {
           animator = GetComponent<Animator>();
       }
   
       //a callback for calculating IK
       void OnAnimatorIK()
       {
           if(animator) {
          
               //if the IK is active, set the position and rotation directly to the goal.
               if(ikActive) {
   
                   // Set the look target position, if one has been assigned
                   if(lookObj != null) {
                       animator.SetLookAtWeight(1);
                       animator.SetLookAtPosition(lookObj.position);
                   }
   
                   // Set the right hand target position and rotation, if one has been assigned
                   if(rightHandObj != null) {
                       animator.SetIKPositionWeight(AvatarIKGoal.RightHand,1);
                       animator.SetIKRotationWeight(AvatarIKGoal.RightHand,1);  
                       animator.SetIKPosition(AvatarIKGoal.RightHand,rightHandObj.position);
                       animator.SetIKRotation(AvatarIKGoal.RightHand,rightHandObj.rotation);
                   }
               }
   
               //if the IK is not active, set the position and rotation of the hand and head back to the original position
               else {          
                   animator.SetIKPositionWeight(AvatarIKGoal.RightHand,0);
                   animator.SetIKRotationWeight(AvatarIKGoal.RightHand,0);
                   animator.SetLookAtWeight(0);
               }
           }
       }
   }
   
   

6. To avoid the right hand passing through the Cylinder GameObject, add an empty child GameObject to the Cylinder GameObject. To do this, right-click the Cylinder GameObject in the Hierarchy window and select Create Empty. Name this empty child GameObject Cylinder Grab Handle.

7. Position and rotate the Cylinder Grab Handle GameObject so that the right hand touches but does not pass through the cylinder.

   

8. Assign the Cylinder Grab Handle GameObject as the Right Hand Obj property of the IKControl script.

9. Assign the Cylinder GameObject as the Look Obj so that the character looks towards the center of the Cylinder when IK Active is enabled.

   

10. Enter play mode. Your character should touch and release the Cylinder GameObject as you enabled and disable the IK Active checkbox. While in Play Mode, change the position and rotation of the Cylinder GameObject to see how the right hand and character look at reacts.