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IK Arm (with 2 or 3 Segments)

This iExpression allows to animate an entire arm by just specifying the point where the hand should be - the expression calculates the rotation of all arm segments accordingly (using Inverse Kinematics, IK). You tell where the arm should move - the expression knows how.

This expression comes in two variants:

  • 2 Segment Arm A normal arm with upper arm and lower arm
  • 3 Segment Arm A arm with upper arm , "middle arm" and lower arm - you can use this, for example, to animate a finger (which in contrast to an arm consists of three segments) or other objects, like the plant in our example.

The arm must consist of the following layers:

  • upper arm (with anchor point at shoulder)
  • lower arm (with anchor point at elbow)
  • hand (with anchor point at wrist)
  • The 3 segment variant needs a middle element in addition - see tutorial for details.

Each arm layer must be parented to the respective next arm segment. So for the 2 segment variant the hand must be parented to the lower arm and the lower arm to the upper arm. In addition there must be a (null-) layer that represents the goal where the arm should point.

When the arm is set up like this, all you need to do is to connect the respective layers into the iExpression and apply it to the rotation of all arm layers and the hand (but not the hand goal).


Inverse Kinematic Arm with 3 Segments - After Effects expression

This "arm" consists of 3 segments - as you can see, the stem of the plant consists of three parts or on other words has "2 elbows" instead of one. Therefore, we used the 3 segment variant of the expression.

Also note that the expression makes sure that the head always points in the same direction no matter how the arm it is attached to rotates. You can still keyframe the head, if you want to rotate it, but don't have to deal with unwanted rotations when you move the arm.

Robot Character IK Rig After Effect expression for inverse kinematics

In this example we applied the "IK Arm with 2 Segments" expression to each of the arms and legs separately.

As a result, the character can now be animated by moving the body and any of the 4 goal layers for hands and feet. To let these kinds of characters walk, also take a look at the Walk Cycle iExpression.

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