Using IK with two forearm bones

Development World Creation
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1

Hi all,

I am trying to setup IK for my character’s arms to be able to move the hands to any position.
Using the Two Bone IK node, this works well using Unreal’s standard human rig but not our character’s rig.

The rig we are using has two bones in each forearm, to be able to properly animate twist.

With this setup, the Two Bone IK node does not work anymore, since there are three bones in the arm:

114360-skeleton_twoboneik.png

FABRIK does not work either since it resolves the bone chain by ‘breaking’ the forearm:

114371-skeleton_fabrik.png

Is there any way to prevent this, for example by ignoring or constraining the second forearm bone for the IK calculations?

Thanks in advance

1 Like
2

Hi squatsch.

did you find any solution for this problem?

3

I had 4 bones, i used virtual bones, like described here

4

I’d rather go with one forearm bone attached to the upper arm and with the hand attached to the forearm bone. For realistic twists you could create multiple twist bones attached to the forearm bone and then rig your mesh onto the twist bones instead of the forearm bone. Then you’ll have one bone that drives the whole angular motion and you can rotate the twist bones via an Animation Blueprint. (I am mostly driving the rotation from the hand rotation as this is closest to natural motion. Just watch how your arm is twisting if you are rotating your hand :smiley: The twist occurs to rotate the hand.)
The Result would be like this: UpperArm → Forearm (ForearmTwist1, ForearmTwist2) → Hand (attached to “Forearm”) => Mesh rigged to Forearm Twists instead of “Forearm”

EDIT: You may take a look at the Biped Skeleton System from 3ds max (just google it). It uses the exact technique that I’ve described.

5

I had the skeleton and mesh already made, had no choice in the matter.

6

Then you’ll know it better if you are doing something new ^^