Editing the lower arm twist

00:04

If we rotate the wrist,

00:06

you see we get quite a harsh twist here.

00:09

If you look at your own arm as you twist your wrist,

00:12

you will see it's the forearm that twists, not the actual wrist.

00:17

So that's where this twist joint comes in.

00:19

We need to pass the twist down to this joint. Instead,

00:24

the other axes are fine. It's just the twist, we need to correct

00:28

and we can use a similar set up to how we control the shoulder

00:33

start by creating a new locator

00:36

and snap it to the wrist position

00:39

using the match transformations tool.

00:42

Let's move it back to

00:44

and rename it to hand, aim left.

00:48

This is going to be used just like the upper arm aim locator above.

00:53

But instead this will trigger the twist rather than remove it.

00:58

We don't need any follow joints this time.

00:60

Instead just parent the locator to the hand left joint.

01:05

So now it will follow as a hand rotate.

01:08

And because it's parented to the main skin joint,

01:11

this will work regardless of if we are using IK or FK to animate the arm.

01:17

What we need now is for the twist joint to be locked to the locator.

01:20

So we can use another aim constraint.

01:23

Again, we can tell it to always look at the elbow.

01:26

So it maintains the right orientation.

01:29

So select the elbow or lower arm joint.

01:34

And now we need the hand twist joint here,

01:38

open the aim constraint options again

01:41

and change the world up object to the hand aim locator this time.

01:46

Although we still need the Y axis to point to the elbow,

01:50

it needs to be looking behind the joint instead.

01:53

So change the aim vector from one to minus one

01:58

and apply that

02:00

if I move the locator, now

02:02

the twist joint follows,

02:05

you can only just see it moving,

02:08

let's show the rotational axis.

02:11

So that's a bit clearer

02:14

this time. If we rotate the wrist,

02:17

you see the twist joint isn't moving,

02:20

that is until we twist it

02:22

and then it starts to follow the locator.

02:25

So we are checking for the twist this time.

02:29

What we can do now is pass that rotation up to the forearm, twist joint.

02:34

Let's go back to the node editor.

02:37

We can use the same multiply divide node.

02:39

So open this up,

02:42

we can remove these,

02:44

bring in the hand twist joint

02:46

and the lower arm twist joint

02:49

move these up here.

02:51

Now connect the hand twist joints, Y rotation to input one Y

02:57

and then output Y to the lower arm twist joints rotate Y attribute

03:05

the forearm. Now follows as the wrist twists, but again, it's too much.

03:09

So we need to reduce it.

03:11

Let's half the amount again. So change this to 0.5.

03:17

So the lower arm now has a nice gradual twist.

03:22

The beauty of this setup is we can position the hand anywhere we like.

03:26

And the lower arm follows

03:28

if we simply use the wrist joint to drive

03:31

the lower arm joint and connect to them directly.

03:34

Without the aim constraint in the locator,

03:37

which would have been an option,

03:39

there would be poses where it wouldn't work if

03:41

the joint hit gimbal lock or the rotations clashed.

03:45

And this would result in the twist joint, flipping or rotating the wrong direction.

03:51

This way, the rotations are dictated by the locator's position.

03:55

So it makes it much more stable and more predictable.

03:59

Ok. So that's the arm twist joints rigged

04:02

and we have some much nicer and more natural defamation in the upper and lower arm.

04:06

Now,

04:08

this setup can easily be applied to the leg now too.