Adding an IK solver for the arm

00:03

We now need an I

00:05

handle. So go to the skeleton menu

00:07

and then down to create

00:09

I

00:09

handle

00:10

and open the options.

00:13

Just reset this,

00:14

we can leave the default options here.

00:17

The solver we need is the rotate plane solver.

00:22

This will create an IK chain

00:24

but give you the option of being able to also rotate it,

00:28

which we need. So we can control the elbow's position.

00:32

Now select the upper arm I care joint

00:36

and then the hand I care joint,

00:38

an I

00:39

handle is then created for us.

00:43

We can move this around and the arm follows.

00:46

So this is how IK works.

00:49

It evaluates the joints, rotations from the hand up to the arm

00:53

with FK.

00:54

The arm is evaluated the other way.

00:57

So from the upper arm down to the hand,

00:60

hence its name forward kinematics.

01:04

OK. Let's rename this,

01:06

call it arm left I can handle.

01:10

Now parent it to the arm. I care control,

01:13

just add it to the selection and press P.

01:17

We can use the control now to move the arm rather than the I care handle.

01:23

Rotating doesn't work yet. But we will come to that later.

01:28

You see

01:29

we can move the body now and the hand tries to stay in its original position.

01:35

The problem is we have no easily accessible control over the elbow.

01:40

So let's add that next

01:43

down here, we have these point to control icons.

01:47

So where is the arm one?

01:50

Ah Here it is arm PV. Control.

01:53

The PV just stands for Paul Vector.

01:56

Although you may want to rename these to elbow control

01:59

just so it's more obvious what they are.

02:02

First, let's move it. So it matches the elbow's position and orientation.

02:07

Let's switch to wireframe so we can see it.

02:11

So this is pointing or it will point towards the elbow

02:16

with that selected, also select the ICA

02:19

handle

02:21

and go to constrain Paul vector to create a Paul vector constraint.

02:26

What this will do is use the plane created along the arm.

02:30

So this triangle here

02:32

and try to keep the elbow aiming at the control

02:36

if we move the arm control. Now

02:38

you see the arm is flipping because the elbow control is too close.

02:43

If we move it behind the arm instead,

02:46

you see the arm twisting to stay pointed towards it.

02:50

Not just that though, it also gives us more control when it's behind the arm.

02:55

The problem is as we move it, the arm twists slightly

02:60

and this is just down to how the skeleton was built. And the positions of the joints,

03:05

the only real way to get around this is to

03:07

have the arms completely straight when viewed from the front.

03:10

So they're perfectly level.

03:13

But this doesn't always work with the character models or other elements of the rig,

03:18

but it's easy enough to get around.

03:21

So you see the elbow has changed position,

03:24

you can see it clearer if I show the model again.

03:27

Ideally,

03:27

we need there to be no movement as the animator switches between the IK and FK arms,

03:33

they should both be in the default position

03:36

when the rig is in its neutral pose.

03:39

So what do we do?

03:41

Well,

03:41

we need to record the movement and then apply that to the control

03:45

as an offset to correct the movement and move the elbow back into position

03:51

first, create a temporary locator.

03:54

No need to bother renaming it because we're gonna delete it soon

03:58

and use the match transformations tool to move it to the elbow joint.

04:03

Let's make this a little bigger

04:07

and now duplicate it. So we have two locators.

04:12

We now need to move these back behind the arm.

04:15

Let's open the move options and use this step snap option to move it an exact amount.

04:22

This is just so that we can move all these controls the same distance.

04:26

So set this to relative

04:28

and we will leave it set to 30

04:31

and move these back.

04:33

Now parent one of the locators to the elbow I care joint.

04:39

This now follows as the arm moves,

04:43

select the arm pull vector control and move this back now

04:48

to speed things up, we can quickly just match the locators, transforms

04:53

what we have now is a difference between the locators

04:56

and this is the distance we need to move the control to correct the offset.

05:02

Now,

05:02

you could work out the exact distance between

05:04

the locators using some of Mayer's distance tools,

05:08

but sometimes it's just easier to eyeball it.

05:11

So

05:12

move the control up until the locators lie in the same place.

05:17

Turn off snap.

05:21

OK. That looks about right

05:23

now, that's in position. We can delete those locators

05:27

if you want, you can delete the pole vector constraint,

05:32

make sure the arm is reset.

05:34

So no rotations are on the elbow

05:37

and then reapply it.

05:40

But you may find this doesn't make a huge difference.

05:42

So you may not need to reapply the constraint.

05:46

So we can now again, use this to control the elbow

05:49

if we move the arm, control the elbow points towards the Paul vector control.

05:54

And if I switch between IK and FK,

05:57

you see the arm doesn't move now.

05:59

So we have kept the IK arm in its default pose.