Evaluating and adding stretch control to the spine

00:03

If we pull the shoulder control, which is the IK spine control,

00:08

you see that the torso stays in place,

00:11

which looks odd.

00:14

So ideally we want the spine to stretch so it moves with the control.

00:19

OK. Let's show the rig systems

00:22

and we need to find the curve we used when setting up the IK spine.

00:27

Now open the node editor

00:30

and bring the curve in.

00:33

Usually we hide the shape nodes but this time we need them.

00:37

This is the one we need.

00:38

So let's remove these instead

00:41

and open this.

00:43

And now we need a new node. So press tab and create a curve info

00:48

node,

00:50

call it spine curve info.

00:53

Next connect the curves world space zero attribute to the curve info

00:58

nodes, input curve attribute.

01:01

So what this node does is tell us a lot of information about the curve

01:07

which you can see in the attribute editor.

01:10

The one we are interested in is the arc length which tells us the length of the curve.

01:16

We can use this to check when the spine is stretching

01:20

and then use that information to also stretch the joints. So they follow.

01:25

So first we need a new multiply divide node.

01:28

Let's open this

01:31

and rename it to spine scale factor,

01:35

connect arc length to input one X.

01:40

So here we have the curve length,

01:43

we are going to use this node to compare its stretched length against its base length

01:48

to do this, copy the value down to input two X.

01:53

OK. Let's look at it in the attribute editor,

01:57

change the operation to divide.

01:60

This will divide the first value by the second

02:03

and give us a result,

02:05

the result will be one

02:07

when the values are equal. And then this will adjust as the curve stretches.

02:12

So what we can do then is use that value to then adjust the scale of the joints

02:17

because the joint's default value is one.