Stretchy spine with volume preservation [part 1]

During this hot days, I'm studyng new rigging techniques.

I'm starting with the torso and here is a little tutorial on how to create a simple stretchy spine with volume preservation.
Some simple steps:

Create a joint chain with at about 6 or 7 joints, and add some geometry parented on all joints to watch the result. Rename all!
Then create two extra single joints aligned to the start and the end of the chain. Call that start_joint and end_joint. Add two geometry and parent joints under that geometry.

Now it's time to create an IKSpline form start joint to end joint
Skin two extra joints created before to the curve created with the IKspline and...voilà!!! Move the two big geomtry and something appened!

But there's a problem: the chain doesn't stretch. Ok...

Use the arcLenght tool to measure the arc lenght of the curve and take note of the "relax lenght"

Now open the hypergraph and create "multiplyDivide" node.
Simply write createNode multiplyDivide inside the command line
Connect the archLenght with the multiplyDivide input1X, set the opereation to divide, an put the relax lenght inside the input2X

Now connect the outputX of the multiplyDivide to all joint scaleX and you have the stretchy spine! Awesome!!!

For the volume preservation you have to open the expression editor and put some code line (bad!!).
Is quite symple: you have to get the value of the scale (multiplyDivide.outputX) and put this value, inverted, inside the scaleY and the scaleZ of all joints.
I've used the pow function to get some nice result

Now the spine stretch and preserve the volume but... don't twist!!!! But this in another lesson...

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