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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...voilĂ !!! 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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