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I have a gum squeeze bulb rigged with an armature (simple two bones), when I turn the bone the top of the bulbs vertex group is attached to I get a nice bending of the whole bulb. Please see the attached images.

Now I want to have the same behavior accomplished by shape keys. I found the Corrective Shape Keys addon, but can't get it doing a good job. The resulting mesh is a mess. Also I found the option to apply the armature modifier as a shape key, which gives me exactly the same result as with the armature, but instead with a single shape key. That's too good to be true. So I found, that when I want to bend the bulb in +X/-X and +Y/-Y after the first time using the armature modifier as shape key, my rigging has gone and I can't do the -X/+Y/-Y motions with the "use armature modifier as shape key" process...

Help is greatly appreciated! straight bulb: 0° bent bulb by 45°

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  • $\begingroup$ Is there any reason you need this to be done with shapekeys? Shapekeys are a bad fit for this kind of motion, an armature is probably your best fit here. $\endgroup$
    – Sazerac
    Aug 30, 2016 at 2:10

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I got this working by using the "Corrective Shape Key" addon. So I posed the armature in the end position I wanted. Then I selected the mesh object (bulb) and hit the "create duplicate for editing" in the Shape Keys group of the objects data tab. Selecting both (copied first and then original) objects and hitting "Join as Shapes" from the same menu creates a new shape key with the copied objects shape as final mesh deformation of this shape key.

I then did this with all four directions the bulb is able to bend. This work nicely with one nasty exception. The motion is not exactly a copy of the armatures motion, the top of the bulb changes length during bending, which in the armature is forbidden. The bulb is actually attached with top and bottom lips to metal parts. So now when I make the angle sweep, a gap between the bulbs lips and the upper metal part opens and closes again. I hope to get around this issue soon... first step second step

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    $\begingroup$ The reason for the change in motion is that applying shape keys performs a simple linear interpolation between the points in each shape key that is applied, so any rotational motion will be distorted. This is a fundamental limitation to how shapekeys work, so generally with large ranges of motion, other options are used or combined with shape keys. $\endgroup$
    – Sazerac
    Aug 30, 2016 at 2:08

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