Extracting the difference between two blend shapes(Maya)?

Rigging
1

Heya Guys. Im not sure if this was discussed anywhere here in the forums but im going to ask anyway.

Ok so Im currently setting up a face rig with joints and im working on a few facial blend shapes.
My questions is as follows: How do i create a new blendshape of the difference of the original shape and the corrective shape?

I’m trying to use the layers in mudbox to do so and not being successful at all.
Could someone maybe help me with some sort of script to do this kind of setup.

2

It’s not quite what you’re after, but try abSymMesh: http://www.creativecrash.com/maya/downloads/scripts-plugins/modeling/poly-tools/c/absymmesh

It lets you select a base shape and a blend and then select just the moved verts. You could fiddle around with the options until you worked out how to do a difference. You can revert part of a mesh to the base shape.

3

Here is another tool you might find helpful: http://www.chadvernon.com/blog/resources/cvshapeinverter/

Someone wrote an excellent blog post about what this tool is doing, but I can’t remember who or find the link. Basically, if you realize you can set a blendshape to -1 weight, then you can start to do some blending to figure out differences.

4

I’m currently at this same stage with a facial rig I’m working on right now. Using layers in mudbox is definitely one way of getting it done, but if that isn’t working for you, it’s pretty simple to do without a script.

Once you have your corrective shape (for instance, I did a corrective sculpt for the jaw-open), duplicate the default base geometry and and the shape that your corrective is for. In my case, I’d have the base head, the jaw-open, and the jaw-open-corrective. Apply the posed shape and the corrective shape as blendshapes to the base head, set the corrective shape to a weight of 1 and the posed shape to -1. That’s the difference between the corrective and the original shape. Delete the history on that, and apply it as a blendshape back onto the rig geometry. It’s a really quick process, but I guess you could write a quick script to do it if you’ve got a lot of shapes.

Hope that helps!