cgmBlendshape 101

To anyone who’s worked with coding blendshape stuff it can be tedious especially when you bring in inbetweens.  Thankfully, Autodesk is fixing a lot of that with 2016 extension 2 if you missed that update but there are still folks using older versions and it doesn’t resolve everything. We have to deal with them a good bit on Morpheus 2 and so we wrote a metaclass to deal with them.

Initial features of the cgmBlendshape metaclass that you can’t easily do with normal api or mc/cmd calls:

  • Most functions work off of index/weight or shape/selection format
  • Easy alias naming
  • Replacing shapes — change out shapes in place keeping inbetweens and connections intact
  • Extract shapes — extract shapes from index/weight calls and supporting multipliers to the delta difference
  • Shape restoration — replace deleted shapes on the fly. Recreate a shape from delta and base information and plug it back in for further editing
  • Subclass to cgmNode to all those functions carry over as well
  • Tested in 2011 and 2016
  • NOTE – this is  wip metaclass and will undergo lots of changes

Before we get into the the specifics of the metaclass, here’s some general lessons learned on blendshapes working through this.

  • A blendshape target has several bits of important information
    • Index — this is it’s index in the blendshape node. Note – not necessarily sequential.
    • Weight — this is the value at which this shape is ‘on’. Usually it is 1.0. Inbetween shapes are between 0 and 1.0.
    • Shape — this is the shape that drives the blendshape channel
    • Dag — the dag node for the shape
    • Alias — the attribute corresponding to its index in the weight list. Typically it is the name of the dag node.
    • Plug — the actual raw attribute of the shape on the node. ‘BSNODE.w[index]’
    • Weight Index — follows a maya formula of index = wt * 1000 + 5000. So a 1.0 weight is a weight index of 6000.
  • The way maya stores info
    • Blendshape data is stored in these arrays in real time so that if you query the data and your base mesh isn’t zeroed out, the transformation happening is baked into that
    • The caveat to that is that targets that have their base geo deleted are ‘locked’ in to their respective data channels at the point they were when deleted. Their delta information is frozen.
    • BlendshapeNode.inputTarget[0].inputTargetGroup[index].inputTargetItem[weightIndex]
      • inputTarget — this is most often 0.
      • inputTargetGroup — information for a particular shape index
      • inputTargetItem — information for a particular weight index
    • Sub items at that index
      • inputPointsTarget — the is the differential data of the point positions being transformed by a given shape target. It is indexed to the inputComponentsTarget array
      • inputComponentsTarget — these are the compents that are being affected by a given shape
      • inputGeomTarget — this is the geo affecting a particular target shape
  • Replacing blendshapes – you can 1) use a copy geo function if the point count is exact to change the shape to what you want or 2) make a function to do it yourself. There’s not a great way to replace a shape except to rebuild that whole index or the node itself. We made a function to do that
  • Once a blendshape node is created with targets, the individual targets are no longer needed and just take up space. Especially when you have the easy ability to extract shapes.
  • Getting a base for calculating delta information. As the blendshapes are stored as delta off of the base, the best way I could find to get that delta was to turn off all the deformers on the base object, query that and then return on/connect the envelopes. I’m sure there’s more elegant solutions but I was unsuccessful in finding one.
    • Once you have that creating a new mesh from a an existing one is as simple as:
      • Taking base data
      • For components that are affected on a given index/weight: add the delta to base
      • duplicating the base and xform(t=vPos, absolute = True) each of the verts will give you a duplicate shape
  • Aliasing weight attributes – mc.aliasAttr(‘NEWNAME’, ‘BSNODE.w[index]’)

Here’s a dummy file I used for testing:

Here’s some code to play with the first iteration. You’ll need to grab the MorpheusDev branch on bitbucket if you wanna play with it till I push it to the main branch.

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