Corrective Blendshapes
To get better deformation on your model you can use corrective blendhapes on top of your skinned mesh. Technically they are underneath and the blendshape should be applied before the skin cluster, something Maya now does by default.
With the introduction of Pose Sculpting creating corrective blendshapes has been simplified. However, blendshapes can not be driven by more than one joint using that system. The shape editor also makes creating blend shapes much easier.
Sculpting the undeformed mesh (a shoulder in default position f.i.) to get a good shape on the deformed mesh (e.g. Arm up) can be hard. Here is a way to sculpt the deformed mesh to create corrective blend shapes.
- Select the Mesh while it is in bind pose default shape)
- Duplicate the Mesh
- Bind the copy to same joints as the original.
- You need to be exact and select only the joints actually in the skin cluster of the original mesh
- In the outliner: select the original mesh, then control select the copy
- you may try to select them in a viewport but they should be exactly overlapping)
- copy the skin weights (Animation → Skin → Edit Smooth Skin → Copy Skin Weights - Use the default settings)
- Hide the original mesh. The copy should now deform identically to the original
- Pose your character and sculpt the correction onto the deformed mesh
- Go back to bind pose
- Delete the history on the copied mesh
- this will delete the skin cluster but keep the changes you sculpted
- Set as the copied mesh as blendshape for the original
- Use a set driven key based on joint angle to blend the new blendshape in and out