Deform bones and Deform bones only

[bartteunis]

Blender bones have a property deform that determines if the bone deforms any vertices/geometry.
The SMF exporter currently doesn't do anything with that info though it should be straightforward and a fairly simple addition to set the weights to zero for those bones that have deform set to False.

Idea taken from the Collada exporter's Deform Bones Only as described under Armature Options.
Exporting only deform bones may be another interesting thing to add.

[bartteunis]

See also the FBX exporter under Armatures. It seems to have the same export option.

[bartteunis]

The glTF exporter has an export option called Export Deformation Bones Only under Animation. It doesn't seem to be documented (yet).

[bartteunis]

To look into for this issue:

• How does an export of only use_deform bones map to a valid SMF rig? (node numbering/indices, etc.)
• Rigify: deform bones are on a single layer, but some of them have non-deform parent bones. How to handle this? How to get valid bone/node chains out of these setups?

[bartteunis]

Some other things to look into (general Blender armatures, Rigify setups, etc.):

• https://blender.stackexchange.com/questions/142720/whats-the-difference-between-def-bones-and-org-bones-in-functionality
• https://devtalk.blender.org/t/rigify-parent-bones-of-some-def-bones/14745
• https://blender.stackexchange.com/a/142727

[bartteunis]

After some first testing it turns out that the hierarchy for a more complex setup could either be:

• "filtered", i.e. maintain the bone hierarchy where there are any links to deform bones (i.e. bones that have use_deform set to True). For e.g. the default Rigify setup this already reduces the number of bones from 222 to 56 (with 35 use_deform bones)
  This'd mean that parts of the bone hierarchy are still exported with it.
  The FBX exporter does this ("Only write deforming bones (and non-deforming ones when they have deforming children).")
• "baked", i.e. "bake" the transforms of the upper parts of the bone hierarchy into the deform bones. Would give a minimum of bones to be exported as only those tagged with use_deform will be exported (35 for a default Rigify rig)
  This will require inserting and extensive use of SMF's detached nodes.