Skeleton hashing

[io7m]

Calculate a deterministic hash value from a compiled skeleton.

The hash value can then be recorded for both skeletons and their associated meshes (stamped into the meshes as SMF metadata). This allows for checking that a skeleton has not been modified without the associated meshes also having been re-exported.

[io7m]

This raises the question of what constitutes structural compatibility. For example, if a bone in a skeleton is renamed, then the skeleton would still be "compatible" in the sense that the positions and numbers of bones have not changed.

[io7m]

A skeleton is compatible if meshes bound to that skeleton will still continue to work after a change is made to the skeleton.

This means that:

• Joint indices have not changed
  • This implies that joint names have not changed, because the generation of joint indices is tied to joint names
  • This also implies that the hierarchy of joints has also not changed, because the generation of joint indices is tied to the tree structure
  • This also implies that no joints have been added, because even if joints are added at the leaves of the tree, the joint indices are generated in breadth-first order and therefore the indices will change

This pretty much rules out all structural changes.

What happens if existing joints are moved/rotated/scaled? Does a mesh remain "compatible" with a skeleton when this happens?

[io7m]

What happens if existing joints are moved/rotated/scaled? Does a mesh remain "compatible" with a skeleton when this happens?

No, it doesn't.

[io7m]

If, in effect, absolutely everything is relevant to whether or not a skeleton remains compatible, then everything in the skeleton has to be included in the hash.

The question remains about the specifics of hashing. Inevitably, it's going to be a SHA256 sum of the skeleton data. Need to come up with a well-specified, portable, and deterministic way to turn a skeleton into a string and then hash that string.