Using weight paint, i’ve assigned all the vertices of the head of my character to a single bone that starts at the jaw and points up. Although in texture paint, all those vertices are red, there’s still some deformation when i move the bone around. I joined the eyeballs to the mesh of the body, since i was getting parallax issues having them separate. But it’s obvious to see that there’s deformation since the iris is noticeably not circular anymore, and the bridge of the nose doesn’t line up with the bridge of the helm she’s wearing.
I used bone heat when i parented, perhaps that’s the issue?
Have you tried very extreme poses with the head bone?? Sometimes, weight painting may miss hidden vertices, dunno about the bone heat option. For example, the inside of a mouth won’t be weight painted to a bone when doing weight painting, because it’s inside the mesh. I’m wondering if maybe there may be stray vertices for the nose somewhere inside the mesh that didn’t get weight painted. What does the mesh look like when the head is tilted backwards to the point the head bone is flat or even inverted somewhat?
In Blender 2.4x just because one bone group has been painted to 1 does not mean that it has full control of the vertices The workflow used to be to paint one group up, and then paint all overlapping groups down. That is why I wrote this script when I was first starting, and included this video demonstrating how to use it.
In Blender 2.5 the weight paint tool has a normalize function built-in so it works in real-time. But I just filed a bug for it, so use it with care
Actually, i just figured out that you could manually assign the vertices to the bone itself via vertex group. So i did that and the deformation is gone! So yay for me.
Yeah, when you do the auto-assign in Blender 2.5 and paint everything red, then go into your vertex groups, the results can be surprising!
There is a “Clean” option in the Weight Painting Menu. This sometimes helps clean up some of the extraneous stuff – but sometimes it is too aggressive.
In other words, sometimes, you have no option but to assign things to a group manually…
The workflow used to be to paint one group up, and then paint all overlapping groups down. That is why I wrote 
