In a visual arts-specific article republished on Intel's Visual Computing microsite
and originally published in Game Developer
magazine, Bungie's Steve Theodore discusses skeletal construction
and animation for game characters, looking at "strategies for taming the skeletal hordes without ending up in the graveyard."
Getting skeletons down and doing it early is crucial, says Theodore:
A high quality character mesh represents several weeks of an artist's time. A skeleton, of course, can be assembled in a couple of days. Although the cost of laying down bones may be trivial, the problems that arise when the skeleton has to be changed mid-stream are truly epic.
When you change a character's skeleton, you've invalidated every bit of existing data that depends on the skeleton. This means you may have to reweight its mesh and rebuild every one of its animations. This can be nearly as expensive as redoing them from scratch.
It might require weeks or months of work by a whole team of animators, supported by a character rigger. And if the alteration in the skeleton involves a change in the overall proportions of the model, you may need to rework the game mesh as well, which can also result in creating new UVs and thus new textures.
Most teams recognize the costs of skeleton changes and treat them with the kind of enthusiasm usually reserved for a dinner invitation at Castle Dracula. Locking down your skeletons as early as possible is a natural and sensible precaution, given the time costs and potential for chaos that comes with changing them.
Good process can reduce the need to rework characters that may already have dozens of completed animations, but Theodore says it's almost certain that some accident or glitch will necessitate this at some point. So what to do?
The first and most important line of defense is simply to know when things have gone wrong. It's a great idea to check every animation against the "skeleton of record" when you export it. That's a good way to keep people from walking down blind alleys for too long, and it means that minor problems (like bones that accidentally get renamed) can be fixed right when they occur, instead of showing up as subtle bugs or mysteriously failed imports.
The real horrors, of course, are serious changes to the layout or proportions of the skeleton. These happen for a lot of reasons: a change in the game design, a fatal flaw in the existing skeleton (like an unavoidable gimbal lock that snuck through testing), or an anatomical problem that has to be fixed. When these sorts of situations arise, fixing them is going to be costly. The only question is how much.
The basic strategy for coping with serious skeleton changes is to retarget your existing animations onto the new skeleton, in much the same way that you might apply animations from a mo-cap actor onto a monster. If you already have in-house expertise in motion retargeting, you've got a huge leg up on the problem. Experienced MotionBuilder jockeys can be particularly useful when the bones start going haywire.
You can now read the full animator-focused feature
at Gamasutra (no registration required, please feel free to link to this feature from other websites.)