All ‘virtual’ joints that we place are based on virtual anatomical surface landmarks. By ‘virtual’ I mean the polygonal rendermeshes that our ‘puppet’ will drive. As a rigger, you have to be able to look at the surface anatomy (polygonal mesh) and determine where a joint should be placed, but the solution is not always obvious.
The jaw is one of these tricky situations! Many people think the jaw rotates from the ‘socket’ or ‘fossa’ that the jaw (or mandible — or mandibular condyle) fits into, but this is not the case.
As riggers, we sometimes need to ignore internal anatomy and focus on surface anatomy, which is our final deliverable. This means think about the center of rotation for the entire mass of flesh (or vertices) that we’re moving. For the jaw of a human(oid) this pivot is under the earlobe when viewed from the side.
Rotating directly from the ‘socket’ would result in this incredible ‘derp‘ shown above, but the temporomandibular (TMJ) joint doesn’t work like this when we open our jaw/mouth.
Instead, the jaw/mandible slides forward as the mouth opens, like you see in this ‘live MRI’ slice above, resulting in this sexay jaw open below. You can see her mandible/jaw slide forward as she opens her mouth.
This post is of course ignoring the fact that you can rig the jaw in a way that, through a combination of rotation and translation uses the TMJ as a pivot and rotates around the true jaw mass center of rotation described above. You can do virtually anything, you can drive with your feet; that doesn’t mean it’s a good idea. This post is probably most important for riggers creating characters fast, using tools that generate skeletons from user-placed signposts or locators.
Speaking of signposts and locators, another tool to help you with placement is to constrain a toroid or circle to your joint, it can help you visualize the jaw swing quickly:
For those interest in further investigation, the paper ‘Rotation and Translation of the Jaw During Speech’ by Jan Edwards and Katherine Harris (1990) can be downloaded [here].
Jaw and Teeth Placement (Modeling)
Quite a few people have told me they found this helpful. I would like to add that teeth placement is very important and something you should check before rigging. For modelers, here are some radiograms showing teeth placement in reference to the facial surface anatomy (click to enlarge):
As you see above, we’re primarily interested in 6, 7, 20, and 21. Rhine et al have created charts for caucasoid and negroid Americans of varying builds. (Click below to enlarge)
NOTE: Soft tissue thickness charts for the face are also a great place to gut-check your sub surface scattering maps and profiles!
Up next, placing the teeth so that they are large enough or wide enough is also important. Below, notice the item marked ‘J’, this is the average upper lip line in relation to the upper teeth.
Above is a forensic facial reconstruction proportion guideline. I like to augment that a bit to help with teeth placement. I find that it’s helpful to look at the incisors vs the nostrils, and the pupils vs the lip corners/molars. Here are some examples of that (click to enlarge):