Modelling for Rigging

Alrighty, a friend of mine just asked about how I want a model posed for rigging. He was mainly asking about the angle of the arms, but I got into a bit of a tangent and wrote down a whole list of things. And so I thought I’d post it here.

Of course, everyone rigs differently, and so these guidelines might only be useful to me, but at least I’ll try to say why. At any rate, thinking about the model’s default pose beforehand can help make for a cleaner and more intuitive rig, as well as better deformations.

For starters, the orientation of the arms is more important for helping deformations, which is dependent on what the character is like. If he is a heavy slouch he might never need to raise his arms very high, so they should be lower in the default pose. By contrast, if he’s a basketball player it might be better to have his arms straight out.

The elbows and knees should be bent slightly, again the amount depends on what you think is appropriate for deformation. A more musclebound character might be more relaxed if the elbows are bent a little more. One thing, though, they should never be bent backwards, as this will mess up your IK.

The wrist is a tough one. Some people like to have it oriented to the world, so that IK handles can be zeroed out easily. I find that makes it harder to deal with forearm twist joints, however. Plus, I personally think that when IK hands are zeroed out the should be flat and facing forward, and I don’t think I’ve ever seen anyone model that way. So in my opinion, the wrist should be oriented the same way as the elbow.
OK, so when I’m making arm joints, I’ve got shoulder, elbow, wrist. The wrist joint is going to have to control twist joints up the forearm, which means that it should have one axis pointing that direction anyway. The easiest and cleanest way I think to do that is to zero out all rotation and jointOrient attributes of the wrist. So if our clean arm rig is oriented that way, the model should be oriented that way too.
That means that the wrist shouldn’t be bent at all, and it should be twisted so that the thumb points at the shoulder. Not at the shoulder, really, but on the same plane. Like if the wrist was aim constrained down the forearm, the shoulder should be it’s up axis. (There I go using a technical example to explain something technical. What a dork.) The whole arm should make one plane, that the hand is flat on.

For fingers, and any long joint chain for that matter, the amount of bend is not as important as the consistency that each one is bending. They should all be bent the same amount, so when you set the same value on all the joints you get a predictable result.

The mouth should be mostly closed, but the lips not quite touching. If you get points that are at the same position, it will mess with mirroring skin weighting.
The Eyelids should be about halfway closed (assuming they are part of the same mesh) because they crease when they are open, and it’s harder to model and skin them that way, and then get them all smoothed out when they’re closed. Of course all this stuff varies with the character.

Feet should be flat on the ground, and the character should be standing over the origin. The feet could be off-axis like the hands are, but I think it’s more important for the feet IK to be zeroed out than it is for the hands.

As for clothes and accesories, it’s ok to have a lot of separate meshes. Just make sure that overlapping parts have a similar amount of detail. For example, if a belt is a separate mesh from the pants, it should have an almost identical number of spans around as the waist, since they are so close together. That way when they deform, they will deform in almost exactly the same manner, and interpenetration wont be as much of an issue. For non-deforming things like buttons that need to be stuck onto skinned mesh, make sure they are close to a vertex. A button that is inbetween two verts will end up floating or penetrating, because it’s not corresponding to a definate point. So if possible stick it right on top of a vertex.

Make sure there’s enough resolution to do what you want, especially in the face. Don’t be afraid of detail, it’s not as bad as you think. Having more spans down a twisting arm will actually make skinning easier, believe it or not. Try to keep consistent squares. Long skinny polygons are harder to skin. So don’t be afraid to throw a couple edgeloops in there, especially places that move a lot. Conversely, if you do have a lot of uneccesary detail in an area that is mainly just going to compress, then you might want to lighten it up a bit.
For an area like the face, remember that if you want wrinkles, the edges need to be there even when they’re not wrinkling.

Alrighty. So that’s what I got for now. I don’t think anyone reads this yet, but if you happen to stumble by and have something to add, or if you think I’m totally wrong, say something!

//EDIT Aug 23 05
Eyeballs: should be modelled as spheres pointed straight ahead and parallel. If they need to be deformed or squashed or turned to fit in the head, they still need to be rigged pointing straight ahead, and they need to be spherical so they can turn. So if nothing else, don’t freeze transforms on them.

That was fun. 🙂

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3 Responses to Modelling for Rigging

  1. Ali says:

    oh we read it alright 🙂

  2. Morgan Loomis says:

    Javier Solsona has another modelling method posted on his site, which I haven’t tried yet but sounds pretty cool.

    Also, I was thinking about the eyelids some more, and I changed my mind. Halfway closed is good, because you want that to be the zero position anyway.

  3. Morgan Loomis says:

    Oh, and hi Ali. I guess I couldn’t possibly have managed to sneak this in under your nose. 🙂

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