Rigging and bone orientation

Question

So, this is more of a practice and method question rather than a technical one; I've found some information on the subject, but I'm not sure I understand the system. To give you a quick background, I've exported the default character model from Unreal Engine 4 and am creating a rig for it in blender so I can do my animations there.

As I was mucking with bone orientation and pole targets and such, I realized I didn't know the philosophy behind this question: how should I orient my bones?

In this example, the bones which form the spine have their Z axis pointing towards the front of the character. This changes for the highlighted head bone, which has a Z axis in line with the global Z. As I was setting all this up I realized I didn't know if every axis should be facing uniformly, if they should all try and orient globally, if some should face some way and others should face differently, and so on.

Phrased specifically - what systems or rules of thumb can I adhere to when adjusting bone orientation in a rig, or where can I find more information on this subject?

Any help is appreciated, thanks everyone :)

P.S. This is all hopefully headed back into UE4. I don't know if the destination software of the animations has to do with how I should handle all this, but that's my plan in case it does.

Answer 1

There was some good input from Nathan Vegdahl on this exact subject in the bf-animsys mailing list last year. This is all subjective, of course, but it makes it easiest for animators if the axes throughout your rig are consistent first and foremost. This is a copy/paste from his email(s) to the list:

The main rules-of-thumb that I try to follow are: - The x-axis should be the "primary" axis of rotation for any given control. - Rotation in the "primary" direction on the x-axis should result in a positive rotation angle (meaning that if it were euler, the x component of the rotation would be positive). - For bones that are free-floating and/or don't have any particular relationship to other controls (the root control is a good example), align all of their axes with the world axes.

In the first two rules, "primary" is often subjective or ambiguous, depending on the control, so you just have to use your best judgement and experiment what it feels like to use it as an animator.

It's important also to note the reason for doing any of this at all. In all 3d animation software, keeping bones aligned well helps keep the rig better organized for the rigger. It's also important for animators when e.g. working in the f-curved editor. But in Blender it's particularly important for animators, because one of the major interaction models for tranforms is via hotkeys, using the x, y, and z keys to specify axes.

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Oh, I should also say: the choice of x vs z axis as the "primary" rotation axis is arbitrary. I choose x because it's the first letter in "xyz". But z is just as good. What matters is consistency within the rig (or within the project if it's a larger project).

I generally follow Nathan's conventions when I rig in blender, although the same principles follow in any software I use. Again, this is all subjective, so your mileage may vary and you might find a better convention for your particular rig. It is also important to take into account your rotation orders on your controls. XYZ might be better suited for a primary axis of Z+ in your rig, whereas an order of YZX might be better suited with X+ as primary axis.

bf-animsys January 2014