Teach me about arm twisting

acitone · Oct 2012

Hi people I'm having a hard time figuring how to do handle the hand movement, twisting to be exact. For ie I turn my character's hand upward, like a person checking their own palm for scars - the wrist totally gets twisted see pic :

Isaiah Sherman · Oct 2012

This should go in the Technical section. They'll help you out!

slipsius · Oct 2012

you need to add more joints. There should be 2 extra joints between the elbow and the wrist. they are called twist joints. Then you just apply the weights. so the wrist would be 100%, the next joint up would be 66%, then 33% then 0.

But, you also make it so they only twist. they dont rotate up and down, like the hand does.

Not exactly the greatest explanation, but im sure if you googled twist joints, it would come up with something.

reverendK · Oct 2012

also - and this doesn't answer your question, it's really just a thought - that is actually a really unnatural movement for a person to make. try bending your own wrist that far in that direction - it hurts - generally most of the work for whatever purpose that motion could serve is done with the elbows.
/unhelpful comment

Elyaradine · Oct 2012

Yeah, I think a lot of the problem is alleviated when you realise your wrist itself can only rotate on two axes, and that for the third one (the one that appears to be causing your twist) your entire forearm/radius rotates.

ZacD · Oct 2012

slipsius wrote: »

you need to add more joints. There should be 2 extra joints between the elbow and the wrist. they are called twist joints. Then you just apply the weights. so the wrist would be 100%, the next joint up would be 66%, then 33% then 0.

But, you also make it so they only twist. they dont rotate up and down, like the hand does.

Not exactly the greatest explanation, but im sure if you googled twist joints, it would come up with something.

Expanding on this point.

You can bend your hand in 2 axis, forward and back, left and right. When you actually go to rotate your wrist, you are rotating your whole forearm. And you can really only rotate 140 degrees before you have to rotate more than just your forearm. There are 2 bones, your radius and ulna, in your forearm that actually cross when you are rotating the wrist.

Basically in your example you are ignoring the bones in your forearm and breaking them.

I'm not a rigger so I'm not sure how they exactly approach this issue, but you just have to think of it as 2 separate joints that work together.

Adam L. Gray · Oct 2012

Well, TeeJay beat me to it, it would seem!

Like so. The bone between the elbow and wrist should be locked as to only rotate on one axis, as your forearm would. So slipsius pretty much said it already, although I've only ever used one extra joint as opposed to two.

ZacD · Oct 2012

I was talking about how the physical bones in your hand work, I have no idea about the actual rigging, I probably should of drawn real bones instead of the symbol.

slipsius · Oct 2012

Ive seen it rigged both ways. Zac's way and Teejays. Teejays is how I was talking about though. I think its the more common way because I think its easier to set up for autoriggers.

Thanks for illustrating my point though, guys. I was gonna draw it at lunch

Mark Dygert · Oct 2012

It looks like you're using biped? It has twist bones built in.
Go into figure mode, structure, twist links and add 2-3 twists to the forearm.

Or you might be using CAT? It also has twist bones. While in setup mode, click on the forearm, and set the number of segments to 2-3.

They both behave the same way. When you rotate the hand, each joint will rotate a little less than the one before it, spreading the twisting motion down the forearm.

They will behave like slipsius, TeeJay and others pointed out. The bones that ZacD pointed out are technically anatomically correct but are prone to more problems than they solve and I don't know too many people that bother to set them up, especially when most rigs include twists natively and you really don't want to bother setting up and managing another system of bones, it just makes things more complicated than it needs to be.

acitone · Oct 2012

OK guys I'm going to try and tackle this, thanks. I must admit I can't wrap my little head around it completely right off the bat cause I'm mainly trying to understand the weighting method when it comes time to skin, which i'll do right after adding the twist bones. And yes I'm using CAT.

Edit : so ok, do you weight any of the mesh to the twist bones or are they just there for the functionality?

Mark Dygert · Oct 2012

You weight mostly to the twist bones down by the wrist and blend it more to the main forearm bone the closer you get to the elbow.

The idea is to spread the wrists rotation down the forearm to combat the candy wrapper effect at the wrist.

(You can probably ignore the bicep twist bones, most games do...)

Most of the time the automatic weights that it applies handles the blend pretty well but I'm not sure how well those weights will be calculated now that you are adding the bones after the fact. Next time around make sure they are there when you first apply skin and it will be much easier.

acitone · Oct 2012

Coolness, this is totally awesome. I'm blown away by the logic of this and how it works. Definitely the right solution. And for all bone setups. Thanks for the wisdom

If you guys don't mind I'm saving these demonstrations for reference in future

Teach me about arm twisting

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