View Full Version : ForeArm Twist Problem

07 July 2002, 04:06 PM
Hi All,

I have a character which is wearing a t-shirt so his arms are naked. I split to 2 my forearm but it still remains.
How can I stop twisting my ForeArm ? Is there any tutorial about this ?

It is an emergency situation !

Thanks in advance...

07 July 2002, 05:41 PM
I split to 2 my forearm but it still remains. How can I stop twisting my ForeArm ?
Whoa there! Just 'cause you're having a problem with your character, you don't need to go mutilating yourself:D

But seriously... have you searched the forums yet? I know a couple of decent solutions have been posted on the wrist-twist problem. It would also help us if we knew what software you're working in.

07 July 2002, 05:50 PM
I am starting to like the ole' reversed radius/ulna bone set-up.

create two bones parented to the wrist. target them to the elbow and then when you twist the wrist you get nice forearm movement.
I hope this helps.

07 July 2002, 12:26 AM
Originally posted by KOryH
I am starting to like the ole' reversed radius/ulna bone set-up.

create two bones parented to the wrist. target them to the elbow and then when you twist the wrist you get nice forearm movement.
I hope this helps.

You know KOryH, a picture speaks a thousand words, could you please post a screenshot of this wrist/elbow bone structure or perhaps a simple diagram if you have a moment (or if anyone else can picture this in their mind) ?

Thanks :D

08 August 2002, 01:16 PM
yes please !!

08 August 2002, 05:17 AM
if you have two bones in the forearm

one for the elbow rotation (flexion) and one for the wrist twist (pronation and supanation, for those movement experts)

then you can sey up as many nulls as you like to spread the twist in the wrist evenly along the forarm to ensure that at the elbow there is 0% twist and the wrist there is 100% twist.

You can use script or driven keys or whatever to do this.

Then these nulls can be what you use to bind the skin, or a lattice or whatever. But it means you have a number of things along the length fo the forearm ie if you have four dummies, 0 % , 25%, 50% and 100% wrist twist. and you have a lot more things to control the skin deformation.

Then you also have the up and down movement of the wrist (flexion and extension) and the side to side movement. that needs to be localised only to the very joint of the wrist.

08 August 2002, 12:29 PM
a little illustration could be helpfull :)

08 August 2002, 01:55 AM
This is only using FK, in Max 5


Orange bone, only for flexion in elbow, bending
yellow bone, only for pronation, supanation, twist in forearm
Green bone, for lateral & flexion, remaining movement of wrist

blue dummies from elbow

1: 0% twist, no wrist movement
2: 25% twist, no wirst movement
3: 50% twist, no wirst movement
4: 100% twist, no wirst movement
5: 100% twist, WITH wirst movement

these dummies are used to bind the annoying orange skin

the IK rig to control the bones, I havent worked out 100%

I usually use another method and make an anatomically correct radius which uses look at to get the rotation components split up out of the wrist

08 August 2002, 02:06 AM
here it is without the annoying skin

Oh yeah, You can also split up the skin movement of the upper arm the same way

have a look at you arm when it moves the twisting motion of the skin (external internal rotation) almost exclusively takes place in the upper arm and not around the shoulder itself.

Does anybody know why IK doesnt respect axis locks that you have put in place in under the hierachy panel, its starting to tick me off. or if there is a good work around?

08 August 2002, 02:48 AM
Hi very cool... but that is that is the normal way to do it IMO... I think I messunderstud u then :hmm:

Here is a example from one of my cartoon characters, but only with two bones in the twist:

in your demonstration it looks like the forarm also bend in the "Z" axsis... like the Right one here:

is that possible??? (i don't think so, but correct me if I'm wrong!!)

another very interesting thing is the elbow joint, any good tricks of how get the "bally" look to stay in the right place, its kinda pissing me off... just like my stupid elbow setup here:

else then Thread

btw. my armsetup works with IK/FK (3dsmax)

08 August 2002, 03:48 AM
Devil the rotation is only in the length, I used FFD for the deformation of the skin and this can sometimes pass the defomation along.

I think this is the standard method.

I dont think you understand, in your example the forearm is twsiting at the elbow, it should only bend at the elbow and twist in the next joint down.

in my example note the red fin is always the the plane of the elbow joint. where as the yellow fin twists along the axis of the forearm.

here is something else:

OK here is one with a radius and an Ulna

The problem with this is that muscles are attached to these bones and the movement of the skin is based on the deformation of the muscles

anyhow here goes, the radius is on the thumb side of you wrist and it rotates around the ulna

hence the pivot for the hand is on the pinky side of the hand

you put a dummy on the thumb side of the hand linked to the hand and get the hand end of the radius to track the position of this dummy. Hence the radius will roll over the ulna, just like in the real world.

But how you get this to translate into the skin you are deforming is different.

You need to measure the angle between the radius and the ulna by using a cross product. In reality you dont even need an radius, you just need a away of isolating the twist from the other movements in the wrist.

Then using the previous method you can spread this twist along the length of the forearm.


08 August 2002, 03:53 AM
Thanks... I'll theck it out later... 6 oclock at the morning here... ZZzzzz

08 August 2002, 04:21 PM
have a questoin: what method do u all use for deforming the skin with muscles atached to skeleton. (for maya if possible).
10x in advance.

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