View Full Version : FK arm movements

09 September 2002, 11:09 PM
Hi guys,

first of all, im mostly a modeller/texturer so please excuse this question as it may seem simple to an animator.

I've started my first attempt at animating with FK arms, ive used IK arms before, but not FK. When rotating them into place the hip movement/shoulder movement counteract it (which is what it is supposed to do with FK is it not) but it creates a very, very jerky arm movement that is highly undesired :) is this normal and you have to counter rotate from the hips, or is there an easy fix??


09 September 2002, 02:41 AM
I'm not sure what sort of character setup you are using, but it sounds like you have some sort of IK through the torso/spine if you are getting movements there when you rotate the arms. I use a setup that only uses IK for the legs...the upper body is all FK. Therefor there is no influence on the hips when I rotate the arms.

Without knowing your setup, the only thing I can say is to look at the hip area and make sure the bone/s (or object/s) that is the parent of legs (assuming you're still using IK legs) and make sure that an option that makes it "unaffected by descendants". That's the option in Lightwave...I'm not sure what other packages call it, but there should be something there along that line.

Not sure if this will help, but it's all the advice I can give.

09 September 2002, 03:53 AM
Rogue: Thanks for the tips, however i wrote my problem wrong :P when i rotate the hips, i have to counteract with the arms. Which is normal for FK, but with FK is it normal to have to rotate the arms back into place everytime the hip moves. The only IK have is in the legs... i was just curious if thats normal for FK.

Sorry for the confusion!! i was tired :)

09 September 2002, 05:03 AM
...only way I can think to fix that is to turn off the inherit rotation on the arm bones. That way when you rotate the hips the arms shouldn't rotate...though you probably will have some movement in the position.

09 September 2002, 05:07 AM
hmmm i tried that too, and its still giving me trouble, i think the best solution would be to make em IK, thanks for the input guys!!

ed: i heard you work at elliott... very cool!! so do i :) where abouts are you locateD?

09 September 2002, 05:30 AM
I'm the guy with the Lava lamp on his desk =)

09 September 2002, 03:21 AM
Lava lamps are cool! :)

Jeffo: I'm not sure what you mean. With an FK setup with the spine and arms, the arms will move accordingly because of parenting. There would be no countering of the arms since they don't have a goal object. If you need for the hands to stay at a certain location when moving or rotating the hips, you're best bet is to use IK for the arms.

I prefer an FK setup on the arms and spine....I have much better control for gesturing. However, if my character needs to hold onto a static object (like a hand rail), I'd have to use IK. If you can come up with an FK/IK solution, that'd give you the best of both worlds. It takes some planning to switch from FK to IK and can lead to results that you're not satisfied with (time for a cut shot), but can save some time.

I didn't realize you had the "lord of the rings" style character. That's a cool character. I'm assuming you're setting him up for animation purposes. Looking forward to seeing him animated.

09 September 2002, 04:30 AM
Thanks rogue! :)

yeah i do indeed tend to aniamte him, actuall the problem arose when i tried to do a quick run cycle with him, when i had the hips and spine rotated for the cycle the arms would pass through the body at certain spots, so i had to rotate em away from the body, which then caused the two to fight each other i think. Does this make any sense?? sorry i guess its kind hard to understand without actually seeing it :P

09 September 2002, 01:40 PM
I've sometimes had to rotate the arms out a bit during a run cycle as well. To get the arms out in front like that, you'll probably have to do some rotating to get the arms away from the body. I don't know what you mean by fighting each other without seeing your animation, but I certainly understand that problems arise when animating....been there, done that. :)

09 September 2002, 03:50 PM
haha thanks for the tips.

Heres the situation, ill try and render an avi out when i get home.
I rotate the hip/spin movements how i want them, and then the arms clip through the chest, so i rotate the arms out so they avoid that, but once i do that, the arm rotation becomes REALLY snappy, and it jerks all around, and in the graph it looks normal. Hard to explain and understand i know :) thats why ill try and get an avi for ya soon.


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