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Old 08-01-2014, 10:06 AM   #1
AlexanderL
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Alexander Lee
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Female arm IK anatomy

In Maya, an IK arm automatically bends such that all of the joints in the chain remain in the same plane as they were originally. Therefore, if you wish to make the elbow bend in the correct direction, the arm must be perfectly straight, such that when the elbow is fully bent, the wrist will approach the position of the shoulder.

However, this does not really match with real anatomy in the case of the female arm. The female arm has a significant "carrying angle." That is, the forearm is skewed laterally in relation to the upper arm.
carrying angle illustration

If I represent this accurately in the rig, the IK elbow will bend in the wrong direction. In order for the IK arm to bend in the right direction, the carrying angle would have to be 0. Is there any way to solve this?
 
Old 08-01-2014, 12:53 PM   #2
Cactus Dan
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Howdy,

Well, I wouldn't go by an "illustration" because we all know how artists love to exaggerate things for emphasis.

Something that you may not realize is just how much the humerus' twist plays a part in the posing of the arm. I think in what that illustration is trying to show, to pose the forearm in a "carrying angle" the humerus will have to twist significantly, thereby keeping the bones on an IK plane.

Take a close look at your own arm and the arms of as many other people around you, and specifically when the elbow is bent 90. You'll notice that the upper arm and forearm remain on a plane, and when you move your arm around, keeping the elbow at 90, you'll see exactly how much twisting the humerus does.

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Old 08-01-2014, 10:46 PM   #3
AlexanderL
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thanks for the response!

I actually based my understanding of the carrying angle on observations of real women, not that illustration, which I just found while writing the post.

The fact is that in women (and to a lesser extent in men), the forearm is skewed laterally in relation to the upper arm, even when the arm is fully extended such that the elbow is locked.
Thus it can't be due to the axial rotation of the humerus. The lateral skew is most pronounced when the hand is supinated, and becomes diminished when the hand is pronated. The axis of skewing (anteroposterior axis) is perpendicular to the axis of elbow articulation (tranverse axis).

(note that the attached imaged is a cropped version of a photo from Scott Eaton's excellent online anatomy course)
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File Type: png femaleCarryingAngle.PNG (72.3 KB, 13 views)
 
Old 08-01-2014, 11:30 PM   #4
Cactus Dan
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Howdy,

Well, you may be over thinking it (a little OCD maybe? I suffer from that, too ) because no matter how you slice and dice it, we're talking about 3 points of a triangle, and mathematically, given 3 points in 3D space, a plane in which those 3 points reside can be defined.

I'd simply adjust the pole vector of the IK to match the angle of the arm, if that's how your model has been created.

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Cactus Dan
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File Type: png femaleCarryingAngle_Triangle.png (76.7 KB, 7 views)
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Old 08-02-2014, 04:00 AM   #5
AlexanderL
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ah yes I should definitely try that. I will have to see if that works in Maya. Thanks!
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Old 08-02-2014, 10:19 AM   #6
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Dude the elbow is a hinge joint.. there is no such thing as a skew on it, its always the rotation of the humerus and angle of the forearm bones radius and ulna that make your so called "skew".
Also there is NO anatomical difference in male and female arms, there may be slight differences for the individual person, but definitely not on a level that requires different rigging.

For a working arm IK system the elbow joint shall not have ANY rotation values, and joint orients ONLY in one axis: the bend axis, so to indicate in which direction the solver will bend the arm.
Then the polevector should be placed on the cerated vector plane like Dan stated. if you just follow those instructions nothing bad happens.
 
Old 08-02-2014, 01:24 PM   #7
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Alexander, I think you're mistaking with the fact that women's elbows are sometimes bending a bit backward, because as Cactus Dan said, what you picture describe is nothing else than the pole vector action. Your picture simply shows an arm in a comfortable natural position, whereas in rigging, we prefer to align the 3 arm bones on the XZ plane to avoid technical issues with joints orients and IK Handles. It's the animators job to pose the character in a natural position.

just to make things clear :


Last edited by Pahuska : 08-02-2014 at 01:37 PM.
 
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