View Full Version : Shoulder deformation
09-08-2010, 06:14 AM
To try and keep a short back-story... I'm currently adapting to the opengl matrix palette extension (OpenGL ES 1.1 for Android) and working on "rigging" my model for animations. I'm not going for a particularly complex rig, hopefully having influence from only two "bones" per vertex, but I may go 3 if necessary.
Now, with what could probably have been a question that stood on it's own, what I'm not certain of, is exactly how much clipping is reasonable in the underarm and upper torso/side area when arms are brought down. I'm reviewing all of the tutorials and tips that I can on a proper shoulder mesh, but I haven't seen too many examples of a mesh that showed arms in both positions, i.e. outstretched and down to the side.
Thanks to everyone
09-08-2010, 07:19 AM
... what I'm not certain of, is exactly how much clipping is reasonable in the underarm and upper torso/side area when arms are brought down.
It really depends on the model how thick their arms are, what their wearing and what shape of torso they have. After skinning a bunch of characters you end up with a pretty good sense where to place the clavicle/bicep joint.
If you place it down by the arm pit you'll normally get very little clipping but the shoulder will stretch quite a bit.
If you place it higher closer to the bump in the shoulder and away from the torso it will be easier to control the shoulder but the arm will clip into the torso.
I'm reviewing all of the tutorials and tips that I can on a proper shoulder mesh, but I haven't seen too many examples of a mesh that showed arms in both positions, i.e. outstretched and down to the side.
Normally the characters I work on have their arms at a 45 degree angle. This is helpful for a few reasons.
1) It's half way between the two extremes for this joint, T-Pose (0 degrees) and arms at side (90 degrees). Typically arms spend the majority of their time in 45-90. Rarely do they go 45-0 and its really uncommon to go above the T-Pose and when they do, you get the clavicle involved (http://dl.dropbox.com/u/2336353/ShoulderJoint.jpg). Also compacting the shoulder texture is very easily disguised but stretching isn't.
2) In a T-Pose its hard to define the shoulders and they're often overlooked or left up to the skinner/animator to define with whatever loops are available. Shoulders are a pain to skin even if the modeler thought about the shoulders, even harder when they didn't. 45 helps them remember the shoulders without you having to get out the "remember the shoulders stick".
3) In the 45 degree pose the shoulder is normally defined and gives you a good idea of where to stick the joint. I totally understand its easier to model arms straight out, but before it gets passed to be skinned it should be posed at 45 and the shoulders cleaned up.
When placing the start of the bicep joint, I normally start off by drawing a line between the shoulder and the armpit then place it in the middle of that line. Depending on the shape of the torso, you might have to move it farther away from the body, to control the clipping or move it closer to the torso to close the armpit gap. In either case weighting between the three bones, Spine, Clavicle and bicep is going to make up a lot of the difference but its not as important as where you place the joint.
Also check out: http://www.hippydrome.com/LArmUD.html
It offers some great insight into joint placement and articulation with some great animated images.
09-08-2010, 10:50 AM
Vig you are a gentleman and a scholar. Working for something that will be in a mobile phone environment, I'm caught between trying to push the envelope a bit and kicking myself into remembering... "you're working for phones... don't overdo it." Loving that last link, btw... really great images there. I will absolutely be looking at working on this in a 45 degree angle as well, that is a marvelous idea. Seeing as I'm coder, designer, modeler, animator and everything else... whatever makes it easier for me, me and me is great.
I see exactly what you mean about stretching vs. clipping on the clavicle/bicep joint. I think with your advice I should be able to find a happy medium. Here's a little shot of where it's at... it's deforming reasonably well but needs some fine tuning still.
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