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Malkalypse
02-12-2009, 07:35 PM
I've been working on a large project for a company that does patient education media, and as part of this have modelled a spine and ribcage model. What I would like to do now is figure out the correct way to rig this.

I can't treat the ribcage as a single solid object in this case, each of the ribs will need to articulate with both the spine in the back, and the sternum in the front. I already have the individual spine segments articulated, but have no idea how to make the ribs move with them and stay connected to the sternum in the front, or what controls the angle of the sternum compared to the ribcage.

Has anyone seen, or even done, anything like this before? What kind of a setup should I be thinking about?

I've already asked about the actual anatomy in the artistic anatomy/figurative art thread, and hopefully someone there will be able to offer some insight, but here I am mostly concerned with finding a practical solution to rigging individual ribs and a sternum.

dunkelzahn
02-13-2009, 09:25 AM
I did something similar for a medical visualization company. The trick is to think of the rib cage as something like a wicker basket holding the inner organs. They are flexible and are being held by a spinal bone on one side and the sternum on the other, while being able to deform.

I did the rig by creating a row of bones from a spinal column to the sternum for each rib and having them controlled by a spline. Smooth bind the ribs to the bones so they flex with the bones. I guess you could also do a direct skin wrap and bind them to the spline. That would be more efficient. You´ll have to constrain the other end of the spline to the sternum and then you would have some sort of animation rig.

I advise to build an control object that rotates all spinal bones at once for the three axis of movement. I didn´t back then ("I´ll just animate it by hand, no prob") and it turned into a lot of work. You could also setup all sorts of additional rigging and skinning corrections onto the control object.

Here a link to some of the stuff I did:

http://flexikon.doccheck.com/Gliederung:_Skelettmuskulatur_des_Menschen

I know it´s in german but you should be able to check out the latin words. Click the links under "Rücken(Dorsum)". You should end up with some wiki tables that lead you to an article for each muscle with a flash animation where you can switch between front and side view as well as a +/- button to fade muscles in and out.

Have fun ;)

Soulcialism
02-13-2009, 02:19 PM
I did this once for cinematics in blitz: the league 2 when i was working at midway games, i approached it almost exactly like dunkel did (mmm...dunkel...i could go for a dunkel right now). i set up a spline ik for the spine so that there were 3 controls for lower/mid/upper parts of the spine. the tricky part for me was setting up the clavicle so that the top of the rib cage didn't clip through the clavicle when the back was arched a lot and the shoulders were rotated forward. i ended up using sort of 'corrective' multiply nodes to fix that.

dunkelzahn
02-13-2009, 03:32 PM
Yeah, clavicles are a bit tricky. I rigged that something like:

Sternum -> Clavicle -> Scapula

As well as

Sternum -> Clavicle -> Upper Arm -> Lowe Arm -> Blablabla

This way, you got full control over the clavicle, Shoulder blade (scapula) and the rest of the arm. Looks a bit weird, but a Ph.D. explained to me, that it´s anatomically correct this way. Basically it´s how muscles and bones add up to form a rotator cuff.

Cheers

Chris

BTW: Over here it´s far too early for a beer right now. But I´ll drink one tonight to the human rib cage ;)

Malkalypse
02-17-2009, 07:24 PM
Dunkel, thanks for the link. The animation for the longissimus shows what I am trying to do, but I am not really able to see how you did it from the animation. For the spine rigging, I currently have control objects set up to operate the lumbar, thoracic, and cervical spine, with each vertebra shifting for a portion of the amount.

I did the rig by creating a row of bones from a spinal column to the sternum for each rib and having them controlled by a spline. Smooth bind the ribs to the bones so they flex with the bones. I guess you could also do a direct skin wrap and bind them to the spline. That would be more efficient. You´ll have to constrain the other end of the spline to the sternum and then you would have some sort of animation rig.

Maybe you could help me understand that a bit better. In the past I tried using a row of bones from the spinal column to the sternum, and just used each bone to control a rib. But, you say that you control those bones with a spline? I don't quite understand what you mean?

dunkelzahn
03-02-2009, 12:57 PM
Hey Malkalypse,

sorry to have kept you waiting so long, but anyway here´s my 2 cents.

If I understand correctly you created a chain of bones for each of the vertebrae and then apply the control objects for the 3 spinal areas (Use List controllers in Max and driven keys for Maya) so the spines move smoothly.

Now you can create a row of bones for each of the ribs (or use a the make live function of Maya and use those to create a wrap controller). The splines would be responsible for the spline IK setup, along which the ribs controlling the bones would flow.

There is a tutorial her which roughly describes the process:

http://www.animationartist.com/2003/08_aug/tutorials/softbody_tutorial.htm

The tricky thing is that the ribs need to be attached to the sternum as well as to the vertebraes. For Maya I used a point constraint to have the ribs coming from the vertebrae connect to the sternum. I can´t tell you in detail , but I had the bones being able to stretch and squash, so there would be a deformation along the ribs. I think the spline IK setup can do that automatically, but I am not sure.

For the final animation of the ribs I´d recommend Maya´s cluster function (coupled with the paint cluster tool) to set up a deformation system set on top of your bones deformation, worked well for me. just make sure to center the cluster pivot point :)

Have fun

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