View Full Version : FK Legs Change COG?

10 October 2010, 08:47 PM
Hey everyone,

My friend brought up a good point today while he was doing an animation demo. He was trying to animate a boy swinging from his hands, climbing on a monkey bar. So, by doing this, shouldn't his COG change from his Pelvis to his Collar? I just want someone elses opinion. So, I could write something for when you switch to FK legs, the spine woulds joint orientation would change directions.

Let me know what you think,


10 October 2010, 11:03 AM
The 'center of gravity' certainly wouldn't change to the position of collar, then again it probably never was around the pelvis either. 'COG' actually is a pretty poor term since Gravity will affect all mass equally in one direction, 'center of mass' is more appropriate. Your COM changes only whenever you move anything on your body. Physically speaking it's the point the force applied to the whole body pivots around, provided there are no external influences. Standing with your hands pointing all the way up or hanging with your legs all the way down; the COM doesn't change - only the external point to 'guide' this force (contact the feet make, grip the hands have).

If you spin around in a desk chair and move your legs up you'll slow down, put them down and you'll speed up again. There won't be more force applied to that rotation, the point that guides the force (the single-axis pivot of the chair) won't change, but the center of mass will. By moving your legs out the com will extend and it'll take more force to spin around.

I realize this does not answer your question, but it's important not to get the two confused. In my opinion when rigging for character animation you're only concerned with the points that constrain movement, the concept of center of mass is something the animator needs to keep in mind while animating.

The animation you want to create is quite annoying to do from the hips up since it's just a never ending stream of counter rotation, so it definitely isn't a bad idea to look into different rigging solutions. A switch that would reverse the hierarchy of the FK spine from the collar down will only work for shots that do not require a switch from rack to ground.

In our rig the spine is freeform in all cases. The chest never follows the pelvis or the other way around, which might sound like it takes more time but in practice our IK spine actually seems to speed things up, just requires a better eye from the animator. It's great for situations like this, especially when there is a switch from ground -> rack or the opposite.

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10 October 2010, 11:03 AM
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