When creating character rigs for video games we always set the root of the rig and all the kinematic chains to the pelvis of the character. Is their a specific reason that we do that or is it by convention?
What is the reasoning behind this?
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asked May 7, 2015 at 7:59
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1 Answer
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As far as I know, this is because the humans centre of mass is near the pelvis. This is just a convention, but almost everyone does it. But it makes sense to choose a point that is at the middle (not a foot for example) and doesn't move too much.
Let's say you choose the foot as root. If you want to do a walk animation, you'll move the foot and everything moves with it. You have to move it back then. You won't have this situation that much, if you choose the pelvis.
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2\$\begingroup\$ Relatively speaking, no point moves that much when it's the center of its' own frame of reference ;) \$\endgroup\$ May 7, 2015 at 12:14
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4\$\begingroup\$ Try doing a sport which requires lots of balance, but learning it as an adult, when you are paying conscious attention to what is happening in your body. You will notice that it is your pelvis you need to keep stable in the position you want, and the rest of the body follows. It is probably mathematically possible to do it another way in a simulation, but the human body is designed for it being the easiest way. So I think this answer is the right one, as far as there is an answer beyond "convention". \$\endgroup\$– rumtschoMay 7, 2015 at 12:32
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3\$\begingroup\$ @JohnDemetriou, probably because if you bend down to pick something (small/light) up, you bend at the waist (pelvis) and not somewhere in the spine. /disclaimer lift with the knees for heavy/large things :) \$\endgroup\$ May 7, 2015 at 16:48
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1\$\begingroup\$ From a game development perspective, wouldn't it make more sense to have the root of the whole skeleton be at (0, 0) (that would be on the ground)? The rest of the skeleton would be as you describe, with the pelvis being the only child of the root and all other bones attached to the pelvis. This is how I do it and it makes the code easier to write since I only need to position the player at the correct position on the ground and not have to worry about how high his pelvis is. \$\endgroup\$ May 7, 2015 at 17:47
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1\$\begingroup\$ But what would be the child element of this root? The pelvis? You have the same thing then. And having the real skeleton non continous could lead to some strange behaviour. And it is unnecessary data that needs to be calculated and send to the gpu. \$\endgroup\$– Tobias BMay 7, 2015 at 17:51