btRigidBody: center of mass transform vs. world transform

[ejtttje]

I was initially under the impression that the rigid bodies handled having a center of mass at an offset from the rigid body's location, due to functions such as setCenterOfMassTransform vs. the inherited setWorldTransform. (This would be handy for robotics, where we define the origin of a link at the joint, not at the center of mass...)

However, after some debugging, setCenterOfMassTransform seems to be directly setting the world transform instead of a separate offset. Some documentation here would be helpful... thanks!

EDIT: and by "documentation", I'm thinking of the doxygen page, which is kind of barren for btRigidBody:
http://www.continuousphysics.com/Bullet ... dBody.html

[pico]

Hi,

you can use a compound shape to set an own center of mass. One of the bullet demos shows how todo this.

Maybe you also want to check out this thread (scroll down to the end):
http://www.bulletphysics.com/Bullet/php ... er+of+mass

[Erwin Coumans]

I agree, it can be misleading. For a Bullet btRigidBody, the world transform == center of mass. If you need to shift the center of mass, you need to use a btCompoundShape, and shift the collision shape relative to the local rigid body origin. The reason for both names, is that the collision detection can be used independently (btCollisionWorld), and for collision detection "center of mass" has no meaning.

This should be made more clear in the Wiki and Bullet manual and doxygen/source code.
I'll make some updates for upcoming Bullet 2.73 manual.

Thanks,
Erwin

By the way, there is an IRC discussion channel #bulletphysics at freenode.net

[ejtttje]

Thanks. I guess btCompoundShape is the way to go to offset the collision shape from the center of mass. Given that the rigid body is defined to be at the CoM, I'm afraid I'm still not quite clear on why there is separate functions for world transform vs. CoM transform, but maybe the new documentation will explain that more... it looks like setCenterOfMassTransform does some extra "stuff", but I don't quite follow it.

Should I always use setCenterOfMassTransform on my rigid bodies then, and avoid setWorldTransform()?

PS I'm on chat.us.freenode.net #bulletphysics... there's some people in the channel, but seems everyone is AFK.