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Why joints have only one pose?

asked 2017-04-19 15:24:43 -0500

lubiluk gravatar image

I noticed that in a joint we can only specify an attachment offset for a child. For parent the attachment of the joint seems to be always in the parent's frame origin. Why is that so? Is it useless to have parent offset? Is the length/offset/distance of a newly created joint determined at the moment of it's creation?

I'm working with SDF 1.4 and trying to make joints dynamically from a plugin.

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answered 2017-04-19 15:46:26 -0500

Each element on SDF is defined with respect to a specific reference frame:

  • Links are defined on the parent model's frame
  • Joints are defined on the child link's frame

So let's say you have two links in a model connected by a joint. The links starting configuration is fully described by their own pose elements. The joint doesn't change any of the link poses at startup. What the joint offset changes is how the movement of these links will be constrained once the physics starts acting on them.

Is the length/offset/distance of a newly created joint determined at the moment of it's creation?

Yes, the joint pose is defined the moment the joint is created. The joint does not change how the links are positioned in the world, it just adds a constraint to their movement from then on. I hope this makes sense.

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Asked: 2017-04-19 15:24:43 -0500

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Last updated: Apr 19