Refactor Bullet plugin implementation

[joxoby]

The previous implementation was using instances if btRigidBody to represent links and instances derived from btConstraint to model the joints.

This implementation uses instances of btMultiBody multi-link models and btRigidBody for single-link ones.

btMultiBody is the Bullet implementation of Featherstone's algorithm, which uses a reduced coordinates representation.

Related ign-gazebo PR: gazebosim/gz-sim#1223

[joxoby]

I've updated the base branch to ign-physics5, so we should update the label to fortress.

[azeey]

Thanks for the contribution @joxoby. I see that this uses Bullet 3.17, but that version of Bullet is not available on Ubuntu Bionic or Focal. Is having 3.17 a strict requirement for the PR?

[joxoby]

Thanks for the contribution @joxoby. I see that this uses Bullet 3.17, but that version of Bullet is not available on Ubuntu Bionic or Focal. Is having 3.17 a strict requirement for the PR?

No, it's not. I reverted the change.

[dgoel]

Just checking if there's any plan to review this PR?

[bhaskara]

Hi all, checking on the status of this. I see it was removed from "In Progress" a few days ago?

[azeey]

I haven't had time to review this, but I think it might have been removed from "In Progress" due to the migration to the new project board under the gazebosim org.

btw, @joxoby I see CI is failing. Anyway you can fix the failing tests?

[mxgrey]

Thanks for all the effort that went into this PR. It was very informative to see the way you approached the integration. Seeing the challenges you dealt with helped us identify some issues that made gz-sim incompatible with physics engines that don't support incremental construction of models.

We're closing this PR in favor of gazebosim/gz-sim#1560 and #373

With those two PRs we're taking a different strategy than what was used here:

• Instead of a ConstructSdfMultibody feature that conceptually overlaps with ConstructSdfModel, we are changing the behavior of gz-sim to, by default, have Models constructed in a single call to ConstructSdfModel, and then allow for incremental additions to those models using ConstructSdf[Link|Joint|Collision] if the physics engine happens to implement it. Now the only required ConstructSdf_ feature will be ConstructSdfModel.
• Instead of migrating the existing bullet plugin over to the Featherstone API, we are allowing users to choose whether they use the classic or the Featherstone implementation by selecting between the two plugins. The classic bullet plugin will allow incremental construction of models whereas the Featherstone will not offer that feature in exchange for having better performance.