API for LINK property "AddForceAtRelativePosition"

[addy1997]

This ticket is raised with reference to this #375 tutorial. I couldn't find a suitable replacement for the "AddForceAtRelativePosition" property of Link API.

Desired behavior

I am writing a wind plugin for pheonixDrone for calculating wind force.
In classical Gazebo, it can be written like this

// Calculate the wind force.
  double wind_strength = wind_force_mean_;

  math::Vector3 wind = wind_strength * wind_direction_;

  // Apply a force from the constant wind to the link.

  link_->AddForceAtRelativePosition(wind, xyz_offset_);

Alternatives considered

Nope, couldn't find suitable alternatives for this.

[diegoferigo]

This is not currently possible with the existing Link APIs. You need to use either Link::AddWorldForce or Link::AddWorldWrench. Be very careful when using any of these methods because they need the force / wrench expressed in different frames (xref #201).

In order to express a force in different frames, you can check the following logic:

https://github.com/ignitionrobotics/ign-gazebo/blob/26832764749291b6ea22b525262136ecbb63fbb9/src/Link.cc#L291-L297

Another example is this downstream project.

[addy1997]

@diegoferigo , thanks.