Visualize contacts

[chapulina]

Gazebo-classic has an option to enable visualization of all contacts in the world as blue spheres with a green line indicating the force's direction and magnitude. These help users debug their model's surface properties and identify contact points while stepping through simulation.

Ignition Gazebo already has a Contact system which populates the ContactSensorData component.

Here's Gazebo-classic's ContactVisual implementation.

[mcres]

I'll take this, as this is partially addressed in #229.

My idea is writing a GUI plugin with a checkbox that allows the user to turn on and off the visualization of the contacts. This plugin would be linked to a source file which could get the contacts in one of the following ways:

1. Continually check for new ContactSensorData in the Update method.

2. Subscribe to the contacts already being published by the Contact system.

3. Modify the Contact system itself and somehow connect the GUI plugin with it (I'm thinking about getting the Contact System instance with the ECM?).

After getting the info, contacts will be visualized with the /marker service.
However, currently the contact messages only return the position, so for the moment the contacts direction would have to be some default value. Work is going on in gazebosim/gz-physics#40.

I belive no. 3 is the best option, but any feedback will be appreciated!

[mabelzhang]

Hey @azeey, since you're the most familiar with the Contact plugin, is there a preference how we want to hook up the contact visualization to it (see suggestions 1-3 above)? Thanks!

[azeey]

Option 1 seems reasonable to me. The GUI plugin should be able to get the contact sensor data from the GUI ECM (almost a copy of the server ECM). We might have to use _contacts->SetData and _ecm.SetChanged in https://github.com/ignitionrobotics/ign-gazebo/blob/9ab1cc3f17173c57028f3c460121a36cbdb0c8d7/src/systems/physics/Physics.cc#L1722

[mcres]

@azeey Would it be currently possible to get the info about all of the contacts in the simulation without requiring the link to have a Contact sensor (i.e. looking for ContactSensorData)? Given this TODO comment, I understand that it is not.

I think that @chapulina meant getting all the contacts, but I don't know if that's possible without modifying too much the physics plugin.

[mcres]

Following the conversation with @azeey : the physics plugin fills the components::ContactSensorData for entities with both ContactSensorData and Collision components.

This means that getting information of <collision> contacts that don't have a <sensor name="contact_sensor" type="contact"> is easily achievable by creating a components::ContactSensorData in the corresponding entity, which in turn will be filled by physics.

[chapulina]

Closed by #234