Split transport scene manager into a plugin outside Scene3D

[chapulina]

🎉 New feature

Part of #137 , alternative to #217

Needs gazebosim/gz-rendering#320

Summary

This is a step towards reducing duplication between 3D scenes. Initially, @ahcorde and I had talked about taking an approach similar to the one used by plotting. That is providing an interface that is wrapped by an ign-gui plugin, and also wrapped by an ign-gazebo plugin, which adds Gazebo-specific functionality on top. @ahcorde started working on this approach in #217, and I also iterated a bit on top of it on 2fc70f2. But while at it, I realized that this approach may not be the best fit for Scene3D, because there's actually a simpler way.

My new idea is to leverage the Render event to perform all scene update tasks such as adding, removing and updating visuals. The Scene3D plugin itself should be very minimal, just focusing on running the render loop and providing some orbit controls. I'm leaving this minimal functionality on the new MinimalScene plugin, while all other functionality should be pushed to new plugins, like the TransportSceneManager that's being proposed here.

This approach allows us to keep Ignition GUI's Scene3D almost the same and add all new functionality on top. I think we could do the same with ignition::gazebo::Scene3D, see gazebosim/gz-sim#813. That is:

• copy all the logic that is not already in ignition::gui::Scene3D to a new plugin, GazeboSceneManager
• GazeboSceneManager will:
  • not work by itself. Like TransportSceneManager, it must be loaded alongside ignition::gui::MinimalScene
  • have no code that paints to the window (like texture IDs, instantiating a rendering thread, etc). That's the gui::MinimalScene's responsibility.
  • install a filter to the Render event and perform all rendering calls within that callback.
• Because we've been actively developing it, gazebo::Scene3D may have some functions and bugfixes that we'll need to port to gui::MinimalScene. We just need to be careful to push too much functionality into gui::MinimalScene. Let's keep that plugin lean. Any well scoped feature should go into a separate plugin that ties into the render event.

Test it

Try the new scene_provider example, see instructions on README added.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #221 (9b2cea1) into main (a9ddfad) will decrease coverage by 1.92%.
The diff coverage is 52.39%.

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
- Coverage   67.08%   65.15%   -1.93%     
==========================================
  Files          25       26       +1     
  Lines        3138     3811     +673     
==========================================
+ Hits         2105     2483     +378     
- Misses       1033     1328     +295     

Impacted Files	Coverage Δ
src/plugins/minimal_scene/MinimalScene.cc	52.15% <52.15%> (ø)
...s/transport_scene_manager/TransportSceneManager.cc	52.29% <52.29%> (ø)
src/plugins/scene3d/Scene3D.cc	48.89% <100.00%> (+0.46%)	⬆️
src/plugins/grid_3d/Grid3D.cc

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[chapulina]

Updates

1. Moved the new approach to a new plugin, MinimalScene, so we can tick-tock the change for users. Scene3D is deprecated on v6 and will be removed on v7.
2. Improved the example so the box moves:

TODO:

• Add tests

[chapulina]

@ahcorde , I'm opening this PR for review. I've added tests, a tutorial, and a migration path.

[ahcorde]

I added a plugin to move the camera with mouse #231, this will remove some code here. should we merge this first?

[ahcorde]

@chapulina, these two PRs are the following ones:

• Added Interactive view control plugin #231
• Added camera tracking #226