Adding lawn.sdf world to have sky and grassy ground plane for rovers

[slgrobotics]

This is a modified default.sdf world file - I added sky, some lightning, grid and painted the ground plane grassy green.

I also added some friction/collision properties to the best of my knowledge.

I used "XML Copy Editor" to "pretty-print" the resulting SDF file - it fixes spaces and replaces single quotes with doubles.

[slgrobotics]

@MaEtUgR - I granted you access to the repository, so that you could do whatever you want with it, if you choose so. Let me know if I can be of help.

[Jaeyoung-Lim]

I think we should not add the sky to the default world, since we want consistent behavior even for low-performance systems.

However, if you want to add the sky to a non-default world, please do so

[fredmarkus]

I agree. I think the default should be as simple as possible for the maximum machines to be able to run it. Plus the sky can be buggy when running on non-dedicated GPU machines (gazebosim/gz-sim#2267).

[slgrobotics]

Ok, if it comes to catering to Celeron class machines with no graphics card - I understand... sad though, as people with standard 2020+ machines would see a dull skyless world by default. Bad marketing ;-)

Meanwhile I can rename this world to "lawn" and add it to the list of gz_worlds in gz_bridge/CMakeLists.txt

The way to see the sky would be "make px4_sitl gz_lawnmower_lawn"

Want to go this way? @Jaeyoung-Lim @frede791 - any other suggestions?

[fredmarkus]

@slgrobotics Yes I think that's good. Nothing to add from my side.

[Jaeyoung-Lim]

@slgrobotics What would you actually test on the lawn mode? I would prefer the worlds we have actually have a use to test vehicle behaviors.

[slgrobotics]

@Jaeyoung-Lim - well, when it comes for testing - all rovers will benefit from collision and friction properties in that ground plane. I test my lawnmower model with it all the time. But overall it just makes a better aesthetics for people who want it. Sky or no sky, current GZ default world isn't very appealing, even compared with the old "classic" one.

[slgrobotics]

I renamed my default.sdf to lawn.sdf to avoid conflicting with the legacy stuff. I also merged recent additions to default world into my lawn.sdf

[PerFrivik]

Looks nice, would it be possible to have some more textures in the green plane? Sometimes I struggle to get a sense of speed when everything is the same color and without any features.

[slgrobotics]

@PerFrivik - At the moment this PR is stale, there's no progress in code review and I'm not sure if it is ever going to be merged. I'd guess adding texture to the plane's visual is not a big deal, but I really don't want to do/disturb anything with it until we all have understanding of its future. As an option, we could have a separate PR on texture after this one is all done.