Add ArcBallCamera example

[ttnghia]

Here is the arcball camera example :)

Brief Usage:

By mouse:

• Left mouse: rotation
• Right mouse: translation
• Middle mouse: zooming

Keyboard shortcuts:

• 'R': reset view
• 'L': toggle lagging (smooth navigation)
• '+'/'-': zooming
• Up/Down/Left/Right: translation

Enjoy.

[mosra]

Thank you, this is great!

I went a bit more in detail on this, because I think this isn't too far from becoming a builtin feature -- especially when we relax the dependency on SceneGraph a bit :)

[ttnghia]

I would prefer to create just one ArcBallCamera object and nothing else, instead of having 3 objects:

ArcBall* _arcball { nullptr };
Object3D*             _cameraObject { nullptr };
SceneGraph::Camera3D* _camera { nullptr };

By doing so, the last 2 objects will be private members of the ArcBall class. How do you think?

[mosra]

Okay, maybe I'm diverging from simplicity of the example a bit too much already, but (to repeat myself) I'd like to have this easy to integrate for people who don't use / want to use the scenegraph.

What about an ArcBall class, containing the algorithmic implementation, and SceneGraphArcBall, being a convenience all-in-one thing containing the camera+object inside? It still feels a bit too rigid for me, as it breaks composability, but as a convenience API why not.

Or if you're fine with the templated update() above, let's do just that, as it's less code overall :)

[ttnghia]

Hah, two-classes sounds good to me. We can have both of them, ArcBall will implement the update(cameraObj) function above, and ArcBallCamera will have an ArcBall object and 2 scene graph camera objects inside as private members.
I'm going to make that. The second class will just be a short header.

[ttnghia]

Done!

[mosra]

If you have time and want to make the merging easier on my side, could you:

• add a PNG 800x600 screenshot to doc/arcball-camera.png, without window borders please
• add a doc/arcball-camera.dox doc page, showing the screenshot, listing key controls and having links to all source files? Similarly to what's in e.g. doc/fluidsimulation2d.dox, doesn't need to be perfect, just putting most of the relevant content there :)
• mention the new WITH_ option in doc/building-examples.dox
• and enable the new WITH_ option in package/ci/ in all *.bat and *.sh files, except the vulkan, android, emscripten and ios ones, so I can see if it builds correctly on most platforms?

This will help me a lot, thanks much! 🙏

[mosra]

Merged as 49a1bbe, thanks a lot! Especially thanks for the docs+CI push, that saved me quite some time. Docs now online here: https://doc.magnum.graphics/magnum/examples-arcball.html

As I mentioned on Gitter, I made the ArcBall class internals use dual quats and as a result ArcBallCamera is independent of used scenegraph transformation implementation. It needed an update on magnum's side to allow this (mosra/magnum@c25b19d), so be sure to pull it before going further :)