[Rd] navigation mode(s) in rgl

[Michael Sumner]

Hello, I've CCd' Duncan Murdoch as the rgl maintainer, but I'm also keen to
hear from the broader community of any insights.

In rgl my understanding is that there's only one rotation-navigation mode,
where you left-click hold and the view pivots abound the centre of the
scene.

In other tools, including Google Earth, that is the default behaviour but
there's also a click-centric mode where the view pivots about the point
clicked on.

I haven't found the right terminology for this, but I call it

1) Data-centric navigation, rotating about the centre of the data in  the
scene (invoked by left-click-drag in rgl and GE and others)
2) Click-centric navigation, rotating about the point clicked on (invoked
by centre-click-drag in GE and others, but not rgl)

My questions:

1) I'd appreciate any guidance on my terminology here, whether I'm making
sense and ask for pointers to resources that explore this properly
2) Is there scope to add this "click-centric" navigation to rgl? I'd
appreciate any pointers to how it could be done - is it an rgl-level
feature, or deeper down?

I know that "click-centric" clicked-on point has a different meaning in
different contexts, in GE clearly it finds the nearest point intersecting
the globe surface since that is a always-present structure, but other tools
must have rules to specify where the pivot point is - either intersecting a
data element or somewhere in the scene.

The data-centric mode is fine for small scenes with a limited scope, but
when the extent covered by data is large it's quite unwieldy to focus in on
specific parts of the scene. I know this could be controlled by
pushing/popping elements in the scene but the navigation mode obviously
offers more flexibility.

Thank you.

Cheers, Mike.

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