revisit rotation limits

[jfirebaugh]

gl-native currently disallows rotation at z<3. This prevents several use cases that it should support:

• Showing interactive rotated low zoom maps, even with world edges, if so desired.
• Showing a non-interactive 90° or 180° ("South Up") map at low zoom level.
• Headless rendering of low zoom level rotated static maps.

This is also currently causing headless test failures on fill-translate-anchor tests, which apply 90° rotation at z0.

I think we should remove the current rotation limits in Transform and replace them with something that applies constraints at a higher level, is off in headless contexts, can be opt-out in interactive contexts, and shares behavior with gl-js.

[incanus]

Related back issues: #242 #279

Totally up for adding switches for this, but mobile use cases are going to want it as-is, plus the eventual addition of animations per #296.

[ljbade]

I would like to have the option to rotate when zoomed out purely because Google Maps can and we want to be better than Google Maps.

[incanus]

I guess it's minor though.

[ljbade]

Just fill northern part with sea colour, and southern part with ice colour. Might not work with certain styles or satellite imagery.

The main problem with locking rotation is that there is no feedback, e.g. 'bounce' when trying to drag past the end of a scroll box. Thus the user thinks they can't rotate the map at all, or something is wrong e.g. we are not reading their rotate gesture.

[ljbade]

@incanus Another argument for no rotation - we should not force a user into a particular direction as being 'up'.

Some Australians like to think that we are at the top of the world and that all maps should look like this:

Why should we deny them their preference?

[ljbade]

Two more againsts - it causes bugs/confusion, see:

• Rotation isn't persisted #463
• Rotation is disabled when you are close to the antimeridian #618

[incanus]

Let's try to keep discussions about limitations to rotation (whether zoom or antimeridian) in this ticket. Closing #618.

I absolutely take your point about Australian preference @ljbade. But check out how Apple Maps handles this:

(Smooth video version here)

The animation style is more of a hint than anything, and the antimeridian issue is actually really a zoom issue. If you zoom way in on the Southern Pacific, then rotate, then pan across the ocean, rotation is allowed. It's only at lower-numbered zooms that this rotates back to north.

We could always add a preference for this, but I'm thinking that based on the kinds of visualizations that are likely to be used by these tools, especially on mobile, restricting rotation to higher-numbered zooms is ok. And I really like the elegance of the user cue in Apple's style of animation.

[ljbade]

Hmm, I think we should make this a option you can turn on and off.

Apple's animation style does help to show what is happening to the user.

But the antimeridian lock is just dumb because there is no reason for it, the map wraps around so there is no edge to the map there and it is really annoying in New Zealand.

[ljbade]

Or at least add a dashed line to the mbgl maps along the antimeridian to show there is a map seam there.