fix: 1338 - longitude +-180 with correct polylines and polygons

[monsieurtanuki]

What

• This PR deals with polylines and polygons (with or without holes) across longitudes +-180 (cf. [BUG] Crossing W/E Extremity With Polyline Results In Unexpected Lines #1338)
• The main issue was that longitudes -179 and 179 were projected in opposite sides of the world, although they are geographically nearby.
• The solution is to build incrementally the list of projected points: if two consecutive projected points are obviously too far away (more than half a world away), we add/remove one world width to the latter point.
• Doing so, we avoid jumping around -180 and +180, and we ensure the continuity among the points.
• The PR doesn't deal with the "teleportation" effect. This will be done in another PR, once we're confident with that "add/remove a world width" strategy.

Screenshots

Polyline: before / after

Polygon with hole: before / after

Impacted files

• crs.dart: new methods getHalfWorldWidth and projectList
• painter.dart: refactored using pre-computed List<Double>
• polygon.dart: added an example around longitude 180
• polyline.dart: added an example around longitude 180
• polyline_layer.dart: we don't cull polylines that go beyond longitude 180
• projected_polygon.dart: using new method Projection.projectList
• projected_polyline.dart: using new method Projection.projectList

[josxha]

Thanks for the pull request @monsieurtanuki! I did some quick testing and your added examples work like intended (thanks for adding them).

Some additional thoughts:
The main use case of line wrapping are paths that cross the -180/180 longitude for example when we want to display flights. We need to figure out a solution when the user follows a polyline or polygon accross the world border. Currently the line suddenly disapears. This, of course, is not in scope for this this pull request. (:

line.wrapping.mp4

[josxha]

As much as I like this name, a declarative name would be more understandable in the future. 😄

[monsieurtanuki]

Some additional thoughts: The main use case of line wrapping are paths that cross the -180/180 longitude for example when we want to display flights. We need to figure out a solution when the user follows a polyline or polygon accross the world border. Currently the line suddenly disapears. This, of course, is not in scope for this this pull request. (:

Oh it's a bug then! Probably related to the "teleportation" issue.
I'll have a look at it, and I may fix that in the current PR.
I keep you posted.

[josxha]

Oh it's a bug then! Probably related to the "teleportation" issue.
I'll have a look at it, and I may fix that in the current PR.

I actually don't think it's an bug. The line just moves to the new world that comes into center. So the line is still active but basically on the other side of the world. If you plan to look into this as well how it can get improved that's awesome of couse. (:

[monsieurtanuki]

@josxha Your description is correct: it's the "teleportation" issue (I can see a unique polyline jumping from world to world on the same zoom 0 map) but when the zoom is high - the "other" world is not visible.

I still think it's a bug, probably not a new bug, but a fair user expectation is to see at least one polyline.

Working on it. The current PR is relatively small (7 files changed), so I'll see how review-able it would become when including that bug fix.

[monsieurtanuki]

@josxha Fixed! One visible copy of the polyline, with a small or a large zoom.