A little explanation is probably necessary to understand some of the main choices:

• To add multiple line segments I could have either split polylines into multiple polyline objects or, as I did, introduce support for multiple annotations intoMarkup. I went for the latter since I thought it would be the least effort.
• I didn't put too much effort into cleaning up the code that performs polyline updates, it's probably not very efficient
• With the version of MapLibre in use the only way to control the stacking of annotations is to create multiple managers, since each gets signed its layer id and since we can specify above/below which other layers it should go.
• The routine that fetches the correct Line/Segment/Fill Manager for the given Z index uses a TreeMap to compute the layers stacking. The idea is that the underlying tree's inherent ordering properties make it easy to insert a layer at any position in the layer stack, determining the layer above/below quickly. The situation might get a bit messy when apps insert layers like crazy but my intuition is that the layer ordering should stay visually consistent.

I finally implemented geodesic polyline. Disclaimer: for this part I used AI extensively since I don't fully understand the math. If you'd rather exclude this let me know.

At this point I don't think there's a lot more functionality we're missing that can be implemented. Tile overlays are basically impossible to add, and respecting the user's requested start/end cap style is very cumbersome with the MapLibre API. So I think I'm done, once somebody gets the change to take a look I can clean it up and get it ready for upstreaming if you'd like to get these changes in.

Tile overlays are basically impossible to add,

Someone asked me over email why this is the case. As I was gathering the info to provide an answer I actually found a potential hacky way to implement it. Maybe next month if I have some time I can play with the idea.

This is my response I gave the person, for whoever may be interested:

[...]

I'm not sure whether the docs I'm providing are for the exact corresponding version of MapLibre but they still give an idea of how things work.

The way you add overlays in MapLibre/MapBox is by adding layers; the relevant layer is the RasterLayer, which can load images from various sources. However, the source can either be a bunch of static files you provide ahead of time, or a web server that serves imagery over HTTP.

On the other hand a Google Maps v2 API tile provider is a very simple class that implements a method that must return a tile given x+y+zoom.

A few links:

• https://docs.mapbox.com/android/maps/api/9.5.1/
• https://docs.mapbox.com/android/maps/guides/styles/work-with-layers/
• https://docs.mapbox.com/style-spec/reference/sources/
• https://developers.google.com/android/reference/com/google/android/gms/maps/model/TileProvider

The reason why I think it might be possible is because I found out you can actually implement your own HTTP transport for HTTP tiles. So it may be possible to implement a custom HTTP transport that, when requesting images from an unlikely domain name, forges a response by calling the GMaps TileProvider locally, while forwarding the request to a proper HTTP client otherwise.

https://github.com/maplibre/maplibre-native/blob/main/platform/android/MapLibreAndroidTestApp/src/main/java/org/maplibre/android/testapp/utils/ExampleHttpRequestImpl.kt

@depau I had come to a similar rough conclusion w.r.t. implementing tile overlays, so overall I agree with this approach. Thanks for giving it further thought! Please do it in a separate PR though 😃

I'm intending on reviewing your changes from this PR but alas, time is short.

No rush, it doesn't look like the next CalyxOS release is going to come any time soon so I'll have to make do with black lines on Garmin Connect anyway :)

Sure, I'll do that in another PR, it shouldn't too hard to integrate any layer changes with these changes (assuming the overall design doesn't change too much over here)

Thank you!