Cannot select annotations that lie on a tile boundary

[jadar]

We have a bug where we're not able to select/tap some of our annotations on the map in our iOS app. After some head scratching, we found out that the annotation just happened to lie on a tile boundary! Stepping through the Mapbox code, it appears that annotationTagsInRect:, (called in annotationTagAtPoint:persistingResults:,) returns a std::vector of size 0 when there is clearly an annotation within the rect, except the annotation lies between a few tiles.

Steps to reproduce

1. Create and add/display a UIView-backed annotation on an MGLMapView
2. Place the annotation between a couple tiles. The lat/lon we can reproduce the issue with are (36.38149043210595, -119.59605216979982).
3. Try to tap on it.

Expected behavior

The annotation is detected and the relevant delegate methods are called.

Actual behavior

No delegate methods are called.

While the screenshot says "THIS WILL WORK", it actually means it won't work.

Configuration

Mapbox SDK versions: 4.2.0
iOS/macOS versions: iOS 11.4
Device/simulator models: iPhone 8 and iPhone X
Xcode version: Xcode 9.4.1

[ChrisLoer]

Hi @jadar thanks for the report! Is there any chance you could post some example code reproducing the problem? What you're describing is a class of bug we've had before, and we believe we've fixed. There may be some subtle difference in how you're exercising the functionality that still causes the problem to manifest.

[jadar]

@ChrisLoer Here is a bare-bones implementation that reproduces the bug. If you zoom in to where the scale shows ~100ft, it won't show a callout. If you zoom out to where the scale shows ~600 ft, the callout shows.
https://gist.github.com/jadar/7a8f8454b78a4168f5724c5c61de6b6b

[ChrisLoer]

Hi @jadar I'm sorry but I'm still not seeing the problem. Here's what I see when I tap on the annotation created with your code pasted into my example app (I moved the location to null island i.e. [0,0] just to be sure it was on a tile boundary, but I couldn't reproduce with your original coordinates either):

[jadar]

Hmm, I can't seem to reproduce it at [0,0] either. When I use the coordinates I gave, it works up until zoom level 16. 15 and below work correctly.

I don't know if it changes anything, but I'm using this style, and the 4.2.0 version of Mapbox.
mapbox://styles/mapbox/satellite-streets-v9
https://github.com/mapbox/mapbox-gl-native/releases/download/ios-v4.2.0/mapbox-ios-sdk-4.2.0-dynamic.zip

[jadar]

It also seems that with [0.000000000000001, 0.00000000000001], it doesn't work at any zoom level.

[ChrisLoer]

Got it, thanks! I can reproduce now at [0.000000000000001, 0.00000000000001]. I'll take a closer look tomorrow and hopefully get to the bottom of it.

[ChrisLoer]

This is a good bug! 😅

The problem is that when the AnnotationManager decides which tile the annotation at [0.000000000000001, 0.00000000000001] should go in, it correctly chooses the "northeast" tile, based on LatLng coordinates:

mapbox-gl-native/src/mbgl/annotation/annotation_manager.cpp

Lines 142 to 147 in 8400355

	LatLngBounds tileBounds(tileID);
	symbolTree.query(boost::geometry::index::intersects(tileBounds),
	boost::make_function_output_iterator([&](const auto& val){
	val->updateLayer(tileID, *pointLayer);
	}));

However, it then translates into tile coordinates:

mapbox-gl-native/src/mbgl/annotation/symbol_annotation_impl.cpp

Lines 18 to 21 in 8400355

	LatLng latLng { annotation.geometry.y, annotation.geometry.x };
	TileCoordinate coordinate = TileCoordinate::fromLatLng(0, latLng);
	GeometryCoordinate tilePoint = TileCoordinate::toGeometryCoordinate(UnwrappedTileID(0, tileID), coordinate.p);
	layer.addFeature(id, FeatureType::Point, GeometryCollection {{ {{ tilePoint }} }}, featureProperties);

TileCoordinate::fromLatLng(0, latLng) returns [0.5, 0.5] (using a coordinate system that represents the whole world in web mercator projection with positions from 0 to 1). You can see the low bits have already been lost (it should really be something like [.5000000000001, .4999999999999]. TileCoordinate::toGeometryCoordinate then returns [0,8192] -- that is, the point that's just off the bottom edge of the tile (the tile goes from [0,0] to [8191,8191].

I haven't actually traced the problem from here all the way back to the annotationsInRect issue, but I'm fairly confident this is the root issue.

@mourner, do you have any ideas about how we should fix this? The quick and dirty fix would be to just build a little tolerance into getTileData() and allow duplicates along the tile edges, but it seems like it would be more consistent to transform to some sort of integer-based coordinate system at the point we insert into the SymbolAnnotationTree, so that we don't have to worry about precision issues after that?

[mourner]

@ChrisLoer yeah, it might make sense to use a global integer coordinates system based on web mercator. Uint32 only allows for values up to z19 (32 - log2(8192)), so this would be need to be Uint64.

[ChrisLoer]

Fixed in #12570.