Compute simplification tolerance based of given projection. Now the u…

…ser-provided tolerance will have similar meaning independent of the user-selected projection.

lib/src/layer/polygon_layer/polygon_layer.dart

@@ -67,6 +67,7 @@ class PolygonLayer extends StatefulWidget {
 
 class _PolygonLayerState extends State<PolygonLayer> {
   List<_ProjectedPolygon>? _cachedProjectedPolygons;
+  double? _effectiveTolerance;
   final _cachedSimplifiedPolygons = <int, List<_ProjectedPolygon>>{};
 
   @override
@@ -82,6 +83,7 @@ class _PolygonLayerState extends State<PolygonLayer> {
 
     _cachedSimplifiedPolygons.clear();
     _cachedProjectedPolygons = null;
+    _effectiveTolerance = null;
   }
 
   @override
@@ -96,13 +98,17 @@ class _PolygonLayerState extends State<PolygonLayer> {
       ),
       growable: false,
     );
+    final simplificationTolerance = _effectiveTolerance ??=
+        getEffectiveSimplificationTolerance(
+            camera.crs.projection, widget.simplificationTolerance);
 
     final zoom = camera.zoom.floor();
+
     final simplified = widget.simplificationTolerance == 0
         ? projected
         : _cachedSimplifiedPolygons[zoom] ??= _computeZoomLevelSimplification(
             projected,
-            widget.simplificationTolerance,
+            simplificationTolerance,
             zoom,
           );
 

lib/src/misc/simplify.dart

@@ -1,6 +1,6 @@
 // implementation based on
 // https://github.com/mourner/simplify-js/blob/master/simplify.js
-
+import 'package:flutter_map/src/geo/crs.dart';
 import 'package:latlong2/latlong.dart';
 
 // Custom point due to math.Point<double> being slow. Math operations tend to
@@ -156,3 +156,17 @@ List<DoublePoint> simplifyPoints(
 
   return nextPoints;
 }
+
+double getEffectiveSimplificationTolerance(
+  Projection projection,
+  double tolerance, {
+  LatLng point = const LatLng(45, 90),
+}) {
+  if (tolerance <= 0) return 0;
+
+  final p0 = projection.project(point);
+  final p1 = projection
+      .project(LatLng(point.latitude + tolerance, point.longitude + tolerance));
+
+  return p0.distanceTo(p1);
+}