You can add areas/shapes formed from coordinates to maps using PolygonLayer and Polygons.
Polygons including labels, holes, mixed colors & opacities, and dotted bordersPolygonLayers and Polygonss support hit detection and interactivity.
The PolygonLayer paints its Polygons across all visible worlds by default. This can be changed.
flutter_map includes many performance optimizations built in, especially as of v7. Some are enabled by default, but may be only 'weakly' applied, and others must be enabled manually. There are also some other actions that can be taken externally to improve performance
The following list is ordered from least 'intrusive'/extreme, to most intrusive. Remember to consider the potential risks of enabling an optimization before doing so.
The example application includes a stress test which loads multiple Polygons from an optimized format, with a total of 138,000 points.
The default renderer supports two painter fill methods using different Flutter APIs. These fill methods change the way Flutter decides what is considered within a polygon (and should be filled), and what is outside. This can change the way particularly intersections and overlaps appear visually.
This can be set using the painterFillMethod property and the PolygonPainterFillMethod enum.
Many apps will not need to change from the default method. Before changing the method, profile and test for visual glitches thouroughly.
The PolygonPainterFillMethod.pathCombine option uses lesser-used Flutter APIs: the constructor. This allows for more intricate s, such as difference and union.
This gives the correct/intended/best visual results on native platforms. It's the default on native (non-web) platforms.
However, on the web, it causes major visual glitches due to a .
Additionally, it has slightly worse performance (especially at scale) than . The hit to performance is unlikely to be significant or even noticeable in many applications, but applications drawing many polygons may see a slow of about 2ms (as tested in the example app's stress test).
The PolygonPainterFillMethod.evenOdd option uses more simple Flutter APIs: the .evenOdd
On the web, inverted filling may not work as expected in some cases. It will not match the behaviour seen on native platforms.
Avoid allowing polygons to intersect, and avoid using holes within polygons. See the second image below for a demonstration of the issues.
This is due to multiple limitations/bugs within Flutter. See and for the problematic bug reports.
Inverted filling (invertedFill) allows a color to be applied to all parts of the map outside a polygon. Transparently filled polygons will reveal the layers beneath without the inverted fill color.
'flutter_map' doesn't provide any public methods to manipulate polygons, as these would be deemed out of scope.
However, some useful methods can be found in libraries such as 'latlong2' and . These can be applied to the input of Polygon's points argument, and the map will do it's best to try to render them. However, more complex polygons - such as those with holes - may be painted inaccurately, and may therefore require manual adjustment (of holePointsList, for example).
PolygonLayer(
polygons: [
Polygon(
points: [LatLng(30, 40), LatLng(20, 50), LatLng(25, 45)],
color: Colors.blue,
),
],
),pointsPolygonpolygonssimplificationTolerance parameter. Increasing this value (from its default of 0.5) results in a more jagged, less accurate (lower quality) simplification, with improved performance; and vice versa. Many applications use a value in the range 1 - 1.5. To disable simplification, set simplificationTolerance to 0. Simplification algorithms reduce the number of points in each line by removing unnecessary points that are 'too close' to other points which create tiny line segments invisible to the eye. This reduces the number of draw calls and strain on the raster/render thread. This should have minimal negative visual impact (high quality), but should drastically improve performance.
For this reason, polygons can be more simplified at lower zoom levels (more zoomed out) and less simplified at higher zoom levels (more zoomed in), where the effect of culling on performance improves and trades-off. This is done by scaling the simplificationTolerance parameter (see below) automatically internally based on the zoom level.
This pathway may be slower than the standard pathway, especially when used on a large scale but with simplification disabled, or used on an especially small scale.
It is intended for use when prior profiling indicates more performance is required after other methods are already in use.
Rarely, some visible artefacts may be introduced by the triangulation algorithm.
Polygons (and similar other features) are usually drawn directly onto a Canvas, using built-in methods such as drawPolygon and drawLine. However, these can be relatively slow, and will slow the raster thread when used at a large scale.
Therefore, to improve performance, it's possible to optionally set the useAltRendering flag on the PolygonLayer. This will use an alternative, specialised, rendering pathway, which may lead to an overall performance improvement, particularly at a large scale.
There's two main steps to this alternative rendering algorithm:
Cut each Polygon into multiple triangles through a process known as triangulation. flutter_map uses an earcutting algorithm through dart_earcut (a port of an algorithm initially developed at Mapbox intended for super-large scale triangulation).
Draw each triangle onto the canvas via the lower-level, faster drawVertices method. Borders are then drawn as normal.
This gives the best performance, and works on the web. This is the default when targeting the web.
However, it yields unintended results in certain edge cases when polygons intersect when Inverted Filling is used, or when polygon holes can intersect with other holes.
.evenOdd setting
