[BUG] Alternative polygon renderer in 7.0 renderers multiple holes incorrectly

[Elleo]

What is the bug?

If a polygon contains multiple holes and the useAltRendering property is set on the polygon layer, then the holes are cut out incorrectly (with vertices from different holes seemingly connecting to each other)

Expected output:

Actual output:

How can we reproduce it?

The following code can be used to reproduce this issue:

          FlutterMap(
            options: const MapOptions(
              initialCenter: LatLng(64.025, -1.132),
              initialZoom: 16,
            ),
            children: [
              PolygonLayer(
                useAltRendering: true,
                polygons: [
                  Polygon(
                      points: const [
                        LatLng(64.0283, -1.13083),
                        LatLng(64.0282, -1.130172),
                        LatLng(64.024794, -1.130862),
                        LatLng(64.0247, -1.13126),
                        LatLng(64.0247, -1.13183),
                        LatLng(64.025, -1.13274),
                        LatLng(64.0254, -1.13352),
                        LatLng(64.0259, -1.134093),
                        LatLng(64.027859, -1.135774),
                        LatLng(64.027915, -1.134542),
                        LatLng(64.028168, -1.132229),
                        LatLng(64.028284, -1.131658),
                        LatLng(64.0283, -1.13083)
                      ],
                      holePointsList: const [
                        [
                          LatLng(64.026675, -1.133183),
                          LatLng(64.026601, -1.133234),
                          LatLng(64.026534, -1.133158),
                          LatLng(64.026534, -1.13302),
                          LatLng(64.026601, -1.132944),
                          LatLng(64.026688, -1.13303),
                          LatLng(64.026675, -1.133183)
                        ],
                        [
                          LatLng(64.026226, -1.134005),
                          LatLng(64.0262, -1.134068),
                          LatLng(64.026149, -1.134059),
                          LatLng(64.02616, -1.133918),
                          LatLng(64.026211, -1.133933),
                          LatLng(64.026226, -1.134005)
                        ],
                        [
                          LatLng(64.025838, -1.13236),
                          LatLng(64.02578, -1.132496),
                          LatLng(64.025723, -1.132513),
                          LatLng(64.025669, -1.132476),
                          LatLng(64.025631, -1.132298),
                          LatLng(64.025695, -1.132169),
                          LatLng(64.025752, -1.132159),
                          LatLng(64.025804, -1.132202),
                          LatLng(64.025838, -1.13236)
                        ]
                      ],
                      label: "No 3",
                      color: Colors.lightBlue.withOpacity(0.5),
                      borderColor: Colors.lightBlue,
                      borderStrokeWidth: 1),
                ],
              ),
            ],
          ),

Do you have a potential solution?

No response

Platforms

Windows 11, iOS 17

Severity

Obtrusive: Prevents normal functioning but causes no errors in the console

[JaffaKetchup]

It's probably misinterpreting the output of dart_earcut, or maybe the code in dart_earcut is wrong. But more likely is the first one, as I had a hard time understanding the output myself (despite it being me that ported that library!).

[JaffaKetchup]

It probably starts at this line, which then throws the whole triangulation off:

flutter_map/lib/src/layer/polygon_layer/polygon_layer.dart

Lines 177 to 180 in b69a0d7

	// Not sure how just this works but it seems to :D
	holeIndices: culledPolygon.holePoints.isEmpty
	? null
	: [culledPolygon.points.length],

I knew there would be issues when I wrote it, but none of my test cases included multiple holes!