From 6a4234d2f37858b019c849eae19876c857d7680f Mon Sep 17 00:00:00 2001
From: Brendan Duncan <brendanduncan@gmail.com>
Date: Fri, 20 Dec 2024 15:39:06 -0700
Subject: [PATCH] Fix fillPolygon to better handle concave polygons

---
 lib/src/draw/fill_polygon.dart   | 71 ++++++++------------------------
 test/draw/fill_polygon_test.dart | 21 +++++++++-
 2 files changed, 37 insertions(+), 55 deletions(-)

diff --git a/lib/src/draw/fill_polygon.dart b/lib/src/draw/fill_polygon.dart
index 994b0db4..cb9ca11f 100644
--- a/lib/src/draw/fill_polygon.dart
+++ b/lib/src/draw/fill_polygon.dart
@@ -64,65 +64,28 @@ Image fillPolygon(Image src,
   xMax = min(xMax, src.width - 1);
   yMax = min(yMax, src.height - 1);
 
-  final inter = List<num>.filled(40, 0);
-  final vi =
-      List<int>.generate(numVertices + 1, (i) => i < numVertices ? i : 0);
-
-  for (var yi = yMin, y = yMin + 0.5; yi <= yMax; ++yi, ++y) {
-    var c = 0;
-    for (var i = 0; i < numVertices; ++i) {
-      final v1 = vertices[vi[i]];
-      final v2 = vertices[vi[i + 1]];
-
-      var x1 = v1.x;
-      var y1 = v1.y;
-      var x2 = v2.x;
-      var y2 = v2.y;
-      if (y2 < y1) {
-        var temp = x1;
-        x1 = x2;
-        x2 = temp;
-        temp = y1;
-        y1 = y2;
-        y2 = temp;
-      }
-
-      if (y <= y2 && y >= y1) {
-        num x = 0;
-        if ((y1 - y2) == 0) {
-          x = x1;
-        } else {
-          x = ((x2 - x1) * (y - y1)) / (y2 - y1);
-          x = x + x1;
-        }
-        if (x <= xMax && x >= xMin) {
-          inter[c++] = x;
+  // Function to fill a complex polygon using the ray casting algorithm
+  for (var yi = yMin, y = yMin + 0.5; yi <= yMax; ++yi, y += 1.0) {
+    for (var xi = xMin, x = xMin + 0.5; xi <= xMax; ++xi, x += 1.0) {
+      var intersections = 0;
+      for (var vi = 0; vi < numVertices; ++vi) {
+        final v1 = vertices[vi];
+        final v2 = vertices[(vi + 1) % numVertices];
+        // Ray casting: cast a ray to the right (x increasing)
+        if (v1.y <= y && v2.y > y || v2.y <= y && v1.y > y) {
+          // Ray intersects the edge (vertical check)
+          final vt = (y - v1.y) / (v2.y - v1.y);
+          if (x < v1.x + vt * (v2.x - v1.x)) { // Horizontal check
+            intersections++;
+          }
         }
       }
-    }
-
-    for (var i = 0; i < c; i += 2) {
-      var x1f = inter[i];
-      var x2f = inter[i + 1];
-      if (x1f > x2f) {
-        final t = x1f;
-        x1f = x2f;
-        x2f = t;
-      }
-      final x1 = x1f.floor();
-      final x2 = x2f.ceil();
-      for (var x = x1; x <= x2; ++x) {
-        drawPixel(src, x, yi, color, mask: mask, maskChannel: maskChannel);
+      // Even number of intersections means inside
+      if (intersections & 0x1 == 1) {
+        drawPixel(src, xi, yi, color, mask: mask, maskChannel: maskChannel);
       }
     }
   }
 
-  /*for (var i = 0; i < numVertices; ++i) {
-    final v1 = vertices[vi[i]];
-    final v2 = vertices[vi[i + 1]];
-    drawLine(src, x1: v1.xi, y1: v1.yi, x2: v2.xi, y2: v2.yi,
-        color: color, mask: mask, maskChannel: maskChannel, thickness: 1);
-  }*/
-
   return src;
 }
diff --git a/test/draw/fill_polygon_test.dart b/test/draw/fill_polygon_test.dart
index 5aab9ddd..e6415d49 100644
--- a/test/draw/fill_polygon_test.dart
+++ b/test/draw/fill_polygon_test.dart
@@ -17,11 +17,30 @@ void main() {
       ];
 
       fillPolygon(i0, vertices: vertices, color: ColorRgb8(176, 0, 0));
-      //drawPolygon(i0, vertices: vertices, color: ColorRgb8(0, 255, 0));
+      drawPolygon(i0, vertices: vertices, color: ColorRgb8(0, 255, 0));
 
       File('$testOutputPath/draw/fillPolygon.png')
         ..createSync(recursive: true)
         ..writeAsBytesSync(encodePng(i0));
     });
+
+    test('fillPolygon concave', () async {
+      final i0 = Image(width: 256, height: 256);
+
+      final vertices = <Point>[
+        Point(50, 50),
+        Point(50, 150),
+        Point(150, 150),
+        Point(150, 50),
+        Point(100, 100),
+      ];
+
+      fillPolygon(i0, vertices: vertices, color: ColorRgb8(176, 0, 0));
+      drawPolygon(i0, vertices: vertices, color: ColorRgb8(0, 255, 0));
+
+      File('$testOutputPath/draw/fillPolygon2.png')
+        ..createSync(recursive: true)
+        ..writeAsBytesSync(encodePng(i0));
+    });
   });
 }