Shape rendering and intersection with Path and Path.combine.

path_combine_intersect.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Container(
            width: 600,
            height: 600,
            color: Colors.grey[200],
            child: MyWidget(),
          ),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    Path p = Path();
    p..addPolygon(poly1, true)..addPolygon(poly2, true);
    Rect bounds = p.getBounds();

// Fit it to any parent dimensions
// Based on https://youtu.be/vvI_NUXK00s
    return FittedBox(
      fit: BoxFit.fill,
      child: SizedBox(
        width: bounds.width + 2 * bounds.left,
        height: bounds.height + 2 * bounds.top,
        child: CustomPaint(
          painter: PathPainter(poly1, poly2),
        ),
      ),
    );
  }
}

class PathPainter extends CustomPainter {
  const PathPainter(this.polyA, this.polyB);

  final List<Offset> polyA;
  final List<Offset> polyB;

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()..style = PaintingStyle.fill;

    Path p1 = Path()
      ..addPolygon(polyA, true)
      ..close();

    Path p2 = Path()
      ..addPolygon(polyB, true)
      ..close();

    canvas.drawPath(p1, paint..color = Colors.yellow);
    canvas.drawPath(p2, paint..color = Colors.cyan);

    //  Path.combine is not implemented on web
    //  https://github.com/flutter/flutter/issues/44572
    try {
      Path interP1P2 = Path.combine(PathOperation.intersect, p1, p2);
      canvas.drawPath(interP1P2, paint..color = Colors.red);
    } on UnimplementedError {
      print('Path.combine is not implemented in this context.');
    }
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;
}

final poly1 = <Offset>[
  Offset(170, 19),
  Offset(243, 17),
  Offset(274, 69),
  Offset(213, 150),
  Offset(146, 155),
  Offset(139, 80),
];

final poly2 = <Offset>[
  Offset(195, 60),
  Offset(345, 60),
  Offset(345, 140),
  Offset(195, 140),
];