Flutter Custom Paint (masking arc with open path)

main.dart

import 'package:flutter/material.dart';
import 'dart:math' as math;

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        backgroundColor: Colors.white,
        body: Container(
          color: Colors.black.withOpacity(.8),
          child: Center(
            child: SizedBox(
              height: 300,
              width: 300,
              child: CustomPaint(
                painter: MyWidget()
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class MyWidget extends CustomPainter {
  
  @override
  void paint(Canvas canvas, Size size) {
    final center = Offset(size.width / 2, (size.height / 2) + 20);
    final radius = math.min(size.width, size.height) / 2 - 5;
    final ovalRect = Rect.fromCenter(center: center, width: radius * 2, height: radius * 2);
    final arch1 = Path()..addArc(ovalRect, math.pi, math.pi);
    final paint1 = Paint()
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 20
      ..color = Colors.amber;
    
    final arch2 = Path()..addArc(ovalRect, math.pi, (math.pi / 2) + .05);
    final paint2 = Paint()
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 23
      ..color = Colors.transparent
      ..blendMode = BlendMode.srcOut;
        
    final arch3 = Path()..addArc(ovalRect, math.pi, (math.pi / 2));
    final paint3 = Paint()
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 21
      ..color = Colors.purple;

    canvas.saveLayer(Rect.largest, Paint());
    canvas.drawPath(arch1, paint1);
    canvas.drawPath(arch2, paint2);
    canvas.restore();
    canvas.drawPath(arch3, paint3);
  }
  
  
  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;
}