Rounded any acute corner

main.dart

import 'dart:ui';

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Custom Painter',
      theme: ThemeData(
        primarySwatch: Colors.pink,
      ),
      home: MyPainter(),
    );
  }
}

class MyPainter extends StatefulWidget {
  @override
  _MyPainterState createState() => _MyPainterState();
}

class _MyPainterState extends State<MyPainter> {
  var _sides = 3.0;
  var _radius = 100.0;
  var _radians = 0.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Polygons'),
      ),
      body: SafeArea(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Expanded(
              child: CustomPaint(
                painter: ShapePainter(_sides, _radius, _radians),
                child: Container(),
              ),
            ),
            Padding(
              padding: const EdgeInsets.only(left: 16.0),
              child: Text('Sides'),
            ),
            Slider(
              value: _sides,
              min: 3.0,
              max: 10.0,
              label: _sides.toInt().toString(),
              divisions: 7,
              onChanged: (value) {
                setState(() {
                  _sides = value;
                });
              },
            ),
            Padding(
              padding: const EdgeInsets.only(left: 16.0),
              child: Text('Size'),
            ),
            Slider(
              value: _radius,
              min: 10.0,
              max: MediaQuery.of(context).size.width / 2,
              onChanged: (value) {
                setState(() {
                  _radius = value;
                });
              },
            ),
            Padding(
              padding: const EdgeInsets.only(left: 16.0),
              child: Text('Rotation'),
            ),
            Slider(
              value: _radians,
              min: 0.0,
              max: math.pi,
              onChanged: (value) {
                setState(() {
                  _radians = value;
                });
              },
            ),
          ],
        ),
      ),
    );
  }
}

// FOR PAINTING POLYGONS
class ShapePainter extends CustomPainter {
  final double sides;
  final double radius;
  final double radians;
  
  ShapePainter(this.sides, this.radius, this.radians);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint()
      ..color = Colors.teal
      ..strokeWidth = 5
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.round;

     var path = new Path();
    path.lineTo(0.0, size.height - 20);
    path.quadraticBezierTo(0.0, size.height, 20.0, size.height);
    path.lineTo(size.width - 20.0, size.height);
    path.quadraticBezierTo(size.width, size.height, size.width, size.height - 20);
//     path.lineTo(size.width, 50.0);
//     path.quadraticBezierTo(size.width, 30.0, size.width - 20.0, 30.0);
//     path.lineTo(20.0, 5.0);
//     path.quadraticBezierTo(0.0, 0.0, 0.0, 20.0);
   
    
    path.close();
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}