main.dart

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

void main() => runApp(new MaterialApp(
      home: new MyApp(),
    ));

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Stack(
        overflow: Overflow.visible,
        children: <Widget>[
          new Container(
            decoration: new BoxDecoration(
              image: new DecorationImage(
                image: new ExactAssetImage('assets/pics/newbook.jpg'),
                fit: BoxFit.cover,
              ),
            ),
            child: new BackdropFilter(
              filter: new ui.ImageFilter.blur(sigmaX: 5.0, sigmaY: 5.0),
              child: new Container(
                decoration: new BoxDecoration(color: Colors.white.withOpacity(0.0)),
              ),
            ),
          ),
          new Positioned(
            top: 150.0,
            left: 60.0,
            child: new ClipPath(
              clipper: new customclipper(),
              child: new Container(
                height: 350.0,
                width: 300.0,
                color: Colors.white70,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class customclipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    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);
    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => false;
}

and a path with no biezer curves;

var path = new Path();
path.lineTo(0.0, size.height);
path.lineTo(size.width, size.height);
path.lineTo(size.width, 50.0);
path.lineTo(0.0, 5.0);