CropRect.dart

import 'package:flutter/material.dart';

final 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(
        body: Center(
          child: CropRect(
            child: Image.network(
              "https://miro.medium.com/max/1200/1*mk1-6aYaf_Bes1E3Imhc0A.jpeg",
              fit: BoxFit.fill,
            ),
            clipper: TriangleClipper(),
            size: const Size(200, 200),
          ),
        ),
      ),
    );
  }
}

class CropRect extends StatelessWidget {
  final CustomClipper<Path> clipper;
  final Size size;
  final Widget child;

  const CropRect({Key key, this.clipper, this.size, this.child})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return SizedBox(
      width: clipper.getClip(size).getBounds().width,
      height: clipper.getClip(size).getBounds().height,
      child: Stack(
        children: <Widget>[
          Positioned(
            height: size.height,
            width: size.width,
            child: ClipPath(
              clipper: clipper,
              child: Image.network(
                "https://miro.medium.com/max/1200/1*mk1-6aYaf_Bes1E3Imhc0A.jpeg",
                fit: BoxFit.fill,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

class TriangleClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    final path = Path();
    path.moveTo(10, 10);
    path.lineTo(100, 10);
    path.lineTo(100, 110);
    path.lineTo(10, 110);
    path.close();
    return path;
  }

  @override
  bool shouldReclip(TriangleClipper oldClipper) => true;
}