How to create a picture area in Flutter?

Question

In their example https://flutter.dev/docs/cookbook/plugins/picture-using-camera we can take pictures using the camera plugin. I modified the code and move into a separate widget. My main goal is to try to implement a picture area (like QRCode style) and take the picture and if necessary tell the user to corp the image or my be app will do it automatically. There is a barcode_scan plugin. It shows an area to scan the barcode. I like to implement that part to take pictures of an item.

https://codecanyon.net/item/qr-code-barcode-scanner-and-generator-for-ios-swift-with-admob/screenshots/20280521?index=1 screenshot2 has a 4 half square bracket on 4 edges. I like to make it similar but capture images only.

I am new to this plugin and corp idea. How can I create a picture area in Flutter so the user can center the item in that area and take a picture.?

Widget _buildPictureArea(BuildContext context) {
  return new Container(
    width: 200,
    height: 200,
    child: FutureBuilder<void>(
      future: _initializeControllerFuture,
      builder: (context, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          // If the Future is complete, display the preview
          return CameraPreview(_controller);
        } else {
          // Otherwise, display a loading indicator
          return Center(child: CircularProgressIndicator());
        }
      },
    ),
  );
}