toImage method

Future<Image> toImage ({double pixelRatio: 1.0 })

Capture an image of the current state of this render object and its children.

The returned ui.Image has uncompressed raw RGBA bytes in the dimensions of the render object, multiplied by the pixelRatio.

To use toImage, the render object must have gone through the paint phase (i.e. debugNeedsPaint must be false).

The pixelRatio describes the scale between the logical pixels and the size of the output image. It is independent of the window.devicePixelRatio for the device, so specifying 1.0 (the default) will give you a 1:1 mapping between logical pixels and the output pixels in the image.

The following is an example of how to go from a GlobalKey on a RepaintBoundary to a PNG: 
class PngHome extends StatefulWidget {
  PngHome({Key key}) : super(key: key);

  @override
  _PngHomeState createState() => _PngHomeState();
}

class _PngHomeState extends State<PngHome> {
  GlobalKey globalKey = GlobalKey();

  Future<void> _capturePng() async {
    RenderRepaintBoundary boundary = globalKey.currentContext.findRenderObject();
    ui.Image image = await boundary.toImage();
    ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
    Uint8List pngBytes = byteData.buffer.asUint8List();
    print(pngBytes);
  }

  @override
  Widget build(BuildContext context) {
    return RepaintBoundary(
      key: globalKey,
      child: Center(
        child: FlatButton(
          child: Text('Hello World', textDirection: TextDirection.ltr),
          onPressed: _capturePng,
        ),
      ),
    );
  }
}

See also:

Implementation 

Future<ui.Image> toImage({double pixelRatio = 1.0}) {
  assert(!debugNeedsPaint);
  return layer.toImage(Offset.zero & size, pixelRatio: pixelRatio);
}