Basic flutter to image of canvas.

main.dart

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

void main() async {
  const double width = 800;
  const double height = 600;
  final ui.PictureRecorder recorder = new ui.PictureRecorder();
  final ui.Canvas canvas =
      new ui.Canvas(recorder, Rect.fromLTWH(0, 0, width, height));

  canvas.drawRect(
      Rect.fromLTWH(0, 0, width, height),
      Paint()
        ..style = PaintingStyle.fill
        ..color = ui.Color.fromRGBO(128, 0, 0, 1.0)
        ..isAntiAlias = false);
  final ui.Picture picture = recorder.endRecording();
  ui.Image image = await picture.toImage(width.floor(), height.floor());
  ByteData data = await image.toByteData();

  // Should be 128 as specified in our Paint fill.
  print("First pixel's R: ${data.getUint8(0)}");
}