WatermarkOverImageExample.dart

import 'dart:io';
import 'dart:typed_data';

import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';

import 'package:image/image.dart' as ui;

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  File _originalImage;
  File _watermarkImage;
  File _watermarkedImage;
  final picker = ImagePicker();

  Future getOriginalImage() async {
    final pickedFile = await picker.getImage(source: ImageSource.gallery);
    setState(() {
      _originalImage = File(pickedFile.path);
    });
  }

  Future getWatermarkImage() async {
    final pickedFile = await picker.getImage(source: ImageSource.gallery);
    setState(() {
      _watermarkImage = File(pickedFile.path);
    });
  }

  @override
  void initState() {

    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(title: Text("Watermark Example"),),
        body: Center(
          child: Column(
            children: <Widget>[
//<--------------- select original image ---------------->
              _originalImage==null? FlatButton(
                child: Text("Select Original Image"),
                onPressed: getOriginalImage,
              )
                  : Image.file(_originalImage),

//<--------------- select watermark image ---------------->
              _watermarkImage==null? FlatButton(
                child: Text("Select Watermark Image"),
                onPressed: getWatermarkImage,
              )
                  : Image.file(_watermarkImage),

              SizedBox(height: 50,),
//<--------------- apply watermark over image ---------------->
              (_originalImage!=null)&&(_watermarkImage!=null)?
              FlatButton(
                child: Text("Apply Watermark Over Image"),
                onPressed: () async {

                  ui.Image originalImage = ui.decodeImage(_originalImage.readAsBytesSync());
                  ui.Image watermarkImage = ui.decodeImage(_watermarkImage.readAsBytesSync());

                  // add watermark over originalImage
                  // initialize width and height of watermark image
                  ui.Image image = ui.Image(160, 50);
                  ui.drawImage(image, watermarkImage);

                  // give position to watermark over image
                  // originalImage.width - 160 - 25 (width of originalImage - width of watermarkImage - extra margin you want to give)
                  // originalImage.height - 50 - 25 (height of originalImage - height of watermarkImage - extra margin you want to give)
                  ui.copyInto(originalImage,image, dstX: originalImage.width - 160 - 25, dstY: originalImage.height - 50 - 25);


                  // for adding text over image
                  // Draw some text using 24pt arial font
                  // 100 is position from x-axis, 120 is position from y-axis
                  ui.drawString(originalImage, ui.arial_24, 100, 120, 'Think Different');


                  // Store the watermarked image to a File
                  List<int> wmImage = ui.encodePng(originalImage);
                  setState(() {
                    _watermarkedImage = File.fromRawPath(Uint8List.fromList(wmImage));
                  });

                },
              )
                  : Container(),

//<--------------- display watermarked image ---------------->
              _watermarkedImage!=null?
              Image.file(_watermarkedImage)
                  : Container(),


            ],
          ),
        ),
      ),
    );
  }

}