write text on an image xamarin android

Question

Hi dears,
I want to write text on an image then save it, but my code has runtime error.

error : System.PlatformNotSupportedException: 'Operation is not supported on this platform.'

using System.Drawing;


public void Watermark(string picturePath, string text)
        {

            PointF firstLocation = new PointF(10f, 10f);
            PointF secondLocation = new PointF(10f, 50f);

            Bitmap bitmap = (Bitmap)Image.FromFile(picturePath);//load the image file

            using (Graphics graphics = Graphics.FromImage(bitmap))
            {
                using (Font arialFont = new Font("Arial", 10))
                {
                    graphics.DrawString(text, arialFont, Brushes.Blue, firstLocation);
                }
            }

            bitmap.Save(picturePath);
        }

Answer 1

Hello,​

Welcome to our Microsoft Q&A platform!

I used the following code to write text to image. You can make a test(I do not save the image to output path, I set it to imageview directly).

   public class MainActivity : AppCompatActivity  
       {  
           Android.Graphics.Bitmap bitmap;  
           Android.Graphics.Bitmap mutableBitmap;  
           protected override void OnCreate(Bundle savedInstanceState)  
           {  
               base.OnCreate(savedInstanceState);  
               Xamarin.Essentials.Platform.Init(this, savedInstanceState);  
               // Set our view from the "main" layout resource  
               SetContentView(Resource.Layout.activity_main);  
               Watermark();  
               ImageView imageView1 = FindViewById<ImageView>(Resource.Id.imageView1);  
               imageView1.SetImageBitmap(mutableBitmap);  
           }  
            
           public void Watermark()  
           {  
               bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.hamburger); // Load your bitmap here  
     
     
              mutableBitmap = bitmap.Copy(Android.Graphics.Bitmap.Config.Argb8888, true);  
               Canvas canvas = new Canvas(mutableBitmap);  
     
              // Canvas canvas = new Canvas(bitmap);  
               Paint paint = new Paint();  
               paint.Color= Android.Graphics.Color.Black;  
               paint.TextSize=50;  
               canvas.DrawText("Some", 50, 50, paint);  
     
           }  

Here is running screenshot.

======================
Update========================

I need to save picture after watermark, Is there any way?

If you want to write picture to external path,

If you want to support Android 11 or later devices, t you can var sdCardPath = GetExternalFilesDir(string.Empty).Path; path

If you want to support Android 10 or before, you can use "/storage/emulated/0/Download/" this path, and add android:requestLegacyExternalStorage="true" to <application> tag in your AndroidManifest.xml

For these details, you can refer to the following code.

https://stackoverflow.com/questions/66490122/how-to-save-db-to-external-storage-so-user-can-see-it-in-xamarin-forms/66491973#66491973

Here is my test code.

   private void Button1_Click(object sender, System.EventArgs e)  
           {  
                
     
              var sdCardPath = GetExternalFilesDir(string.Empty).Path;  
               //"/storage/emulated/0/Download/"  
               var filePath = System.IO.Path.Combine(sdCardPath, "test.png");  
               var stream = new FileStream(filePath, FileMode.Create);  
               mutableBitmap.Compress(Android.Graphics.Bitmap.CompressFormat.Png, 100, stream);  
               stream.Close();  
           }  

Here is running screenshot.

for `` var sdCardPath = GetExternalFilesDir(string.Empty).Path;

for "/storage/emulated/0/Download/"

Best Regards,

Leon Lu

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