create thumbnail of large BitmapImage (Android/Xamarin)

ThumbnailBitmap.cs

public Bitmap GetThumbnail(Uri uri, int thumbnailSize)
  {
      var input = this.ContentResolver.OpenInputStream(uri);

      BitmapFactory.Options onlyBoundsOptions = new BitmapFactory.Options();
      onlyBoundsOptions.InJustDecodeBounds = true;
      onlyBoundsOptions.InDither = true; //optional
      onlyBoundsOptions.InPreferredConfig = Bitmap.Config.Argb8888; //optional
      BitmapFactory.DecodeStream(input, null, onlyBoundsOptions);
      input.Close();
      
      if ((onlyBoundsOptions.OutWidth == -1) || (onlyBoundsOptions.OutHeight == -1))
          return null;

      int originalSize = (onlyBoundsOptions.OutHeight > onlyBoundsOptions.OutWidth)
          ? onlyBoundsOptions.OutHeight
          : onlyBoundsOptions.OutWidth;

      double ratio = (originalSize > thumbnailSize) ? (originalSize / thumbnailSize) : 1.0;

      BitmapFactory.Options bitmapOptions = new BitmapFactory.Options();
      bitmapOptions.InSampleSize = GetPowerOfTwoForSampleRatio(ratio);
      bitmapOptions.InDither = true; //optional
      bitmapOptions.InPreferredConfig = Bitmap.Config.Argb8888; //optional
      input = this.ContentResolver.OpenInputStream(uri);
      Bitmap bitmap = BitmapFactory.DecodeStream(input, null, bitmapOptions);
      input.Close();
      return bitmap;
  }

  private static int GetPowerOfTwoForSampleRatio(double ratio)
  {
      int k = Integer.HighestOneBit((int)Math.Floor(ratio));
      if (k == 0) return 1;
      else return k;
  }

from: http://stackoverflow.com/questions/3879992/get-bitmap-from-an-uri-android