I try the sample WriteableBitmapEx.TextExample.csproj.
The text appears coarse, without Anti-Aliasing, either if the method signature used ends with Aa: bmp.DrawTextAa(formattedText, 0, 100, Colors.Black,7);
Is this a bug? Are you aware of it?
In the meantime: Do yo plan to add an angle to the text, like to be able to write on an angle of 90 degree left or right?

