http://www.mehdiplugins.com/english/finethreshold.htm

Here is another method:
When you install Xreme or Pro, some Mehdi plugins are installed automatically. The Fine Threshold is not, but I believe is free so download and try it.

1. Apply it in Xara to a copy of your photo
2. Do an Arrange > Create Bitmap to create a mask which can then
3. be applied to your original photo via the BitMap Gallery (Apply Transparency).