A mortise mask leaves the composition image exposed, and covers everything around it - like a stencil.
How This Is
Done --
Follow all the basics for making a common mask with these exceptions --
Cut toward, but not beyond the outside
edge of the image's outline... and
Discard the cut-out, but keep the "hole." The portion surrounding the "hole" is
the mask!
Place this over, or around the composition image.
