Compute region outline by differencing two dilations.

        Parameters
        ----------
        inner : int
            Size of inner outline boundary (in pixels)

        outer : int
            Size of outer outline boundary (in pixels)

def outline(self, inner, outer):
        """
        Compute region outline by differencing two dilations.

        Parameters
        ----------
        inner : int
            Size of inner outline boundary (in pixels)

        outer : int
            Size of outer outline boundary (in pixels)
        """
        return self.dilate(outer).exclude(self.dilate(inner))