generating a mask that has in,out and on the boundary values as specified by me

Hi

I wanted to generate a second mask that has in,out and on-the-boundary value for the mask that i pass in. The mask i pass in is an uint8 image with minimum 0 and maximum 255, where the region of interest marked in 255.

The mask passed in is attached.

Ideally, I would want the second mask to have values inside the region of interest at -25, values outside the region of interest at 25 and on the boundary at 0. Right now I don't even know how to mark the boundary of the ROI.

thanks