transform mask from circle to ring

hello all,

i have a number of ROIs made with roipoly and i want to transform them to a ROI ring shape when i can set the thickness of the ring manually, the i dea is to keep the original ROI make a small dupliicate and place it inside and get the ring shape, if someone can help with that or have a better way to do it. image modified to clarify the final ROIs wanted.

regards

Answer 1

[Image Analyst]

Once you have the first mask, you can erode it by some then erase that from the original:

radius = 2; % Thickness of ring.
se = strel('disk', radius, 0);
smallerMask = imerode(mask, se);
% Erase it from original.
mask = mask & ~smallerMask;
% Or alternative way to do it that gives the same result:
% mask(smallerMask) = false;

If the thickness is known to be just one pixel, then you can use bwperim()

ringMask = bwperim(mask);

[abdelillah douhi]

thank you very much @Image Analyst works like a charm