# ROI of a ring area?

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xi on 18 Jan 2021
Commented: Matt J on 19 Jan 2021
How do I define an ROI of a ring area, i.e, a big cirle and small cirle inside, and I want to select the area in between.
Do I have to define two ROIs first and name ROI3 = ROI1 -ROI2?

Matt J on 19 Jan 2021
For example,
[x,y]=deal(1:1000);
dsq=(x-400).^2 + (y(:)-300).^2;
ROI = (50^2<=dsq & dsq<=100^2 );
imshow(ROI)
xi on 19 Jan 2021
Thanks for your reply. However, what I'm looking after is an interactive ROI definition and managment tool. Users can create an ROI object by drawing ROI on top of an image using the 'freehand', 'polygon','circular', 'rectangle'... What's common is that, they all define a sets of edge points and close up to form an enclosed region, but what if I want to define a ring shape ROI object? Not necessarily two circles, can be any shape of a connected region with a hollow area in the middle.
Matt J on 19 Jan 2021
You will probably have to build that out of two separate ROIs.