# Is it possible to erode only the outermost layer of a circle?

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Teshan Rezel on 31 May 2022
Commented: Teshan Rezel on 2 Jun 2022
Hi folks,
I am looking to erode a circle from the outside in, layer by layer. So far, I have tried to use the following:
x = imerode(x, strel('disk', 1));
but in the case of an image with a higher Euler number than 1, this erodes the holes within the image as well...Is there a way around this please?
##### 2 CommentsShowHide 1 older comment
Teshan Rezel on 31 May 2022
@Walter Roberson Unfortunately, the holes are an important part of this exercise. Essentially, I'm trying to model a physical process where something is "eaten" from the outside in (this is the nearest analogy I can find, apologies if it doesn't make sense!)

Image Analyst on 31 May 2022
Another option would be to use bwperim and imfill. Something like (untested)
% Fill holes
% Get outer perimeter.
% Set those pixels to false/0/black.
Teshan Rezel on 2 Jun 2022
This is brilliant, thank you!

Image Analyst on 31 May 2022
Here are the steps I'd try, assuming you're starting with a binary image
Call bwboundaries to get the outer layer. Of course, tell it to ignore interior holes - there's an option for that.
Have a for loop where you visit every (x,y) location and set it's value to false.
Something like (untested)
bxy = boundaries{1};
rows = bxy(:, 1);
cols = bxy(:, 2);
for k = 1 : length(rows)
end

Walter Roberson on 31 May 2022
negate the image so the holes and background are true and the white is false. bwareafilt requesting the largest. That will be the background. negate the background to get the filled foreground. Subtract the original image from the filled one to get the location of the holes. Erode the filled image. Subtract the holes.
You might have to filter differently if you have multiple disconnected objects.

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