Detecting almost surrounded background areas [closed] - matlab

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I'm doing image processing on tribological samples. I'm segmenting the wear track from the images, but have a recurring problem:
Setting the threshold too low leads to a complete failure. Setting it too high (as in the image) leads to multiple areas that are almost but not quite enveloped by the label. They mess up the distance transform I'm using to estimate track width, and should be detected and merged to the label.
I have used morphological operations to improve the quality of the label, but do not wish to make the structuring elements bigger due to some side effects in the rest of the image. The curvature of the label prevents me from using convex hulls. The bulk of the label prevents me from using the solidity of the label as an indicator. The unwanted interior objects are not enveloped completely by the label, so they can not be detected via the Euler characteristic etc.
Is there any good way to detect background objects that are 'almost completely' surrounded by foreground objects?

Answer 1

I used watershed to divide the background into separate regions and then bwboundaries to detect how much of a region's boundary is shared with the foreground object:
% generate example image
bw = im2double(rgb2gray(imread('example.jpg'))) == 1;
% dilate binary object to overlap watershed boundaries
bwDil = imdilate(bw,ones(5));
% get watershed labels
D = bwdist(bw);
D = -D;
D(bw) = -Inf;
L = watershed(D);
% get binary regions and remove fg object
R = (L > 0);
R(bw) = 0;
% get boundaries of all regions
BR = bwboundaries(R);
% set boundary ratio - if a regio's shares more boundary with fg object
% than this threshold it considered surrounded
boundaryRatio = zeros(numel(BR),1);
ratioThresh = 0.6;
mask = false(size(bw));
% go through region boundaries and add them to mask if above tresh
for ii = 1:numel(BR)
ind = sub2ind(size(bw),BR{ii}(:,1),BR{ii}(:,2));
boundaryRatio(ii) = nnz(bwDil(ind))/numel(ind);
if boundaryRatio(ii) > ratioThresh
mask(ind) = 1;
end
end
% fill mask
mask = imfill(mask,4,'holes');
% plot
subplot(121);
imshow(bw);
title('fg')
rgb = double(cat(3,mask,bw,bw));
subplot(122);
imshow(rgb);
title('fg with surrounded bg')