Problem trying to access labeled portion of images (updated 7/5)

Question

Ryan 2012-7-3

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https://ww2.mathworks.cn/matlabcentral/answers/42667-problem-trying-to-access-labeled-portion-of-images-updated-7-5

unique(diff(unique(endpts_labeled)))
ans = 
     1
     2

ORIGINAL QUESTION:

I am getting a rare error running the following code on an image of 1px thick skeleton segments:

    V(V<=0) = 0; V(V>0) = 1; % Segments <=2 pixels are removed as well earlier
    [K n] = bwlabel(V,8);
    props = regionprops(K,'Perimeter');
    tort = zeros(1,n);
    endpts = bwmorph(K,'endpoints');
    endpts_labeled = immultiply(K,endpts);
    for m = 1:n
        [rows,cols] = find(endpts_labeled==m);
        p1 = [rows(1),cols(1)]; % Error happens here!!
        p2 = [rows(2),cols(2)];
        d = sqrt((p1(1)-p2(1))^2+(p1(2)-p2(2))^2);
        c = props(m,1).Perimeter/2;
        tort(m) = c/d;
    end

The error happens when the loop tries to access rows(1) when the numel(rows) = 0. That case should never happen though. This error happened one in every 3 or 4 runs previously when I was using a non-labeled version of the image to find the endpoints, but now that I am using the labeled image to find the endpoints, it is only happening once in a blue moon and I cannot figure out why.

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Ryan 2012-7-5

I have updated the question with the .mat data file. I did not add dbstop if error before this happened.

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Answer 1

Anton Semechko 2012-7-6

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编辑：Anton Semechko 2012-7-7

在 MATLAB Online 中打开

The error occurs for m=559. If you look at the object:

figure, imshow(K==559)
set(gca,'xlim',[481 580],'ylim',[500 600])

you will see that its a loop that does not have any end points. This is what ultimately causes the error.

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Ryan 2012-7-7

I will upload the image to show you on Monday at the soonest. There is a chance I suppose for loops because the skeleton is coming from an inf- thin with bwmorph, but I do a thicken and re-thin to eliminate them.

Anton Semechko 2012-7-7

编辑：Anton Semechko 2012-7-7

That will be a different question ... I believe this one can be closed

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Answer 2

Image Analyst 2012-7-4

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编辑：Image Analyst 2012-7-4

在 MATLAB Online 中打开

I don't know how this could happen. If rows is null, then endpts_labeled==m is completely false. And if that's true, then n must be zero and so the loop never gets entered. That is a contradiction. Try "dbstop if error" and then when it errors, see what n is. Alternatively you can try this (untested)

endpts_labeled = int32(zeros(size(K)));
endpts_labeled(endpts) = K(endpts);

instead of this:

endpts_labeled = immultiply(K,endpts);

in case the immultiply is giving you slight mismatches as referred to in the FAQ http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

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Ryan 2012-7-4

Will do. I'll update this next time it catches it.

Geoff 2012-7-4

Make sure you dump all your variables to a MAT file next time numel(rows) is zero =)

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Answer 3

Ryan 2012-7-6

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https://ww2.mathworks.cn/matlabcentral/answers/42667-problem-trying-to-access-labeled-portion-of-images-updated-7-5#answer_52757

编辑：Ryan 2012-7-6

I switched to .* over immultiply() and I haven't run into the error message again. This may not mean I am out of the woods yet, but for anyone else out there that may encounter a similar issue, I wanted the possible solution to be documented.