MATLAB Answers

imfindcircles does not detect obvious circles

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Yannick Hauswirth
Yannick Hauswirth on 23 Sep 2020
Answered: Jalaj Gambhir on 1 Oct 2020
I try to use the function imfindcircles to detect circlesly in an image. To try it out I used the attached test image.
The used code can be seen here:
layer = imread("test1.png");
if size(layer,3)>1
layer = rgb2gray(layer); %if layer is a rgb image turn into grayscale
end
imshow(layer);
[centers, radii] = imfindcircles(layer,[10 2000],'ObjectPolarity','dark', 'Sensitivity',0.9)
viscircles(centers, radii,'Color','b');
I followed the principles of the following Mathworks guide:
In my opinion this should be super easy. There is a big, perfectly circular structure in the top of the shape. Still, the circle is not detected. I varried ObjectPolarity, Sensitivity, and the radii but nothing helped. Contrast also can't be the issue.
Is there something obvious that I'm missing why the circle is not detected?

Answers (1)

Jalaj Gambhir
Jalaj Gambhir on 1 Oct 2020
Hi,
I think it is probably because the circle you want to detect is almost a semi circle (like only half of the circle is visible). A workaround could be to increase sensitivity and min. radius that you want to detect.
You can try:
[centers, radii] = imfindcircles(layer,[80 2000],'ObjectPolarity','dark', 'Sensitivity',0.99);
viscircles(centers, radii,'Color','b');
This works fine.
Hope this helps!

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