imfindcircles does not detect obvious circles

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Yannick Hauswirth
Yannick Hauswirth 2020-9-23
回答： Jalaj Gambhir 2020-10-1
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:
https://ch.mathworks.com/help/images/detect-and-measure-circular-objects-in-an-image.html
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?

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回答（1 个）

Jalaj Gambhir
Jalaj Gambhir 2020-10-1
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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