imfindcircles does not detect obvious circles

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?

Answer 1

[Jalaj Gambhir]

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!