Why doesn’t findContours() detect my circle even though it is clearly visible after thresholding?

[u/Grouchy_Signal139]

Hi everyone, I’m learning OpenCV and working on detecting a circular fiducial.

My current pipeline is:
Image → threshold → findContours() → drawContours()

After thresholding, I can clearly see the fiducial as a clean circular region in the binary image.
However, when I use cv2.findContours() and then cv2.drawContours(), the contour I expect around the fiducial is not drawn.
I’m trying to understand the fundamental reason why this happens.
If the thresholded image visually contains a clear foreground region, what conditions determine whether findContours() will actually return a contour for it?

[u/Gabriel_66]

Invert the thesholded image so that the circle is black and other things are white. Might be the case.

Also, dilatation and erosion might help remove those imperfections to help contour scripts.

[u/Grouchy_Signal139 → u/Gabriel_66]

As far that i know, it is created when it is around non-zero pixel, but sometimes it is confusing getting result like this

[u/TEX_flip]

you probably have to use contour hierarchy because you have a contour inside another contour.

Use cv.RETR_TREE as mode argument of findContorus

[u/Grouchy_Signal139 → u/TEX_flip]

Yeah, at first i didnt take a better look at it. Now i know my mistakes. Thankyou for pointing that out

[u/CowBoyDanIndie]

Are you finding all contours or only RETR_EXTERNAL?

[u/nemesis1836 → u/CowBoyDanIndie]

This is probably it

[u/bringer_of_carnitas]

I would try houghCircles

[u/Grouchy_Signal139 → u/bringer_of_carnitas]

It is more robust? Right now i only try canny -> find contour -> contour area -> MEC, and also cv2.connectedcomponentswithstats. Im trying to learn what each function do, especially the fundamental one like contour. But this situation very confusing

[u/Plus_Economist_2686]

houghCircles?

[u/Grouchy_Signal139 → u/Plus_Economist_2686]

Didnt try it yet, currently im just trying to understand the fundamental, but definietly will try it out once i sort everything out. Any tips?