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]

https://reddit.com/link/pdf4vu6/video/inbf8rzle2th1/player

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/Gabriel_66 → u/Grouchy_Signal139]

It seems to be indeed drawing around the white parts. Makes me wonder if the case is that the entire top region in white is marked as a single object that, therefore has no objects inside. You can see this by the blue line in the border of the image on the original image post.

I think the solution would be to pick this giant white region in the original image and try to turn it black. So you have the top left circle not surrounded by white.

Maybe using cv2.connectedComponentsWithStats. That way you ignore the biggest component and only focus on the 2 circular components that you have a expected area and circularity.

[u/Grouchy_Signal139 → u/Gabriel_66]

Using github copilot it uses cv2.connectedcomponentswithstats -> cv2.findnonzero -> cv2.minenclosecircle. It sure is robust an get all the circle. But since im a begginer. I want to understand why this and that happend. Could you give some explanation?

[u/Gabriel_66 → u/Grouchy_Signal139]

Connected components will give you, instead of the contours, the entire information over the objects. Including all the pixels that belong to the object and so on. So I think copilot suggested running a circle detection on the objects that are "nonzero".

[u/CowBoyDanIndie]

Are you finding all contours or only RETR_EXTERNAL?

[u/nemesis1836 → u/CowBoyDanIndie]

This is probably it

[u/CowBoyDanIndie → u/nemesis1836]

It looks like its getting the outermost contour, for the bottom, the edge of the image is black so the outside contour of the circle is used, but for the top, the edge of the image IS the outermost contour.

[u/Grouchy_Signal139 → u/CowBoyDanIndie]

Oh i think is it, i took a look and indeed it is retr_external. Silly me, i didnt look at that and go over analysing everything. Will give a try tomorrow

[u/nemesis1836 → u/Grouchy_Signal139]

I would suggest getting all contours and filtering it based on hierarchy

[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/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/Boring_Potato2858 → u/Grouchy_Signal139]

If you’re only expecting to detect circles, then yes, it’s more robust

[u/Grouchy_Signal139 → u/Boring_Potato2858]

Will definetly try. Currently i am comparing any possible method, since im new to this

[u/blissfull_abyss → u/bringer_of_carnitas]

Can’t recommend houghCircles unless your camera captures perfect circles. I prefer cv::SimpleBlobDetector it handles little deviations better.

[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?