How to recognize elips like a circle

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Not quite sure what you mean by this question, are you wanting to detect that black shape in the middle? And do you mean detecting shapes that aren't quite circular?

i want to need detect black shape in the middle which way to follow ?

if it is all the time black as it appears in the picture, then colour segmentation is the simplest way to go.

Look at this tutorial.

I think in this case cv::threshold, cv::findContours and cv::fitEllipse is your friend.

For choosing right contours use cv::areaContour and cv::arcLength to get circularity for example.