You know what the equation for drawing a 2D circle is, so with that knowledge you would calculate a distance from what you suspect in your image is a circle, and a perfect circle.

You should first calculate the bounding box for the shape you suspect is a circle, then calculate the values of a prefect circle with the same bounding box. Calculate the distance for each pixel between the shape and the perfect circle. If the values on average are less than a certain threshold, you have recognized a circle, otherwise it's a different shape.

You should first calculate the bounding box for the shape you suspect is a circle, then calculate the values of a prefect circle with the same bounding box. Calculate the distance for each pixel between the shape and the perfect circle. If the values on average are less than a certain threshold, you have recognized a circle, otherwise it's a different shape.