How to detect circular erosion/dilation

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Hi,

I would like to detect circular erosions and dilations on a line. For dilations, I tried to recursively erode the image and on every recursion, I check width/height aspect ratio. If the ratio is smaller than 4, I assume that it the contour is circular and for each such contour I calculate circle center and radius from moments and area. This is the function that detects circular dilations:

def detect_circular_dilations(img, contours):
    contours_current, hierarchy = cv2.findContours(img, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
    if len(contours_current) == 0:
        return get_circles_from_contours(contours)
    for c in contours_current:
        x, y, w, h = cv2.boundingRect(c)
        if w > h:
            aspect_ratio = float(w) / h
        else:
            aspect_ratio = float(h) / w
        if aspect_ratio < 4 and w < 20 and h < 20 and w > 5 and h > 5:
            contours.append(c)
    return detect_circular_dilations(cv2.erode(img, None, iterations=1), contours)

An example of circular dilation that I want to detect is the following:

Another problem that I haven't solve is the detection of circular erosions. An example of circular erosion is the following:

Here I've marked the circular erosion I would like to detect with red rectangle. There might be some smaller circular patterns (on the left) that shouldn't be treated as actual circular erosion.

Does anyone know what is the best way to detect such circular shapes? For circular dilations, I would appreciate any comment/suggestion in order to potentially make detection more robust.

Thank you!