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Try to detect what you see : a white blob enclosed with dark zone. It is not a real time method

1. convert to gray level (color is not helped ?)
2. basic otsu threshold
3. estimate mean gray level Iback of 0 pixel (0 in binary image)
4. dilate binary image (4 iterations ?)
5. estimate gray level I(b) of each blob but only enclosed region (dilated region)
6. select each blob with I(b)<0.9Iback

It's only an idea.