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Of course you can subtract two images using OpenCV. Something like this should work:

cv::Mat1f img1 = cv::imread("image1.png", cv::LOAD_IMAGE_GRAYSCALE);
cv::Mat1f img2 = cv::imread("image2.png", cv::LOAD_IMAGE_GRAYSCALE);

cv::Mat1f sub = img1 - img2; // Now all pixels != 0 are different pixels

From here on you can proceed further, some possible involved steps:

• blur the output map w. a GaussianBlur(), or use CLOSE as morphological Operator (morphologyEx) to close some of the holes
• use findContours() to find all contours.
• use minEnclosingCircle() to draw a circle around each found contour