Identify the white section of a binary image

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Detect contours. Cycle through contours and build rectangle, which contour fit.

Use some CV lib for this task, for example OpenCV (highly recommeded). You will need do something like that:

// contours of targets
std::vector<std::vector<cv::Point> > contours;

// get contours from binary image
cv::findContours(binImg,contours,CV_RETR_TREE,CV_CHAIN_APPROX_SIMPLE);

// poly contours
std::vector<std::vector<cv::Point> > contours_poly(contours.size());

// bounded rectangles
std::vector<cv::Rect> boundRect(contours.size());

for (int i=0; i<contours.size(); i++) {
    // get polyDPcontour and bound rectangle of it
        cv::approxPolyDP(cv::Mat(contours[i]), contours_poly[i], 3, true);
        boundRect[i] = cv::boundingRect(cv::Mat(contours_poly[i]));
}