How to find rectangle in black region opencv (C++) - c++

Question

I'm newbie OpenCV. I am using opencv for find position attach a stamp in image. The stamp can't overlap with other object in image.
Example binary image with white region is object on image, black region is none object
Example image
Result : Find rect in black region (in white circle) can attach stamp
Result imagge
Please help me find rect in black region same blue rect.
Thank you!

Answer 1

If you know about rectangles count then it can use kmeans for clusterization points.
First get only blue points and binarization they:
cv::Mat img = cv::imread("NQdmi.png", cv::IMREAD_COLOR);
std::vector<cv::Mat> chans;
cv::split(img, chans);
cv::Mat diff;
cv::absdiff(chans[2], chans[1], diff);
cv::threshold(diff, diff, 1, 255, cv::THRESH_BINARY);
cv::imshow("diff", diff);
Only blue points:
Clusterization points and find rotated rectangles:
std::vector<cv::Point2f> points;
for (int y = 0; y < diff.rows; ++y)
{
for (int x = 0; x < diff.cols; ++x)
{
if (diff.at<uchar>(y, x))
{
points.emplace_back(x, y);
}
}
}
cv::Mat pointsKmeans(points.size(), 1, CV_32FC2, &points[0]);
cv::Mat labels;
int clusterCount = 2;
cv::Mat centers;
cv::kmeans(pointsKmeans, clusterCount, labels,
cv::TermCriteria(cv::TermCriteria::EPS+cv::TermCriteria::COUNT, 100, 1.0),
3, cv::KMEANS_PP_CENTERS, centers);
std::vector<cv::Point2f> points1;
std::vector<cv::Point2f> points2;
cv::Mat draw = img.clone();
for (size_t i = 0; i < points.size(); ++i)
{
int clusterIdx = labels.at<int>(i);
if (clusterIdx > 0)
{
cv::circle(draw, points[i], 2, cv::Scalar(255, 0, 0), cv::FILLED, cv::LINE_AA);
points1.push_back(points[i]);
}
else
{
cv::circle(draw, points[i], 2, cv::Scalar(0, 0, 255), cv::FILLED, cv::LINE_AA);
points2.push_back(points[i]);
}
}
auto DrawRRect = [draw](const std::vector<cv::Point2f>& pp)
{
cv::RotatedRect rr = cv::minAreaRect(pp);
cv::Point2f corners[4];
rr.points(corners);
cv::line(draw, corners[0], corners[1], cv::Scalar(0, 255, 0), 2);
cv::line(draw, corners[1], corners[2], cv::Scalar(0, 255, 0), 2);
cv::line(draw, corners[2], corners[3], cv::Scalar(0, 255, 0), 2);
cv::line(draw, corners[3], corners[0], cv::Scalar(0, 255, 0), 2);
};
DrawRRect(points1);
DrawRRect(points2);
cv::imshow("draw", draw);
Result: