Blob detection

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Hello all,

I'm trying to detect the blob in the following image (the one on the bottom left) :

I used the cv::SimpleBlobDetector detector to detect blob in other previous image as here (after inverting the color so the square is actually black) :

And it worked fine.

However with the first image, the code doesn't return any keypoints. Here is the said code :

cv::SimpleBlobDetector::Params params;
params.filterByCircularity = false;
params.filterByArea = true;
params.minArea = 1;
cv::SimpleBlobDetector detector(params);
std::vector<cv::KeyPoint> keypoints;
detector.detect( roi, keypoints);

std::cout << "print keypoints" << std::endl;
for(size_t i = 0 ; i < keypoints.size() ; i++){
    std::cout << "keypoint : " << keypoints[i].pt << std::endl;
}
std::cout << std::endl;
return true;

I've seen this page and I know that by default the blob detector is tuned to detect black circular blobs. Could it be the reason for the failing ?

ANSWER :

Sorry I can't post an answer for now because I'm too new :

One need to set those parameters :

cv::SimpleBlobDetector::Params params;
params.filterByConvexity = false;
params.filterByInertia = false;
params.filterByCircularity = false;
params.filterByArea = true;
params.minArea = 1;