SimpleBlobDetector exception

Question

I'm trying to write a code for Blob detection and I'm following the tutorial here https://www.learnopencv.com/blob-detection-using-opencv-python-c/ I've just copied and pasted the code

Mat im = imread("blob.jpg", IMREAD_GRAYSCALE);

// Set up the detector with default parameters.
SimpleBlobDetector detector;

// Detect blobs.
std::vector<KeyPoint> keypoints;
detector.detect(im, keypoints);

// Draw detected blobs as red circles.
// DrawMatchesFlags::DRAW_RICH_KEYPOINTS flag ensures the size of the circle corresponds to the size of blob
Mat im_with_keypoints;
drawKeypoints(im, keypoints, im_with_keypoints, Scalar(0, 0, 255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS);

// Show blobs
imshow("keypoints", im_with_keypoints);

when I run it I get an error at

drawKeypoints(im, keypoints, im_with_keypoints, Scalar(0, 0, 255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS);

saying "unhandled exception", : Microsoft C++: cv::Exception

How can I solve this?

Answer 1

Ok after going through the tutorial without coding, I've realized that the code that is posted at the beginning of the article does not specifiy the parameters for the detector. After specifying them the program now works fine. Here's the full code:

#include "opencv2\opencv.hpp"

using namespace cv;

``int main() {
//LOAD THE IMAGE
Mat im = imread("../data/blob.jpg", IMREAD_GRAYSCALE);

// Setup SimpleBlobDetector parameters.
SimpleBlobDetector::Params params;

// Change thresholds
params.minThreshold = 10;
params.maxThreshold = 200;

// Filter by Area.
params.filterByArea = true;
params.minArea = 1500;

// Filter by Circularity
params.filterByCircularity = true;
params.minCircularity = 0.1;

// Filter by Convexity
params.filterByConvexity = true;
params.minConvexity = 0.87;

// Filter by Inertia
params.filterByInertia = true;
params.minInertiaRatio = 0.01;


// Storage for blobs
vector<KeyPoint> keypoints;
Ptr<SimpleBlobDetector> detector = SimpleBlobDetector::create(params);

// Detect blobs
detector->detect(im, keypoints);

// Draw detected blobs as red circles.
// DrawMatchesFlags::DRAW_RICH_KEYPOINTS flag ensures
// the size of the circle corresponds to the size of blob

Mat im_with_keypoints;
drawKeypoints(im, keypoints, im_with_keypoints, Scalar(0, 0, 255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS);

// Show blobs
imshow("keypoints", im_with_keypoints);
waitKey(0);
return 0;
}