How to use OpenCV SimpleBlobDetector

Question

Instead of any additional blob detection library, how do I use the cv::SimpleBlobDetector class and its function detectblobs()?

Answer 1

Python: Reads image blob.jpg and performs blob detection with different parameters.

#!/usr/bin/python  # Standard imports import cv2 import numpy as np;  # Read image im = cv2.imread("blob.jpg")  # Setup SimpleBlobDetector parameters. params = cv2.SimpleBlobDetector_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  # Create a detector with the parameters # OLD: detector = cv2.SimpleBlobDetector(params) detector = cv2.SimpleBlobDetector_create(params)   # Detect blobs. keypoints = detector.detect(im)  # Draw detected blobs as red circles. # cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS ensures # the size of the circle corresponds to the size of blob  im_with_keypoints = cv2.drawKeypoints(im, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)  # Show blobs cv2.imshow("Keypoints", im_with_keypoints) cv2.waitKey(0) 

C++: Reads image blob.jpg and performs blob detection with different parameters.

#include "opencv2/opencv.hpp"  using namespace cv; using namespace std;  int main(int argc, char** argv) {     // Read image #if CV_MAJOR_VERSION < 3   // If you are using OpenCV 2     Mat im = imread("blob.jpg", CV_LOAD_IMAGE_GRAYSCALE); #else     Mat im = imread("blob.jpg", IMREAD_GRAYSCALE); #endif      // 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     std::vector<KeyPoint> keypoints;  #if CV_MAJOR_VERSION < 3   // If you are using OpenCV 2      // Set up detector with params     SimpleBlobDetector detector(params);      // Detect blobs     detector.detect(im, keypoints); #else       // Set up detector with params     Ptr<SimpleBlobDetector> detector = SimpleBlobDetector::create(params);      // Detect blobs     detector->detect(im, keypoints); #endif       // 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); } 

The answer has been copied from this tutorial I wrote at LearnOpenCV.com explaining various parameters of SimpleBlobDetector. You can find additional details about the parameters in the tutorial.