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

User · Answer

You may store the parameters for the blob detector in a file  but this is not necessary   Example      set up the parameters  check the defaults in opencv s code in blobdetector cpp  cv  SimpleBlobDetector  Params params  params minDistBetweenBlobs   50 0f  params filterByInertia   false  params filterByConvexity   false  params filterByColor   false  params filterByCircularity   false  params filterByArea   true  params minArea   20 0f  params maxArea   500 0f         any other params you don t want default value     set up and create the detector using the parameters cv  SimpleBlobDetector blob detector params      or cv  Ptr lt cv  SimpleBlobDetector gt  detector   cv  SimpleBlobDetector  create params      detect  vector lt cv  KeyPoint gt  keypoints  blob detector detect image  keypoints       extract the x y coordinates of the keypoints    for  int i 0  i lt keypoints size    i         float X   keypoints i  pt x       float Y   keypoints i  pt y

User · Answer

Note  all the examples here are using the OpenCV 2 X API   In OpenCV 3 X  you need to use   Ptr lt SimpleBlobDetector gt  d   SimpleBlobDetector  create params     See also  the transition guide  http   docs opencv org master db dfa tutorial transition guide html tutorial transition hints headers

User · Answer

creation              cv  SimpleBlobDetector   blob detector               blob detector   new SimpleBlobDetector                 blob detector- gt create  SimpleBlobDetector        change params - first move it to public                blob detector- gt params filterByArea   true               blob detector- gt params minArea   1               blob detector- gt params maxArea   32000      or read   write them with file              FileStorage fs  test fs yml   FileStorage  WRITE                FileNode fn   fs  features                    blob detector- gt read fn       detect              vector lt KeyPoint gt  keypoints               blob detector- gt detect img text  keypoints                fs release       I do know why  but params are protected  So I moved it in file features2d hpp to be public      virtual void read  const FileNode amp  fn       virtual void write  FileStorage amp  fs   const    public   Params params       protected   struct CV EXPORTS Center             Point2d loc    If you will not do this  the only way to change params is to create file  FileStorage fs  test fs yml   FileStorage  WRITE     than open it in notepad  and edit  Or maybe there is another way  but I m not aware of it

User · Answer

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 detector   cv2 SimpleBlobDetector 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  lt  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 lt KeyPoint gt  keypoints    if CV MAJOR VERSION  lt  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 lt SimpleBlobDetector gt  detector   SimpleBlobDetector  create params           Detect blobs     detector- gt 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

[opencv] How to use OpenCV SimpleBlobDetector

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