CODEWITHSUNDEEP
copencvcontour
Vincent Lu
Vincent Lu

Reputation: 1

OpenCV(C): calculating moments FROM contour

Here's the code I have:

#include "highgui.h"
#include "cv.h"  
#include <stdio.h>

// this code finds contours

int main (int argc, char* argv[]) {
  cvNamedWindow( argv[0], 1 );

  IplImage* img_8uc1 = cvLoadImage( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
  IplImage* img_edge = cvCreateImage( cvGetSize(img_8uc1), 8, 1);
  IplImage* img_8uc3 = cvCreateImage( cvGetSize(img_8uc1), 8, 3);

  cvThreshold( img_8uc1, img_edge, 128, 255, CV_THRESH_BINARY );

  CvMemStorage* storage = cvCreateMemStorage();
  CvSeq* first_contour = NULL;
  int Nc = cvFindContours(img_edge, storage, &first_contour,
              sizeof(CvContour), CV_RETR_LIST );

  int n = 0;
  printf("Total Contours Detected: %d\n", Nc);
  CvSeq* c=first_contour;
  for( c; c!=NULL; c=c->h_next ) {
    cvCvtColor( img_8uc1, img_8uc3, CV_GRAY2BGR );
    cvDrawContours(img_8uc3, c, 
           CV_RGB(250,0,0), //red
           CV_RGB(0,0,250), //blue
           0,2,8);
    printf("Contour #%d\n", n);
    cvShowImage( argv[0], img_8uc3 );
    printf(" %d elements:\n", c->total );
    for( int i=0; i<c->total; ++i) {
      CvPoint* p = CV_GET_SEQ_ELEM(CvPoint, c, i);
      printf("     (%d,%d)\n", p->x, p->y); 
    }
    cvWaitKey(0);
    n++;
  }


  // moments
  CvMoments* Moments;
  CvHuMoments *HuMoments;
  // calculate moments; calculate humoments
  cvContourMoments(c, Moments);
  cvGetHuMoments(Moments, HuMoments);

  //print the hu moments
  printf("Hu Moment hu1: %.12f", HuMoments->hu1);
  printf("Hu Moment hu2: %.12f", HuMoments->hu2);
  printf("Hu Moment hu3: %.12f", HuMoments->hu3);
  printf("Hu Moment hu4: %.12f", HuMoments->hu4);
  printf("Hu Moment hu5: %.12f", HuMoments->hu5);
  printf("Hu Moment hu6: %.12f", HuMoments->hu6);
  printf("Hu Moment hu7: %.12f", HuMoments->hu7);


  printf("Finished all contours.\n");
  cvCvtColor(img_8uc1, img_8uc3, CV_GRAY2BGR);
  cvShowImage(argv[0], img_8uc3);
  cvWaitKey(0);

  cvDestroyWindow(argv[0]);
  cvReleaseImage(&img_8uc1);
  cvReleaseImage(&img_8uc3);
  cvReleaseImage(&img_edge);

  return 0;
}

Basically, the first half of the code and the last lines of the code are taken directly from the "Learning OpenCV" book, since that does the contour finding already and it's what I want (not plagiarizing, I'm trying something out), and the middle section of the code (as documented) is dedicated to computing Hu Moments. I want to try to see if any moment values vary or not when a particular image with a contour, is oriented differently or when the size of the image changes.

I thought I wrote this the right way, at least the algorithm seemed right to me, but I keep getting this error message:

OpenCV Error: Null pointer () in cvMoments, file /build/buildd/opencv-2.1.0/src/cv/cvmoments.cpp, line 343 terminate called after throwing an instance of 'cv::Exception' what(): /build/buildd/opencv-2.1.0/src/cv/cvmoments.cpp:343: error: (-27) in function cvMoments

Aborted

Is there an issue of how I used the CvMoments data structure, or did the problem had to do with CvContourMoments() function? The version of OpenCV I used currently is 2.1.0.

I appreciate the help! i can't find solutions like this anywhere on the internet and I am wondering if anyone has a similar problem about this with moment calculations from contours.

Upvotes: 0

Views: 7295

Answers (3)

Mantale
Mantale

Reputation: 1

you forget to reset the c；

Try adding

c=first_contour；

before you use it in 

cvContourMoments(c, &Moments);；

Upvotes: 0

Knighk
Knighk

Reputation: 405

I think you missed to specify what image will be tested. I'm still a newbie also but you can try the Program that I've tested to get the contours (it works :) ) and from there you can add the function to get the moments of the contours. I hope this helps you.

Upvotes: 0

xiaofang
xiaofang

Reputation: 11

I think you should define a instant of struct instead of a pointer.

TRY this:

CvMoments Moments;
CvHuMoments HuMoments;
// calculate moments; calculate humoments
cvContourMoments(c, &Moments);
cvGetHuMoments(&Moments, &HuMoments);
//print the hu moments
printf("Hu Moment hu1: %.12f", HuMoments.hu1);
printf("Hu Moment hu2: %.12f", HuMoments.hu2);
printf("Hu Moment hu3: %.12f", HuMoments.hu3);
printf("Hu Moment hu4: %.12f", HuMoments.hu4);
printf("Hu Moment hu5: %.12f", HuMoments.hu5);
printf("Hu Moment hu6: %.12f", HuMoments.hu6);
printf("Hu Moment hu7: %.12f", HuMoments.hu7);

By the way, cvContourMoments() seems to be the same as cvMoments(). I can only find cvMoments() at http://www.opencv.org.cn/opencvdoc. And variable "c" should be a pointer to IplImage or CvArr.

I'm New. Wish to help.

Upvotes: 1

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