C++ OpenCV Eliminate smaller Contours

Question

I am developing a OpenCV project.

I am currently working on detecting contours of particular ROI (Regoin Of Interest). What I want to achieve is to eliminate all the smaller contours in other words I do not want these smaller contours to be drown at all.

So Far if I have coded this algorithm to do this job:

CODE:

vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    findContours(mBlur, contours, hierarchy, CV_RETR_EXTERNAL,  CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
    //----------------------------------------------------------------------------->
    //Contours Vectors
    vector<vector<Point> > contours_poly(contours.size());
    vector<Rect> boundRect (contours.size());
    vector<Point2f> ContArea(contours.size());
    Mat drawing = Mat::zeros( range_out.size(), CV_8UC3 );
    //----------------------------------------------------------------------------->
    //Detecting Contours
    for( int i = 0; i < contours.size(); i++ )
     {  

        ContArea.clear();
        ContArea.push_back(Point2f(boundRect[i].x, boundRect[i].y));
        ContArea.push_back(Point2f(boundRect[i].x + boundRect[i].width, boundRect[i].y));
        ContArea.push_back(Point2f(boundRect[i].x + boundRect[i].width, boundRect[i].y + boundRect[i].height));
        ContArea.push_back(Point2f(boundRect[i].x, boundRect[i].y + boundRect[i].height));

        double area = contourArea(ContArea);

        if(area > 2000)
        {
           approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
           boundRect[i] = boundingRect( Mat(contours_poly[i]));


           cout<<"The area of Contour: "<<i<< " is: " <<area<<endl;

        }
     }



    /// Draw polygonal contour + bonding rects


   //////////////////////////////////////////////////////////////////////////////////

    for( int i = 0; i< contours.size(); i++ )
    {

        Scalar color = Scalar(255,255,255);
        drawContours( drawing, contours_poly, i, color, 1, 8, vector<Vec4i>(), 0, Point() );
        fillPoly(drawing, contours, Scalar(255,0,0));

    }

The problem here is that it looks like the if(area > 2000) statement is not executed even dough i know some of areas present in image are way bigger than this.

I have been trying a lot of different solutions but this looks to be the most appropriate one to me.

THE KEY QUESTIONS:

Is it Possible to achieve what I want with the given code....? If so can anyone see where I am going wrong with this Else Could someone suggest some kind of solution or a good online source....?

Answer 1

If you want to remove based on the size of ROIs, you can do as follow (based on the example of bounding box of opencv ):

vector< vector< Point > > contours_poly( contours.size() );
vector< Rect > boundRect (contours.size() );
vector< Point2f > centeres ( contours.size() );
vector< float > radiuses ( contours.size() );

// finding the approximate rectangle and circle of contours
for( int i = 0; i < contours.size(); i++ )
  {
    approxPolyDP( Mat ( contours[i] ), contours_poly[i], 3, true );
    boundRect[i] = boundingRect( Mat ( contours_poly[i] ) );
    minEnclosingCircle( ( Mat ) contours_poly[i], centeres[i], radiuses[i] );
  }

The above code snippet gives you the approximate rectangles of contours ( boundRect ), and approximate circles ( centers , radiuses ). After this you must be able to call contourArea(); and if it's smaller than certain threshold, you can eliminate it.

---

If you just want to remove contours that are smaller certain length, you can calculate it's length, look at the answer to similar question , and it seems you can use arcLength() function. I think something like this: double perimeter = arcLength ( Mat ( contours[i] ), true ); .