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You need to do,

See below code,

Mat src=imread("a.png",1);
Mat dst;//(src.rows,src.cols,CV_8UC4);
Mat tmp,thr;

cvtColor(src,tmp,CV_BGR2GRAY);
threshold(tmp,thr,50,255,THRESH_BINARY);

 vector< vector <Point> > contours; // Vector for storing contour
     vector< Vec4i > hierarchy;
     int largest_contour_index=0;
     int largest_area=0;

Mat alpha(src.size(),CV_8UC1,Scalar(0));
findContours( tmp, contours, hierarchy,CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image
    for( int i = 0; i< contours.size(); i++ ) // iterate through each contour.
      {
       double a=contourArea( contours[i],false);  //  Find the area of contour
       if(a>largest_area){
       largest_area=a;
       largest_contour_index=i;                //Store the index of largest contour
       }
      }
drawContours( alpha,contours, largest_contour_index, Scalar(255),CV_FILLED, 8, hierarchy );

Mat rgb[3];
split(src,rgb);

Mat rgba[4]={rgb[0],rgb[1],rgb[2],alpha};
merge(rgba,4,dst);
imwrite("dst.png",dst);

Note:

To keep the edge smooth check out below links,

http://stackoverflow.com/questions/21795643/image-edge-smoothing-with-opencv

http://stackoverflow.com/questions/17161088/how-to-refine-or-blur-or-smooth-just-the-edges

image description

You need to do,

See below code,

Mat src=imread("a.png",1);
Mat dst;//(src.rows,src.cols,CV_8UC4);
Mat tmp,thr;

cvtColor(src,tmp,CV_BGR2GRAY);
threshold(tmp,thr,50,255,THRESH_BINARY);
threshold(tmp,thr,100,255,THRESH_BINARY);

 vector< vector <Point> > contours; // Vector for storing contour
     vector< Vec4i > hierarchy;
     int largest_contour_index=0;
     int largest_area=0;

Mat alpha(src.size(),CV_8UC1,Scalar(0));
findContours( tmp, contours, hierarchy,CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image
    for( int i = 0; i< contours.size(); i++ ) // iterate through each contour.
      {
       double a=contourArea( contours[i],false);  //  Find the area of contour
       if(a>largest_area){
       largest_area=a;
       largest_contour_index=i;                //Store the index of largest contour
       }
      }
drawContours( alpha,contours, largest_contour_index, Scalar(255),CV_FILLED, 8, hierarchy );

Mat rgb[3];
split(src,rgb);

Mat rgba[4]={rgb[0],rgb[1],rgb[2],alpha};
merge(rgba,4,dst);
imwrite("dst.png",dst);

Note:

To keep the edge smooth check out below links,

http://stackoverflow.com/questions/21795643/image-edge-smoothing-with-opencv

http://stackoverflow.com/questions/17161088/how-to-refine-or-blur-or-smooth-just-the-edges

image description

image description