How to find angle of corners detected?

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i want to find angles between the corners detected from a sketched rectangle, i have used harris corner detection and canny edge detection and also have drawn circles around the corners , can i used all of these together to find angles of each corner?

I want to find angles from this image for rectangle classification.

I have so far done this:

Here is a snippet of code (I have saved the vertices of the drawn circle and want to use it with detected edges from canny):

       cornerHarris( detected_edges, dst, blockSize, apertureSize, k, BORDER_DEFAULT );
       normalize( dst, dst_norm, 0, 255, NORM_MINMAX, CV_32FC1, Mat() );
       convertScaleAbs( dst_norm, dst_norm_scaled );

       /// Drawing a circle around corners
       for( int j = 0; j < dst_norm.rows ; j++ )
       {
         for( int i = 0; i < dst_norm.cols; i++ )
         {
           if( (int) dst_norm.at<float>(j,i) > thresh )
           {
             circle( dst_norm_scaled, Point( i, j ), 5,  Scalar(0), 2, 8, 0 );
         count++;
           }
         }
       }

       vector<Point>* circle_points;  
       circle_points= new vector<Point>[count];
       int pt=0;
       for( int j = 0; j < dst_norm.rows ; j++ )
       {
         for( int i = 0; i < dst_norm.cols; i++ )
         {
            if( (int) dst_norm.at<float>(j,i) > thresh )
            {
               ellipse2Poly( Point(i,j), axes, 0, 0, 360, 1, circle_points[pt] ); 
               pt++;
            }
         }
       }