【4Opencv】如何识别出轮廓准确的长和宽

int main( int argc, char** argv )
{
    //read the image
    Mat img = imread("e:/sandbox/leaf.jpg");
    Mat bw;
    bool dRet;
    //resize
    pyrDown(img,img);
    pyrDown(img,img);

    cvtColor(img, bw, COLOR_BGR2GRAY);
    //morphology operation  
    threshold(bw, bw, 150, 255, CV_THRESH_BINARY);
    //bitwise_not(bw,bw);
    //find and draw contours
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    findContours(bw, contours, hierarchy, CV_RETR_LIST, CV_CHAIN_APPROX_NONE);
    for (int i = 0;i<contours.size();i++)
    {
        RotatedRect minRect = minAreaRect( Mat(contours[i]) );
        Point2f rect_points[4];
        minRect.points( rect_points ); 
        for( int j = 0; j < 4; j++ )
            line( img, rect_points[j], rect_points[(j+1)%4],Scalar(255,255,0),2);
    }
    imshow("img",img);
    waitKey();
    return 0;
}

cv::RotatedRect cv::minAreaRect( InputArray _points )
{
    CV_INSTRUMENT_REGION()

    Mat hull;
    Point2f out[3];
    RotatedRect box;

    convexHull(_points, hull, true, true);

    if( hull.depth() != CV_32F )
    {
        Mat temp;
        hull.convertTo(temp, CV_32F);
        hull = temp;
    }

    int n = hull.checkVector(2);
    const Point2f* hpoints = hull.ptr<Point2f>();

    if( n > 2 )
    {
        rotatingCalipers( hpoints, n, CALIPERS_MINAREARECT, (float*)out );
        box.center.x = out[0].x + (out[1].x + out[2].x)*0.5f;
        box.center.y = out[0].y + (out[1].y + out[2].y)*0.5f;
        box.size.width = (float)std::sqrt((double)out[1].x*out[1].x + (double)out[1].y*out[1].y);
        box.size.height = (float)std::sqrt((double)out[2].x*out[2].x + (double)out[2].y*out[2].y);
        box.angle = (float)atan2( (double)out[1].y, (double)out[1].x );
    }
    else if( n == 2 )
    {
        box.center.x = (hpoints[0].x + hpoints[1].x)*0.5f;
        box.center.y = (hpoints[0].y + hpoints[1].y)*0.5f;
        double dx = hpoints[1].x - hpoints[0].x;
        double dy = hpoints[1].y - hpoints[0].y;
        box.size.width = (float)std::sqrt(dx*dx + dy*dy);
        box.size.height = 0;
        box.angle = (float)atan2( dy, dx );
    }
    else
    {
        if( n == 1 )
            box.center = hpoints[0];
    }

    box.angle = (float)(box.angle*180/CV_PI);
    return box;
}