Don't read my rectangle

edit

I adapted this code to read the white rectangle with the word 23-1965

But, it does not detect it sees a lot of other side dishes, but not what interests me. Who tells me where I'm wrong? Also attach the image to be processed

/**
 * Simple shape detector program.
 * It loads an image and tries to find simple shapes (rectangle, triangle, circle, etc) in it.
 * This program is a modified version of `squares.cpp` found in the OpenCV sample dir.
 */
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <cmath>
#include <iostream>

using namespace std;

/**
 * Helper function to find a cosine of angle between vectors
 * from pt0->pt1 and pt0->pt2
 */
static double angle(cv::Point pt1, cv::Point pt2, cv::Point pt0)
{
    double dx1 = pt1.x - pt0.x;
    double dy1 = pt1.y - pt0.y;
    double dx2 = pt2.x - pt0.x;
    double dy2 = pt2.y - pt0.y;
    return (dx1*dx2 + dy1*dy2)/sqrt((dx1*dx1 + dy1*dy1)*(dx2*dx2 + dy2*dy2) + 1e-10);
}

/**
 * Helper function to display text in the center of a contour
 */
void setLabel(cv::Mat& im, const std::string label, std::vector<cv::Point>& contour)
{
    int fontface = cv::FONT_HERSHEY_SIMPLEX;
    double scale = 0.4;
    int thickness = 1;
    int baseline = 0;

    cv::Size text = cv::getTextSize(label, fontface, scale, thickness, &baseline);
    cv::Rect r = cv::boundingRect(contour);

    cv::Point pt(r.x + ((r.width - text.width) / 2), r.y + ((r.height + text.height) / 2));
    cv::rectangle(im, pt + cv::Point(0, baseline), pt + cv::Point(text.width, -text.height), CV_RGB(255,255,255), CV_FILLED);
    cv::putText(im, label, pt, fontface, scale, CV_RGB(0,0,0), thickness, 8);
}

int main(int argc, char** argv)
{
    //cv::Mat src = cv::imread("polygon.png");
    cv::Mat src = cv::imread(argv[1]);
    if (src.empty())
        return -1;

    // Converti in scala di grigi
    cv::Mat gray;
    cv::cvtColor(src, gray, CV_BGR2GRAY);

    // Utilizzare Canny invece di soglia per la cattura di piazze con ombreggiatura gradiente
    cv::Mat bw;
    cv::Canny(gray, bw, 0, 50, 5);

    // Trova contorni
    std::vector<std::vector<cv::Point> > contours;
    cv::findContours(bw.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);   

    std::vector<cv::Point> approx;
    cv::Mat dst = src.clone();


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

        if (contours[l].size() == 4 /*&& contours[l].size() <= 6*/)
        {

                //cout << "# of contour points: " << contours[l].size() << endl ;
            cout << endl ;
                for(unsigned int k=0;k<contours[l].size();k++)
                {
                //cout << "Point(x,y)=" << contours[l][k] << endl;
                cout << contours[l][k] << ";";
                }
                //cout << " Area: " << contourArea(contours[l]) << endl;
        }
    }

    //cv::imshow("src", src);
    //cv::imshow("dst", dst);
    cv::waitKey(0);
    return 0;
}