Histogram and binary image in opencv?

Wednesday, November 29, 2017

Histogram and binary image in opencv?

I would like to compute how many zeros and ones (e.i. 255) in a binary image. The following code to generate a binary image by using Matlab.

I = imread('images.jpg');
level = graythresh(I);
BW = im2bw(I, level);
imshow(BW)
imwrite(BW, 'img.jpg');

This is my C++ code.

#include 
#include 

#include 
#include 
#include 

std::map computeHistogram(const cv::Mat& image)
{
    std::map histogram;

    for ( int row = 0; row < image.rows; ++row)
    {

        for ( int col = 0; col < image.cols; ++col)
        {
          ++histogram[(int)image.at(row, col)];

        }
    }

    return histogram;
}


void printHistogram(const std::map& histogram)
{
    std::map::const_iterator histogram_iter;
    std::cout << "\n------------------\n";
    for( histogram_iter = histogram.begin(); histogram_iter != histogram.end(); ++histogram_iter)
    {
      std::cout << std::setw(5) << histogram_iter->first <<  " : " << histogram_iter->second << "\n";
    }
    std::cout << "------------------\n";
}


int main(int argc, char **argv)
{
    cv::Mat img = cv::imread("img.jpg", CV_BGR2GRAY);
    printHistogram(computeHistogram(img));
    return 0;
}

This is what I got

------------------

    0 : 11451
    1 : 590
    2 : 602
    3 : 498
    4 : 428
    5 : 376
    6 : 387
    7 : 349
    8 : 314
    9 : 278

   10 : 229
   11 : 206
   12 : 212
   13 : 159
   14 : 142
   15 : 112
   16 : 106
   17 : 96
   18 : 86
   19 : 54

   20 : 44
   21 : 37
   22 : 26
   23 : 26
   24 : 14
   25 : 14
   26 : 10
   27 : 4
   28 : 7
   29 : 8

   30 : 2
   32 : 7
   33 : 2
   34 : 3
   36 : 2
   37 : 4
   38 : 1
  211 : 1
  216 : 1
  217 : 1

  219 : 4
  220 : 2
  221 : 2
  223 : 3
  224 : 5
  225 : 2
  226 : 4
  227 : 4
  228 : 5
  229 : 8

  230 : 15
  231 : 19
  232 : 22
  233 : 25
  234 : 36
  235 : 21
  236 : 54
  237 : 54
  238 : 64
  239 : 93

  240 : 113
  241 : 111
  242 : 127
  243 : 163
  244 : 193
  245 : 182
  246 : 202
  247 : 218
  248 : 263
  249 : 289

  250 : 291
  251 : 301
  252 : 403
  253 : 458
  254 : 469
  255 : 29211
------------------

Is this right? What I'm expecting is the number of 0s and 255s.

Answer

The answer is posted as a reply to Otsu's method problem? . So it is now a duplicate I guess.

Notes

Wednesday, November 29, 2017

Histogram and binary image in opencv?

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