Strange behavior displaying matrix opencv

Question

I'm trying to implement the Bayer Color Filter Array in OpenCV!

Here's the code:

#include <cv.h>
#include <highgui.h>
#include <iostream>

using namespace cv;
using std::cout;
using std::endl;

int main( int argc, char** argv )
{
  int rows = 4, cols = 4;

  cv::Mat_<uchar> mask(rows, cols);

  cv::Mat_<uchar> pattern(1, cols);

  for (int i = 0; i < cols; i = i + 2) pattern(0, i) = 1;

  for (int j = 0; j < rows; j = j + 2) pattern.row(0).copyTo(mask.row(j));

  cout << mask << endl;

  namedWindow( "Result" );
  imshow( "Result", mask );

  waitKey(0);

  return 0;
}

The problem is when I comment the lines 23 to 26, the output in the statement of the line 21 is as I expect.

[1, 0, 1, 0;
  0, 0, 0, 0;
  1, 0, 1, 0;
  0, 0, 0, 0]

but when the lines are uncommented, the output become something like that:

[1, 100, 1, 48;
  47, 117, 115, 98;
  1, 100, 1, 48;
  49, 47, 50, 45]

I don't understand what's wrong. I'm printing the values before the commented lines, but somehow it seems they are affecting the matrix from the start.

Answer 1

you are using uninitialized values. Without the namedWindow and imshow stuff you were just lucky.

Try:

using namespace cv;
using std::cout;
using std::endl;

int main( int argc, char** argv )
{
  int rows = 4, cols = 4;

  cv::Mat_<uchar> mask(rows, cols);

  cv::Mat_<uchar> pattern(1, cols);

  // ------------------------------------------
  // initialize your matrices:
  for(int j=0; j<rows; ++j)
      for(int i=0; i<cols; ++i)
      {
          mask(j,i) = 0;
      }
  for(int i=0; i<cols; ++i) pattern(0,i) = 0;
  // ------------------------------------------


  for (int i = 0; i < cols; i = i + 2) pattern(0, i) = 1;

  for (int j = 0; j < rows; j = j + 2) pattern.row(0).copyTo(mask.row(j));

  cout << mask << endl;

  namedWindow( "Result" );
  imshow( "Result", mask );

  waitKey(0);

  return 0;
}