converting 3-channel BGR pic to grayscale BY HAND

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Hello,

I'm having this problem - I have to write a function which converts bgr picture to grayscale one-channel picture. Only things I can use are cv:Mat variables: rows, cols, data and step. I've been trying to solve it this way:

void bgr2gray( const Mat& bgr, Mat& gray )
{
    gray = Mat::zeros(bgr.rows, bgr.cols, CV_8UC1 );

     for (int y = 0; y < bgr.rows; y++) {
      for (int x = 0; x < bgr.cols; x++) {

         unsigned char blue = bgr.data[3*bgr.cols*y + x]*0.114f; //0.114
         unsigned char green = bgr.data[3*bgr.cols*y + x+sizeof(char)]*0.587f;
         unsigned char red = bgr.data[3*bgr.cols*y + x+2*sizeof(char)]*0.299f;

         gray.data[gray.cols*y+x] = red + green + blue;
      }                                                              
   }

}

But still this gives me wrong (not whole) picture with many white vertical lines. Do you know whre I'm mistaken in my thinking?

Thanks for the answer

cz_asm7