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I tried like following,

img is given image in this question.

bitwise_not(img, img_inv);

int totalrows = img_inv.rows;
int totalcols = img_inv.cols;

int hist[totalrows];
int blackspacecount = 0;
bool isbackground = true;

for(int i=0; i<totalrows; i++)
{
    hist[i] = countNonZero(img_inv(Rect(0,i,totalcols,1)));

    if(hist[i] > 0)
    {
        if(isbackground == true)
        {
            blackspacecount++;
            isbackground = false;
        }
    }
    else
    {
        isbackground = true;
    }
}



int blackspaces[blackspacecount][2];

int p = 0;

for(int i=1; i<totalrows; i++)
{
    if ( hist[i] > 0 && hist[i-1] == 0)
    {
        blackspaces[p][0] = i;    // Begin
    }
    if ( hist[i] == 0 && hist[i-1] > 0)
    {
        blackspaces[p][1] = i-1;    // End
        p++;
    }
}