Using Mat::at(i,j) in opencv for a 2-D Mat object

Img_mat is a 3 channeled image. Each channel consists of pixel values uchar in data type. So with split(Img_mat, BGR) the Img_mat is split into 3 planes of blue, green and red which are collectively stored in a vector BGR. So BGR[0] is the first (blue) plane with uchar data type pixels...hence it will be

int dataB = (int)BGR[0].at<uchar>(i,j);
int dataG = (int)BGR[1].at<uchar>(i,j);

so on...

You have to specify the correct type for cv::Mat::at(i,j). You are accessing the pixel as int, while it should be a vector of uchar. Your code should look something like this:

IplImage* img = 0;
img = cvLoadImage("nature.jpg");
if(img != 0)
{
  Mat Img_mat(img);
  std::vector<Mat> BGR;
  split(Img_mat, BGR);

  Vec3b data = BGR[0].at<Vec3b>(i,j);
  // data[0] -> blue
  // data[1] -> green
  // data[2] -> red
}