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

Question

I am using Ubuntu 12.04 and OpenCV 2

I have written the following code :

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

    int data = (RGB[0]).at<int>(i,j)); /*Where i, j are inside the bounds of the matrix size .. i have checked this*/ 
}

The problem is I am getting negative values and very large values in the data variable. I think I have made some mistake somewhere. Can you please point it out.
I have been reading the documentation (I have not finished it fully.. it is quite large. ) But from what I have read, this should work. But it isnt. What is going wrong here?

Answer 1

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...