How to use C-style scan to check pixel neighbors ?

edit

Hey all,

I am trying to scan an image using the efficient C-style pointer access. For every pixel of a 16-bit unsigned image, i

1. look at its right and bottom neighbor and perform some operations if they are not the same.
2. accumulate the color information of the pixel i am at

The problem appears when the pixel is at the border of the image because the neighbor to its right l[j+1] is the first pixel of the next row.

http://docs.opencv.org/2.4/doc/tutori...

Is there a solution for this ?

int n_rows = label.rows;
int n_cols  = label.cols;

for (int i = 0; i < 1; ++i)
{
    ushort* l = label.ptr<ushort>(i);
    cv::Vec3b* c = color.ptr<cv::Vec3b>(i);
    for (int j = 0; j < n_cols * n_rows; ++j)
    {
        //labels
        int mid    = l[j], 
            right  = l[j + 1], 
            bottom = l[j + n_cols];

        //accumulate color
        cluster[mid].color += c[i];

        //do something if (mid label != right label_ || (mid label != bottom label)
        if(mid != right)  //increment_boundary(mid,right);
        if(mid != bottom) //increment_boundary(mid,bottom);
    }
}

This is the unoptimized version

    //label = ushort, color = cv::Vec3b
    cv::Mat label, color;

    //fill up color and label

    //pad label
    cv::copyMakeBorder(label, label_padded, 0, 2, 0, 2, cv::BORDER_REPLICATE);

    //use dimensions of unpadded image
    int n_rows = label.rows,
        n_cols = label.cols;

    for(int row = 0; row < n_rows; row++)
    for(int col = 0; col < n_cols; col++)
    {
        //labels
        int mid    = label_padded.at<ushort>(row, col),
            right  = label_padded.at<ushort>(row, col + 1),
            bottom = label_padded.at<ushort>(row + 1, col);

        //update cluster statistics
        //cluster[mid] += color.at<cv::Vec3b>(row, col);

        if(mid != right) //update boundary
        if(mid != bottom) //update boundary
    }