conversion between c++ class and OpenCV matrix operation - c++

Question

I am trying to convert the following c++ line into OpenCV matrix operation (which is also c++):
double myCode::calculate ( int i, int au )
{
double k;
for ( int j = 0; i < N; i ++ );
{
k += fabs(data[i][j] - means[au][j]);
}
}
I want to define "data" and "means" as openCV matrix type, like:
cv::Mat data ( NUMBER_OF_OBSERVATIONS, N, CV_8UC3 );
cv::Mat means = cv::Mat.zeros ( 5, N, CV_8UC3 );
then repeat the above class for this cvMat type "data" and "means". How can I do that? Especially I don't know how to do the line:
k += fabs(data[i][j] - means[au][j]);
Thanks a lot.

Answer 1

You can simply write
double myCode::calculate ( int i, int au )
{
cv::Scalar res = sum(avg(data(RowRange(i)) - means(RowRange(au))));
return res[0] + res[1] + res[2]; // sum all the channels together
}
Note that RowRange() is not actually the correct syntax - look in OpenCV docs for the proper usage of Range(), but that's the idea.

Answer 2

A simple way to access pixels in OpenCV Mat objects is with the at() operator.
If your data type were 1-channel unsigned char (CV_8UC1), you could simply do this:
k += fabs(data.at<uchar>(i,j) - means.at<uchar>(i,j)); //works for CV_8UC1 type
However, you have 3 channels (R, G, B), dictated by the C3 in your CV_8UC3 datatype. So, here's how do your k += fabs(...) on each channel individually:
//for CV_8UC3 type
k += fabs(data.at<cv::Vec3b>(i,j)[0] - means.at<cv::Vec3b>(i,j)[0]); // Blue Channel
k += fabs(data.at<cv::Vec3b>(i,j)[1] - means.at<cv::Vec3b>(i,j)[1]); // Green Channel
k += fabs(data.at<cv::Vec3b>(i,j)[2] - means.at<cv::Vec3b>(i,j)[2]); // Red Channel
This post offers further explanation about pixel access.