How can I convert a Mat_ to Mat in the following code? - c++

Question

below is a snippet from the opencv SVM tutorial at this link. And in that snippet is this line of code ' Mat sampleMat = (Mat_(1,2) << j,i);'. Instead of using the Mat_ template, I would need to use a regular Mat object. I was hoping someone can show me how to convert the Mat_ to a Mat in the previous line.
I tried Mat sampleMat = (Mat(1,2, CV_32FC1) << j,i); //but get a long page of errors
I tried Mat sampleMat = Mat(1,2, CV_32FC1) << j,i; //same, long page of errors
I just need the code at the link at the top of the page to run without using the Mat_ and only use a Mat in its place...if someone can show me how to write that line I'd appreciate it.
for (int i = 0; i < image.rows; ++i)
for (int j = 0; j < image.cols; ++j)
{
Mat sampleMat = (Mat_<float>(1,2) << j,i);
float response = SVM.predict(sampleMat);
if (response == 1)
image.at<Vec3b>(i,j) = green;
else if (response == -1)
image.at<Vec3b>(i,j) = blue;
}
Edit: Trying to run like below but getting errors
Vec3b green(0,255,0), blue (255,0,0);
// Show the decision regions given by the SVM
for (int i = 0; i < image.rows; ++i)
for (int j = 0; j < image.cols; ++j)
{
Mat sampleMat(1, 2, CV_32F);
float * const pmat = sampleMat.ptr<float>();
pmat[0] = i;
pmat[1] = j;
float response = SVM.predict(sampleMat);
if (response == 1)
pmat[0] = green;
pmat[1] = green;
else if (response == -1)
pmat[0] = blue;
pmat[1] = blue;
}
I figured you'd know enough so I didn't need the errors=)

Answer 1

Set the values directly:
Mat sampleMat(1, 2, CV_32F);
sampleMat.at<float>(0,1) = j;
sampleMat.at<float>(0,2) = i;
or
Mat sampleMat(1, 2, CV_32F);
float * const pmat = sampleMat.ptr<float>();
pmat[0] = j;
pmat[1] = i;
Addendum:
Seeing your loop, you could make it a bit more efficient in the case that SVM.predict doesn't modify sampleMat. You can set the image row just once per row, instead of doing it all the time:
for (int i = 0; i < image.rows; ++i)
{
Mat sampleMat(1, 2, CV_32F);
sampleMat.at<float>(0, 2) = i;
for (int j = 0; j < image.cols; ++j)
{
sampleMat.at<float>(0, 1) = j;
...
}
}