How does this code work?(SVM classification)

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1.40/5 (2 votes)

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Hi I can't understand how does work this line of following code:

Mat sampleMat = (Mat_<float>(1,2) << i,j);

what is "Mat_" ?
Why (1,2)?

C++

Vec3b green(0,255,0), blue (255,0,0);

for (int i = 0; i < image.rows; ++i)
    for (int j = 0; j < image.cols; ++j)
    {
    Mat sampleMat = (Mat_<float>(1,2) << i,j);
    float response = SVM.predict(sampleMat);

    if (response == 1)
       image.at<Vec3b>(j, i)  = green;
    else
    if (response == -1)
       image.at<Vec3b>(j, i)  = blue;
    }

Also why in if-else conditions it uses (j,i) not (i,j) ?

The complete explanation is exist here : http://docs.opencv.org/doc/tutorials/ml/introduction_to_svm/introduction_to_svm.html

Posted 12-Jun-14 2:45am

hor_313

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Legor · Answer 1 · 2014-06-12T03:27:00

Solution 1

Mat is short form for "Matrice" and it is the central type used to store images and other things in openCV. You should really learn those very basics if you want to work with openCV. The documentation of openCV is quite good. See for example here:
http://docs.opencv.org/modules/core/doc/basic_structures.html[^]

With the constructor as shown in your code:
Mat sampleMat = (Mat_<float>(1,2) << i,j);
you create a matrice with 2 elements (1 row, 2 columns) and store the values of i and j into it.

"Also why in if-else conditions it uses (j,i) not (i,j) ?"
You're right it should be indexed with (i, j) and in fact that's what is done in the tutorial you'
ve linked. So it seems you made a mistake there when copying the code.

Posted 12-Jun-14 3:27am

Legor

Updated 12-Jun-14 3:29am

v3

Comments

[no name] 12-Jun-14 9:37am

5ed. Nicely explained.

hor_313 12-Jun-14 10:57am

Thank you Legor!
I only wanted to know about Mat_ type, and your link helped me.
Also I've copied the code from OpenCV document directly and the mistake is in the tutorial code.

[no name] 12-Jun-14 11:00am

You have replied to me! Click the "Have a Question or Comment?" widget to reply the the person who answered!

hor_313 12-Jun-14 14:12pm

Ok, thanks!

hor_313 12-Jun-14 14:12pm

Thank you Legor!
I only wanted to know about Mat_ type, and your link helped me.
Also I've copied the code from OpenCV document directly and the mistake is in the tutorial code.