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Currently, I am trying to grab a Mat and do a simple comparison. But now I am stuck at grabbing previous frame. Here is my code.

for(;;){
        cap >> frame;
        // Clone the current frame:
        Mat original = frame.clone();
        // Convert the current frame to grayscale:
        Mat gray;
        cvtColor(original, gray, CV_BGR2GRAY);
         vector< Rect_<int> > faces;
         Mat face_tmp;
         haar_cascade.detectMultiScale(gray, faces,1.2);    //scaleFactor = 1.2 for stability
         for(int i = 0; i < faces.size(); i++) {
            Rect face_i = faces[i];
            Mat face = gray(face_i);
            Mat face_resized;
            cv::resize(face, face_resized, Size(im_width, im_height), 1.0, 1.0, INTER_CUBIC);
            Mat Diff = face_resized - face_tmp;
            Scalar sumOfface_resized = sum(face_resized);
            Scalar sumOfface_tmp = sum(face_tmp);
            Scalar sumOfDiff = sum(Diff);
            face_tmp = face_resized.clone();

            string String_count = static_cast<ostringstream*>( &(ostringstream() << count) )->str();
            string String_face_i = static_cast<ostringstream*>( &(ostringstream() << i) )->str();
            string filename = String_count + "_" + String_face_i;
            cout << "FileName is : " << filename << endl;
            cout << "Sum of face_resized is : " << sumOfface_resized << endl;
            cout << "Sum of face_tmp is : "     << sumOfface_tmp << endl;
            cout << "Sum of element differentiation is : " << sumOfDiff << endl;
            cout << "xxxxx"  << sum(face_tmp) << endl;
    }

Here is my output:

    FileName is : 0_0
    Sum of face_resized is : [5.3287e+06, 0, 0, 0]
    Sum of face_tmp is : [0, 0, 0, 0]
    Sum of element differentiation is : [5.3287e+06, 0, 0, 0]
    xxxxx[5.3287e+06, 0, 0, 0]
    FileName is : 1_0
    Sum of face_resized is : [5.3287e+06, 0, 0, 0]
    Sum of face_tmp is : [0, 0, 0, 0]
    Sum of element differentiation is : [5.3287e+06, 0, 0, 0]
    xxxxx[5.3287e+06, 0, 0, 0]
    FileName is : 2_0
    Sum of face_resized is : [5.3287e+06, 0, 0, 0]
    Sum of face_tmp is : [0, 0, 0, 0]
    Sum of element differentiation is : [5.3287e+06, 0, 0, 0]
    xxxxx[5.3287e+06, 0, 0, 0]
    FileName is : 3_0
    Sum of face_resized is : [5.3287e+06, 0, 0, 0]
    Sum of face_tmp is : [0, 0, 0, 0]
    Sum of element differentiation is : [5.3287e+06, 0, 0, 0]
    xxxxx[5.3287e+06, 0, 0, 0]

Why my previous Mat always zero? I had defined face_tmp outside for loop already. Where am I missing?