I want to subtract the two successive images taken from the webcam.
as you can see I am doing this inside a while loop. In the last line of the while loop I am setting `frame2 = frame` and so I can subtract them from the next iteration. But the function cv::subtract returns the above error in the terminal.

what am I doing wrong?

```
#include <iostream>
#include "core.hpp"
#include "highgui.hpp"
#include "imgcodecs.hpp"
#include "cv.h"

using namespace std;
using namespace cv;

int main(int argc, char* argv[])
{

    VideoCapture cap(0); ///open the video camera no. 0 (laptop's default camera)

    ///make a writer object:
    cv::VideoWriter writer;

    if (!cap.isOpened())  /// if not success, exit program
    {
        cout << "ERROR INITIALIZING VIDEO CAPTURE" << endl;
        return -1;
    }

    char* windowName = "Webcam Feed(diff image)";
    namedWindow(windowName,WINDOW_NORMAL); ///create a window to display our webcam feed

    ///we need to define 4 arguments for initializing the writer object:
    //filename string:
    string filename = "C:\\Users\\PEYMAN\\Desktop\\ForegroundExtraction\\openCV_tutorial\\2.writing from video to file\\Payman.avi";

    //fourcc integer:
    int fcc = CV_FOURCC('D','I','V','3');

    //frame per sec integer:
    int fps = 10;

    //frame size:
    cv::Size framesize(cap.get(CV_CAP_PROP_FRAME_WIDTH),cap.get(CV_CAP_PROP_FRAME_HEIGHT));

    ///initialize the writet object:
    writer = VideoWriter(filename,fcc,fps,framesize);

    if(!writer.isOpened()){

        cout << "Error opening the file" << endl;
        getchar();
        return -1;
    }

    int counter = 0;
    while (1) {

        Mat frame,frame2,diff_frame;

        ///read a new frame from camera feed and save it to the variable frame:
        bool bSuccess = cap.read(frame);

        if (!bSuccess) ///test if frame successfully read
        {
            cout << "ERROR READING FRAME FROM CAMERA FEED" << endl;
            break;
        }

        /// now the last read frame is stored in the variable frame and here it is written to the file:
        writer.write(frame);

        if (counter > 0){

            cv::subtract(frame2,frame,diff_frame);
            imshow(windowName, diff_frame ); ///show the frame in "MyVideo" window

        }

        ///wait for 10ms for a key to be pressed
        switch(waitKey(1)){

        ///the writing from webcam feed will go on until the user presses "esc":
        case 27:
            ///'esc' has been pressed (ASCII value for 'esc' is 27)
            ///exit program.
            return 0;

        }

     frame2 = frame;
     counter++;
    }

    return 0;

}

```