#include <iostream>

#include "opencv2/highgui/highgui.hpp"

#include <opencv2/core/core.hpp>

#include <opencv2/imgproc/imgproc.hpp>

#include <string>

#include <thread>

#include <mutex>

using namespace std;

using namespace cv;

//The main canvas (a background image)

Mat canvas=imread("canvas.jpg", CV_LOAD_IMAGE_COLOR);

// the mutex for accessing the canvas variable

mutex mu;

// The function that plays a video on a predefined ROI on the canvas

void play(string n, Point p)  //this function could be called n number of times as its own thread

  {

    VideoCapture cap(n);  //open a local video file 

    int fps=cap.get(CV_CAP_PROP_FPS); // get it's frames per-seconds frame rates

    char k; // for exiting the frame fetching loop

    Mat temp;  // a temporary holding variable for each frame from the stream

    while(1)

      {

        unique_lock<mutex> locker(mu);  // lock the mutex (shared memory is the canvas matrix)

        cap>>temp;  // get a frame from the stream to the temporary buffer

        if(temp.empty())// if stream is empty get out of this loop

          { 

                  locker.unlock();  // unlock the mutex first before exiting this loop

                  break;  // exit the loop now.

          }

        // syntax= src_image.copyto(dst_image(Rect(Point(x,y), size(src_image))))

        //dest_image is the shared object between the threads.

        temp.copyTo(canvas(Rect(p.x, p.y, temp.cols, temp.rows)));  // past a frame onto the canvas at location (x,y) and size(of the frame)

        locker.unlock();// done interacting with the shared variable, so unlock the mutex

      }

  }

int main()

  {

          char k; // for breaking out of the main loop ( the display image loop)

          namedWindow("MainWindow", WINDOW_AUTOSIZE); // create the main window

          // create and start the first thread, pass a filename and a coordinates to place the frame on the canvas

          thread P1(play, "video-copy.mp4", Point(0,0));

          // create and start the second thread, they should both be able to access the canvas variable (one at a time ofcourse)

          thread P2(play, "video.mp4", Point(200,300));

          while(1) // keep displaying the canvas as it is changing.

            {

                    unique_lock<mutex> locker(mu);  // lock the mutex again, because we are using the canvas (shared object) again,

                    imshow("MainWindow", canvas); // show image

                    locker.unlock(); // unlock the mutex again.

                    k=waitKey(10);   // flush the buffer and wait for 10 milliseconds

                    if(k==27) //exit the display image loop.

                            break;

            }

          // Join the child threads back with the parrent thread before exiting.

          P1.join();

          P2.join();

          //and finally

          destroyWindow("MainWindow"); // destroy the main window

          return 0; // end the main.

  }

/*   The errors I always get but cant find what is causing it

(MainWindow:3157): GLib-GObject-CRITICAL **: g_object_remove_weak_pointer: assertion `G_IS_OBJECT (object)' failed

[NULL @ 0xb1b06ca0] insufficient thread locking around avcodec_open/close()

[NULL @ 0xb1b07aa0] insufficient thread locking around avcodec_open/close()

[NULL @ 0xb1b06ca0] insufficient thread locking around avcodec_open/close()

[NULL @ 0xb1b07aa0] insufficient thread locking around avcodec_open/close()

[IMGUTILS @ 0xb28fde64] Picture size 0x0 is invalid

[IMGUTILS @ 0xb28fde94] Picture size 0x0 is invalid

[swscaler @ 0xad180300] bad dst image pointers

*/