OpenCV with pthreads and mutexes - c++

Question

I've written a fairly basic C++ program which uses OpenCV library to show an video steam for a IP camera I have.
Since I want to add image processing code in the future, I thought it would be a good idea to use threads to do it. One thread captures the most recent frame and the other thread reads this frame and displays it on screen. I used a pthread_mutex_t to lock the frame variable.
My problem is that the code actually compiles, but when I execute the program nothing happens, it just exists after couple of seconds. I've verified this is not a problem with the VideoCapture object, but I don't have any other idea why this does not work.
This is my code:
#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <pthread.h>
using namespace cv;
using namespace std;
//GLOBALS
VideoCapture vcap;
Mat frame;
pthread_mutex_t *frameLocker;
const string videoStreamAddress = "http://10.0.0.6/mjpg/video.mjpg";
void *Proc(void *arg)
{
for(;;)
{
pthread_mutex_lock(frameLocker);
vcap.read(frame);
pthread_mutex_unlock(frameLocker);
}
}
int main(int, char**) {
frameLocker = new pthread_mutex_t();
vcap.open(videoStreamAddress);
pthread_mutex_init(frameLocker,NULL);
pthread_t *ProcThread;
pthread_create(ProcThread, NULL, Proc, NULL);
for(;;)
{
pthread_mutex_lock(frameLocker);
imshow("Output Window", frame);
pthread_mutex_unlock(frameLocker);
}
delete frameLocker;
}
I'd be glad if you could help me solve this issue.
Thanks,
Matan

Answer 1

I was able to solve this using the following code:
#include <stdio.h>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <pthread.h>
using namespace cv;
using namespace std;
//GLOBALS
VideoCapture vcap;
Mat frame;
pthread_mutex_t frameLocker;
const string videoStreamAddress = "http://IP/mjpg/video.mjpg";
void *UpdateFrame(void *arg)
{
for(;;)
{
Mat tempFrame;
vcap >> tempFrame;
pthread_mutex_lock(&frameLocker);
frame = tempFrame;
pthread_mutex_unlock(&frameLocker);
}
}
int main(int, char**) {
vcap.open(videoStreamAddress);
pthread_mutex_init(&frameLocker,NULL);
pthread_t UpdThread;
pthread_create(&UpdThread, NULL, UpdateFrame, NULL);
for(;;)
{
Mat currentFrame;
pthread_mutex_lock(&frameLocker);
currentFrame = frame;
pthread_mutex_unlock(&frameLocker);
if(currentFrame.empty()){
printf("recieved empty frame\n");
continue;
}
imshow("Output Window", currentFrame);
waitKey(1);
}
}