Visual Studio 2010 OpenCV Multithread Mat does not copy to another Mat - c++

Question

Good afternoon to all!
I've been using Visual Studio 2010 with OpenCV to develop a code for Face recognition. I'm trying to reach the task by two Threads (I need to do it this way, because i´m going to apply it on a bigger project), one (the main) to show the frames and the second to capture (from the Webcam of my laptop) and store the frames on a Mat object (Fast Capture).
The inconvenient here is that the second Thread is capturing the frames, but the main is not showing them. I think there is a problem with copying the Mat from the capture Thread to the Mat on the main thread ("current_frame" seems to be empty after I do the assignation)
Here is the code (I'm using Boost::Thread for Multithreading)
New code with suggestions
Global declarations
#include <iostream>
#include <stdio.h>
#include <boost\thread.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
boost::mutex mtx;
The function
void camCapture(VideoCapture& cap, Mat& frame, bool* Capture)
{
while (*Capture==true)
{
mtx.lock();
cap >> frame;
mtx.unlock();
if(frame.empty())
{
cout<<"No hay captura"<<endl;
}
else
{
cout<<"Frame capturado"<<endl;
}
}cout << "camCapture finished\n";
return;}
The main
int main() {
try{
VideoCapture cap(0); // open the default camera
Mat frame,current_frame, SFI, Input;
bool *Capture = new bool;
*Capture = true;
if (!cap.isOpened()) // check if we succeeded
return -1;
//your capture thread has started
boost::thread captureThread(camCapture, cap, frame, Capture);
while(1)
{
if(frame.empty())
{
cout<<"Frame en hilo principal vacio"<<endl;
}
else{
cout<<"Frame en hilo principal capturado"<<endl;
}
mtx.lock();
current_frame = frame.clone();
mtx.unlock();
if(current_frame.empty())
{
cout<<"Current_Frame vacio"<<endl;
}
else{
imshow("Streaming",current_frame);
if(waitKey(10)==27)break;
}
}
//Terminate the thread
captureThread.join();
}
catch(Exception & e)
{
cout<<e.what()<<endl;
}
return 0;}

Answer 1

according to Boost threads - passing parameters by reference you have to use boost::ref(v) if you want to pass a variable by reference to a boost::thread function. But you could use pointers instead.
In addition you should share the mutex to the thread by passing it as pointer or reference variable instead of using it as global:
#include <iostream>
#include <stdio.h>
#include <boost/thread.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
boost::mutex mtx;
void camCapture(VideoCapture& cap, Mat& frame, bool* Capture)
{
while (*Capture == true)
{
mtx.lock();
cap >> frame;
mtx.unlock();
if (frame.empty())
{
cout << "No hay captura" << endl;
}
else
{
cout << "Frame capturado" << endl;
}
}cout << "camCapture finished\n";
return;
}
int main() {
try{
VideoCapture cap(0); // open the default camera
Mat frame, current_frame, SFI, Input;
bool *Capture = new bool; // better not use a pointer here, but use a bool and pass the address or by reference.
*Capture = true;
if (!cap.isOpened()) // check if we succeeded
return -1;
//your capture thread has started
boost::thread captureThread(camCapture, boost::ref(cap), boost::ref(frame), Capture);
while (1)
{
mtx.lock();
current_frame = frame.clone();
mtx.unlock();
if (current_frame.empty())
{
cout << "Current_Frame vacio" << endl;
}
else{
imshow("Streaming", current_frame);
if (waitKey(10) == 27)
{
// TODO: you should use a mutex (or an atomic type) here, too, maybe setting a bool is thread-safe, but this can't be guaranteed for each hardware!
*Capture = false;
break;
}
}
}
//Terminate the thread
captureThread.join();
// give memory free:
delete Capture;
}
catch (Exception & e)
{
cout << e.what() << endl;
}
return 0;
}