Cannot write to OpenCV frame captured by a different thread - c++

Question

By using the following code, I can successfully capture and return frame from a separate thread to main(). But now I cannot write to the frame/cv::Mat (contour in main()) for display. If I do not use separate thread to for webcam then I can successfully write to the frame/cv::Mat. What am I doing wrong?
VideoCapture cap(0);
class Camera
{
public:
Camera(void);
~Camera(void);
Mat captureVideo(void);
private:
Mat frame;
double dWidth;
double dHeight;
double fps;
};
Camera::Camera(void) {
int isrunning = 0;
usleep(10);
if (!cap.open(2))
{
cout << "Cannot open the video camera.\n" << endl;
}
else
{
isrunning = 1;
}
if(isrunning == 0) {
this->~Camera();
}
cap >> frame;
}
Camera::~Camera(void) {
cap.release();
}
Mat Camera::captureVideo(void) {
cap >> frame;
return frame;
}
Camera cam1;
const int frameBuffer = 20; // Frame buffer
std::vector<cv::Mat> frameStack;
int stopSig = 0; // Global stop signal...
void grabFrame(void) {
Mat frame;
::frameStack.clear();
while(!::stopSig) {
frame = ::cam1.captureVideo();
// 1. Remove one frame from the back, if the stack has more then 2 frames...
if(::frameStack.size() > 2) { //If the framestack has more then 2 frames...
::frameStack.pop_back();
}
// 2. Add a frame at the front of the stack if the stack is not full...
if (::frameStack.size() < ::frameBuffer) {
::frameStack.push_back(frame); // Put new frame on stack on the computer's RAM...
} else {
::frameStack.clear();
}
}
return;
}
int main(int argc, char* argv[])
{
Mat fr, outImg; // Captured single frames...
Mat contour; // Video stream showin countours of objects...
::frameStack.clear();
thread t1(grabFrame);
cv::namedWindow("MyWindow", 1);
while(1) {
if(::frameStack.size() >= 2) {
contour = ::frameStack.back();
circle(contour, Point(150,150),50, Scalar(0,255,255),cv::FILLED, 8);// PROBLEM -> NO EFFECT
putText(contour, "some text", Point(100,100), FONT_HERSHEY_DUPLEX, 1, Scalar(0,143,143), 2);// PROBLEM -> NO EFFECT
imshow("Contour Video", contour);
}
if (waitKey(1) == 27)
{
::stopSig = 1; // Signal to threads to end their run...
frameStack.clear();
break;
}
}
t1.join();
return 0;
}

Answer 1

In your example, you are sharing stopSig, frameStack and frameBuffer variables between the two threads.
When you are using shared memory between threads, you have to properly synchronize the access to that shared memory. One of the reasons why synchronization is needed is to prevent two threads modifying a variable at the same time. Another reason is visibility - a write to a variable by one thread can be cached inside the CPU cache, and a thread running on another CPU core won't see the variable change.
Perhaps the easiest way to synchronize memory access between threads is by using std::mutex. I'm not gonna post the whole code here, but you can read about mutexes in C++ online, for example, here.