Mutex and thread in videoCapture - c++

Question

I have used a worker thread to get the latest frame in real time,the code is as follow.But in my code, there is a problem. The frame is the first frame all the time, It didn't update.As a result, the first frame do the remap(), and the remap result frame do the next loop remap...I don't know why the frame didn't update. If i remove the line remap() or replace this line as dilate(frame, frame..) ,the frame updates all the time. Also, if i copy the frame to image and use the image to do remap(),the frame can update.But why in this case the frame can't update.Can somebody help me?Thank you.
std::mutex mtxCam;
void task(VideoCapture cap, Mat& frame) {
while (true) {
mtxCam.lock();
cap >> frame;
mtxCam.unlock();
}
}
int main() {
Mat frame, image;
VideoCapture cap;
cap.open(0);
cap.set(CV_CAP_PROP_FRAME_WIDTH, 1600);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 1080);
cap >> frame;
thread t(task, cap, frame);
while (true) {
initUndistortRectifyMap(
cameraMatrix, // computed camera matrix
distCoeffs, // computed distortion matrix
Mat(), // optional rectification (none)
Mat(), // camera matrix to generate undistorted
Size(1920 * 1.3, 1080 * 1.3),
// image.size(), // size of undistorted
CV_32FC1, // type of output map
map1, map2); // the x and y mapping functions
mtxCam.lock();
remap(frame, frame, map1, map2, cv::INTER_LINEAR);
frame.copyTo(image);
mtxCam.unlock();
...//image processing loop
}
}

Answer 1

There are two problems here:
1) You pass a single frame and then the video capture is mapped to the same frame every time without clearing it once that frame is processed.
2) You need a signalling mechanism(semaphore) , not a locking mechanism(mutex).
Something along these lines:
while (true) {
frame.clear();
cap >> frame;
semCam.Give();
}
semCam.Take();
remap(frame, frame, map1, map2, cv::INTER_LINEAR);
frame.copyTo(image);
You are dealing with a producer-consumer problem here.
So, Thread 1 produces the frames and Thread2 consumes the frames for image processing.
Thread1 inserts the frames into queue,signals thread2 that frames are ready for processing and waits for thread2 to signal that the frames have been processed.
Algorithm:
Thread 1
FrameProcessed.Wait()
FrameQueue.insert()
FrameQueueReadyForProcessing.Give()
Thread 2
FrameQueueReadyForProcessing.Wait()
ConsumeFrames(FrameQueue.Pop())
FrameProcessed.Give()
unfortunately, C++11 has no out of the box implementation of semaphores.
But you can roll one of your own.
https://gist.github.com/yohhoy/2156481