c++ - why a vector of cv::mat always return the first image? -

i'm reading video frames webcam , storing them in std::vector < cv::mat > variable.

each time want calculate frame difference between 2 consecutive frames, use following code result 0 , 0 matrix !! have 2 threads in program, 1 reading video frames , 1 processing video frames. whenever i'm going write vector use mutex in order prevent further problems in pushing or removing frames.

here pseudo code :

std::vector<cv::mat> image_vec; qmutex image_vec_mutex; main_thread() {  while(1) {  cv::mat frame,reserved_frame;  camera.read(frame);  frame.copyto(reserved_frame);  qmutexlocker locker(&image_vec_mutex);  image_vec.pushback(reserved_frame);  locker.unlock();  cv::imshow("frame",frame);  cv::waitkey(); } } 

and process thread is:

process_thread() {    (; image_vec.size() > 1 ;) {       cv::mat frame1;       image_vec[0].copyto(frame1);       cv::mat frame2;       image_vec[1].copyto(frame2);       cv::subtract(frame1,frame2,result);       qmutexlocker locker(&image_vec_mutex);       image_vec[0].release();       //qdebug()<<"image_vec step2 size  is: "<<image_vec.size()<<"\n";       image_vec.erase(image_vec.begin());       locker.unlock(); } } 

i calculated mean , standard deviation of each frame in process thread , same ! solve problem changing thread process code code

 process_thread() {    (; image_vec.size() > 1 ;) {       cv::mat frame1;       image_vec[0].copyto(frame1);       qmutexlocker locker(&image_vec_mutex);       image_vec[0].release();       //qdebug()<<"image_vec step2 size  is: "<<image_vec.size()<<"\n";       image_vec.erase(image_vec.begin());       locker.unlock();       delay_ms(1000); // more 1 second delay !!!       cv::mat frame2;       image_vec[0].copyto(frame2);       cv::subtract(frame1,frame2,result);  } } 

why happens ? why delay more 1 second should used make code work ? tried delay less 1 second , result previous step ...

i'd appreciate comments on this.

you may need copy or clone frame mat, push vector, change code like

 cv::mat frame;  camera.read(frame);   mat tmp=frame.clone(); //clone frame new mat  image_vec.pushback(tmp); 

otherwise passing same pointer on each pushback.