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Issue with threading and opencv

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  • J.HilkJ J.Hilk

    @Kira how do you save the file, also with Qt ? Than maybe QSaveFile will help

    https://doc.qt.io/qt-5/qsavefile.html#details


    That said, maybe using QFileSystemWatcher to monitor your saved file should also reduce the error frequency ?

    KiraK Offline
    KiraK Offline
    Kira
    wrote on last edited by
    #5

    @J.Hilk : I am using imwrite an opencv function to save the image.
    Actually the problem arise after i start using thread. So i just want to know how the thread is causing the issue.

    jsulmJ 1 Reply Last reply
    0
    • KiraK Kira

      @J.Hilk : I am using imwrite an opencv function to save the image.
      Actually the problem arise after i start using thread. So i just want to know how the thread is causing the issue.

      jsulmJ Online
      jsulmJ Online
      jsulm
      Lifetime Qt Champion
      wrote on last edited by
      #6

      @Kira The thread writing the file should emit a signal when the file was written and closed. The thread consuming files should only read files which were already notified through that signal. This way you would avoid reading and writing same file from two different threads at the same time.

      https://forum.qt.io/topic/113070/qt-code-of-conduct

      KiraK 1 Reply Last reply
      2
      • jsulmJ jsulm

        @Kira The thread writing the file should emit a signal when the file was written and closed. The thread consuming files should only read files which were already notified through that signal. This way you would avoid reading and writing same file from two different threads at the same time.

        KiraK Offline
        KiraK Offline
        Kira
        wrote on last edited by
        #7

        @jsulm : Just one question for clarification.
        If i use two different mutex, will the one mutex will inform about other if i tried to lock ?
        Or should i use same mutex where the conflicting situation is expected ?
        Also i was going through opencv i got the following warning which was not seen before:
        Warning:Premature end of JPEG file

        jsulmJ 1 Reply Last reply
        0
        • KiraK Kira

          @jsulm : Just one question for clarification.
          If i use two different mutex, will the one mutex will inform about other if i tried to lock ?
          Or should i use same mutex where the conflicting situation is expected ?
          Also i was going through opencv i got the following warning which was not seen before:
          Warning:Premature end of JPEG file

          jsulmJ Online
          jsulmJ Online
          jsulm
          Lifetime Qt Champion
          wrote on last edited by
          #8

          @Kira said in Issue with threading and opecv:

          If i use two different mutex, will the one mutex will inform about other if i tried to lock ?

          No.
          But with the solution I suggested there is no need at all for any mutexes. Just use signals/slots as I suggested with queued connection type.

          "Warning:Premature end of JPEG file" - I guess it's because the file is damaged.

          https://forum.qt.io/topic/113070/qt-code-of-conduct

          KiraK 1 Reply Last reply
          2
          • jsulmJ jsulm

            @Kira said in Issue with threading and opecv:

            If i use two different mutex, will the one mutex will inform about other if i tried to lock ?

            No.
            But with the solution I suggested there is no need at all for any mutexes. Just use signals/slots as I suggested with queued connection type.

            "Warning:Premature end of JPEG file" - I guess it's because the file is damaged.

            KiraK Offline
            KiraK Offline
            Kira
            wrote on last edited by
            #9

            @jsulm : Thanks for highlighting the possibility of signal/slots.
            I will have to redesign my approach for implementing the same.
            Just couple of question:
            -> If i queue signal and slot will it affect my overall program.
            ie. Suppose i queue the signal/slots related to this program will it affect the other signal/slots
            written in my program . For ex: delay in execution of the signal/slots of the main program.
            -> How to make a mutex wait for the previous lock if we are not sure how much time it is going to take.

            jsulmJ 1 Reply Last reply
            0
            • KiraK Kira

              @jsulm : Thanks for highlighting the possibility of signal/slots.
              I will have to redesign my approach for implementing the same.
              Just couple of question:
              -> If i queue signal and slot will it affect my overall program.
              ie. Suppose i queue the signal/slots related to this program will it affect the other signal/slots
              written in my program . For ex: delay in execution of the signal/slots of the main program.
              -> How to make a mutex wait for the previous lock if we are not sure how much time it is going to take.

              jsulmJ Online
              jsulmJ Online
              jsulm
              Lifetime Qt Champion
              wrote on last edited by jsulm
              #10

              @Kira said in Issue with threading and opecv:

              Suppose i queue the signal/slots related to this program will it affect the other signal/slots
              written in my program . For ex: delay in execution of the signal/slots of the main program.

              Queued signals do not block anything - they are just in a queue. The only delay you will have is the slot when it is actually called - but that you will have anyway as you have to execute the code anyway, right?

              "How to make a mutex wait for the previous lock if we are not sure how much time it is going to take" - I don't understand this question. A mutex either blocks the thread which wants to lock if the mutex is already locked, or you can continue immediately. So, you already have this "waiting" if the mutex is already locked.

              https://forum.qt.io/topic/113070/qt-code-of-conduct

              KiraK 1 Reply Last reply
              3
              • jsulmJ jsulm

                @Kira said in Issue with threading and opecv:

                Suppose i queue the signal/slots related to this program will it affect the other signal/slots
                written in my program . For ex: delay in execution of the signal/slots of the main program.

                Queued signals do not block anything - they are just in a queue. The only delay you will have is the slot when it is actually called - but that you will have anyway as you have to execute the code anyway, right?

                "How to make a mutex wait for the previous lock if we are not sure how much time it is going to take" - I don't understand this question. A mutex either blocks the thread which wants to lock if the mutex is already locked, or you can continue immediately. So, you already have this "waiting" if the mutex is already locked.

                KiraK Offline
                KiraK Offline
                Kira
                wrote on last edited by
                #11

                @jsulm
                Queued signals do not block anything - they are just in a queue. The only delay you will have is the slot when it is actually called - but that you will have anyway as you have to execute the code anyway, right? : Yes

                Thanks for the help cleared a lot of implementation doubts.
                Just a small question. If i have a thread which acquires the lock and there is one more thread which requests for the locked resource. How will i tell the other thread wait until and unless the lock in the thread have not been unlocked.
                We can currently use the wait time but is there any way to tell when create a notification when the lock is released and tell the other thread to wait for that specific time.

                And one more thing how to highlight specific lines which we are answering. Which you have did for my earlier questions. :)

                jsulmJ 1 Reply Last reply
                0
                • KiraK Kira

                  @jsulm
                  Queued signals do not block anything - they are just in a queue. The only delay you will have is the slot when it is actually called - but that you will have anyway as you have to execute the code anyway, right? : Yes

                  Thanks for the help cleared a lot of implementation doubts.
                  Just a small question. If i have a thread which acquires the lock and there is one more thread which requests for the locked resource. How will i tell the other thread wait until and unless the lock in the thread have not been unlocked.
                  We can currently use the wait time but is there any way to tell when create a notification when the lock is released and tell the other thread to wait for that specific time.

                  And one more thing how to highlight specific lines which we are answering. Which you have did for my earlier questions. :)

                  jsulmJ Online
                  jsulmJ Online
                  jsulm
                  Lifetime Qt Champion
                  wrote on last edited by jsulm
                  #12

                  @Kira said in Issue with threading and opecv:

                  How will i tell the other thread wait until

                  That's exactly what a mutex is doing: it blocks the thread which tries to lock a mutex which is already locked by other thread, as soon as the other thread releases the mutex the OS will wake up one of the blocked (waiting) threads. So, you do not have to tell the thread anything.

                  https://forum.qt.io/topic/113070/qt-code-of-conduct

                  KiraK 1 Reply Last reply
                  2
                  • jsulmJ jsulm

                    @Kira said in Issue with threading and opecv:

                    How will i tell the other thread wait until

                    That's exactly what a mutex is doing: it blocks the thread which tries to lock a mutex which is already locked by other thread, as soon as the other thread releases the mutex the OS will wake up one of the blocked (waiting) threads. So, you do not have to tell the thread anything.

                    KiraK Offline
                    KiraK Offline
                    Kira
                    wrote on last edited by
                    #13

                    @jsulm : Hello i have implemented the save image function in a slot as suggested by you.
                    But the problem is that the image is not being saved. I would like to share the given code:

                    signals:
                    void signalSaveImage(cv::Mat,String row,String column);
                    slots:
                    public slots:
                    void saveImage(Mat finalImage,String row, String column);
                    Slot Definition:

                    void MainWindow::saveImage(Mat finalImage, String row, String column)
                    {
                        cout<<"Inside image save function"<<endl;
                        cout<<"Row"<<row<<endl;
                        cout<<"Column"<<column<<endl;
                        String imagePath = "D:/testN/"
                        String imageFormat = ".bmp";
                        try {
                               cv::imwrite(imagePath+row+"_"+column+imageFormat,finalImage);
                           }
                           catch (cv::Exception& ex) {
                               fprintf(stderr, "Exception in saving image: %s\n", ex.what());
                           }
                           fprintf(stdout, "Saved PNG file with alpha data.\n");
                    
                        cout<<"\n Entering inside save mutex"<<endl;
                    
                        cout<<"\n Completed image save function"<<endl;
                    
                    
                    }
                    

                    Signal to slot connection:
                    connect(this,SIGNAL(signalSaveImage(cv::Mat,String,String)),this,SLOT(saveImage(Mat,String,String)),Qt::QueuedConnection);

                    MainWindow.cpp
                    Emitting signal to save image:
                    emit signalSaveImage(image,to_string(row),to_string(column));

                    But my problem is that my images are not being saved.

                    J.HilkJ 1 Reply Last reply
                    0
                    • KiraK Kira

                      @jsulm : Hello i have implemented the save image function in a slot as suggested by you.
                      But the problem is that the image is not being saved. I would like to share the given code:

                      signals:
                      void signalSaveImage(cv::Mat,String row,String column);
                      slots:
                      public slots:
                      void saveImage(Mat finalImage,String row, String column);
                      Slot Definition:

                      void MainWindow::saveImage(Mat finalImage, String row, String column)
                      {
                          cout<<"Inside image save function"<<endl;
                          cout<<"Row"<<row<<endl;
                          cout<<"Column"<<column<<endl;
                          String imagePath = "D:/testN/"
                          String imageFormat = ".bmp";
                          try {
                                 cv::imwrite(imagePath+row+"_"+column+imageFormat,finalImage);
                             }
                             catch (cv::Exception& ex) {
                                 fprintf(stderr, "Exception in saving image: %s\n", ex.what());
                             }
                             fprintf(stdout, "Saved PNG file with alpha data.\n");
                      
                          cout<<"\n Entering inside save mutex"<<endl;
                      
                          cout<<"\n Completed image save function"<<endl;
                      
                      
                      }
                      

                      Signal to slot connection:
                      connect(this,SIGNAL(signalSaveImage(cv::Mat,String,String)),this,SLOT(saveImage(Mat,String,String)),Qt::QueuedConnection);

                      MainWindow.cpp
                      Emitting signal to save image:
                      emit signalSaveImage(image,to_string(row),to_string(column));

                      But my problem is that my images are not being saved.

                      J.HilkJ Offline
                      J.HilkJ Offline
                      J.Hilk
                      Moderators
                      wrote on last edited by
                      #14

                      @Kira said in Issue with threading and opencv:

                      QueuedConnection

                      forces a copy of the arguments, The MetaSystem doesn't know the type cv::Mat -> can't be transmitted via QueuedConnection.

                      Check your console output you should have warning of unknown metatyps (declare and/or register)


                      Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                      Q: What's that?
                      A: It's blue light.
                      Q: What does it do?
                      A: It turns blue.

                      KiraK 1 Reply Last reply
                      2
                      • J.HilkJ J.Hilk

                        @Kira said in Issue with threading and opencv:

                        QueuedConnection

                        forces a copy of the arguments, The MetaSystem doesn't know the type cv::Mat -> can't be transmitted via QueuedConnection.

                        Check your console output you should have warning of unknown metatyps (declare and/or register)

                        KiraK Offline
                        KiraK Offline
                        Kira
                        wrote on last edited by
                        #15

                        @J.Hilk : I have registered it.
                        #include "mainwindow.h"
                        #include <QApplication>

                        int main(int argc, char *argv[])
                        {
                        qRegisterMetaType< cv::Mat >("cv::Mat");
                        QApplication a(argc, argv);
                        MainWindow w;
                        w.show();

                        return a.exec();
                        

                        }

                        J.HilkJ 1 Reply Last reply
                        0
                        • KiraK Kira

                          @J.Hilk : I have registered it.
                          #include "mainwindow.h"
                          #include <QApplication>

                          int main(int argc, char *argv[])
                          {
                          qRegisterMetaType< cv::Mat >("cv::Mat");
                          QApplication a(argc, argv);
                          MainWindow w;
                          w.show();

                          return a.exec();
                          

                          }

                          J.HilkJ Offline
                          J.HilkJ Offline
                          J.Hilk
                          Moderators
                          wrote on last edited by
                          #16

                          @Kira

                          the data goes most likely out of scope, as the copy constructor of cv::Mat does actually no coping

                          0_1560409629873_2c331f8b-46e4-4408-8567-abe5ac4169e3-image.png

                          I had the same issues in my time with opencv.

                          ended up converting it to a Image before transferring it from the opencv thread to the normal one.


                          Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                          Q: What's that?
                          A: It's blue light.
                          Q: What does it do?
                          A: It turns blue.

                          KiraK 1 Reply Last reply
                          3
                          • J.HilkJ J.Hilk

                            @Kira

                            the data goes most likely out of scope, as the copy constructor of cv::Mat does actually no coping

                            0_1560409629873_2c331f8b-46e4-4408-8567-abe5ac4169e3-image.png

                            I had the same issues in my time with opencv.

                            ended up converting it to a Image before transferring it from the opencv thread to the normal one.

                            KiraK Offline
                            KiraK Offline
                            Kira
                            wrote on last edited by
                            #17

                            @J.Hilk : Just for clarification should i replace this part with cv::clone
                            void saveImage(Mat finalImage,String row, String column);

                            1 Reply Last reply
                            0
                            • KiraK Offline
                              KiraK Offline
                              Kira
                              wrote on last edited by
                              #18

                              @J-Hilk : will try and also i tried to run the program in debug mode and getting the following error.
                              0_1560418863054_65dbf469-08ad-429c-bed7-3a5997fe0246-image.png

                              Debugger redirects to following line of file of ostream:
                              // MANIPULATORS
                              template<class _Elem,
                              class _Traits> inline
                              basic_ostream<_Elem, _Traits>&
                              __CLRCALL_OR_CDECL endl(basic_ostream<_Elem, _Traits>& _Ostr)
                              { // insert newline and flush stream
                              _Ostr.put(_Ostr.widen('\n'));
                              _Ostr.flush();
                              return (_Ostr);
                              }

                              0_1560419407453_ee0e3e1d-16a4-4a24-8b13-2cf3c1db7c66-image.png

                              And the program quits.
                              Can u please explain what may be the relevant reason for error

                              jsulmJ 1 Reply Last reply
                              0
                              • KiraK Kira

                                @J-Hilk : will try and also i tried to run the program in debug mode and getting the following error.
                                0_1560418863054_65dbf469-08ad-429c-bed7-3a5997fe0246-image.png

                                Debugger redirects to following line of file of ostream:
                                // MANIPULATORS
                                template<class _Elem,
                                class _Traits> inline
                                basic_ostream<_Elem, _Traits>&
                                __CLRCALL_OR_CDECL endl(basic_ostream<_Elem, _Traits>& _Ostr)
                                { // insert newline and flush stream
                                _Ostr.put(_Ostr.widen('\n'));
                                _Ostr.flush();
                                return (_Ostr);
                                }

                                0_1560419407453_ee0e3e1d-16a4-4a24-8b13-2cf3c1db7c66-image.png

                                And the program quits.
                                Can u please explain what may be the relevant reason for error

                                jsulmJ Online
                                jsulmJ Online
                                jsulm
                                Lifetime Qt Champion
                                wrote on last edited by
                                #19

                                @Kira Please post the stack trace after crash

                                https://forum.qt.io/topic/113070/qt-code-of-conduct

                                KiraK 1 Reply Last reply
                                0
                                • jsulmJ jsulm

                                  @Kira Please post the stack trace after crash

                                  KiraK Offline
                                  KiraK Offline
                                  Kira
                                  wrote on last edited by
                                  #20

                                  @jsulm : I thought this is the stack trace after crash

                                  jsulmJ 1 Reply Last reply
                                  0
                                  • KiraK Kira

                                    @jsulm : I thought this is the stack trace after crash

                                    jsulmJ Online
                                    jsulmJ Online
                                    jsulm
                                    Lifetime Qt Champion
                                    wrote on last edited by
                                    #21

                                    @Kira said in Issue with threading and opencv:

                                    this is the stack trace after crash

                                    no, it isn't

                                    https://forum.qt.io/topic/113070/qt-code-of-conduct

                                    KiraK 1 Reply Last reply
                                    0
                                    • jsulmJ jsulm

                                      @Kira said in Issue with threading and opencv:

                                      this is the stack trace after crash

                                      no, it isn't

                                      KiraK Offline
                                      KiraK Offline
                                      Kira
                                      wrote on last edited by
                                      #22

                                      @jsulm : This is the only thing which i see after crash with exception triggered.
                                      Is there any other way to do so

                                      jsulmJ 1 Reply Last reply
                                      0
                                      • KiraK Kira

                                        @jsulm : This is the only thing which i see after crash with exception triggered.
                                        Is there any other way to do so

                                        jsulmJ Online
                                        jsulmJ Online
                                        jsulm
                                        Lifetime Qt Champion
                                        wrote on last edited by
                                        #23

                                        @Kira In QtCreator if you start with debugger you will have a stack trace

                                        https://forum.qt.io/topic/113070/qt-code-of-conduct

                                        KiraK 1 Reply Last reply
                                        0
                                        • jsulmJ jsulm

                                          @Kira In QtCreator if you start with debugger you will have a stack trace

                                          KiraK Offline
                                          KiraK Offline
                                          Kira
                                          wrote on last edited by
                                          #24

                                          @jsulm : Are you referring to this:
                                          0_1560428382736_ecc1a67e-9ef8-4aad-ab64-76c3ed8f112e-image.png
                                          Please let me know in case of any issues

                                          jsulmJ 1 Reply Last reply
                                          0

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