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Issue with threading and opencv

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  • 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 Online
                J.HilkJ Online
                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 Online
                    J.HilkJ Online
                    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
                                    • KiraK Kira

                                      @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 Online
                                      jsulmJ Online
                                      jsulm
                                      Lifetime Qt Champion
                                      wrote on last edited by
                                      #25

                                      @Kira Yes. Now you need to go from top to bottom until you hit first source code file from your project and check it at the line mentioned in the stack trace.

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

                                      KiraK 1 Reply Last reply
                                      1
                                      • jsulmJ jsulm

                                        @Kira Yes. Now you need to go from top to bottom until you hit first source code file from your project and check it at the line mentioned in the stack trace.

                                        KiraK Offline
                                        KiraK Offline
                                        Kira
                                        wrote on last edited by
                                        #26

                                        @jsulm : I tried doing it but the error is expected to come after thread running >150 times which is making the debugging process very difficult.
                                        I have also sample test case on github can you please go through it for any possible issues.
                                        https://github.com/HackersSpirit/ImageProcessor

                                        J.HilkJ jsulmJ 2 Replies Last reply
                                        0
                                        • KiraK Kira

                                          @jsulm : I tried doing it but the error is expected to come after thread running >150 times which is making the debugging process very difficult.
                                          I have also sample test case on github can you please go through it for any possible issues.
                                          https://github.com/HackersSpirit/ImageProcessor

                                          J.HilkJ Online
                                          J.HilkJ Online
                                          J.Hilk
                                          Moderators
                                          wrote on last edited by
                                          #27

                                          @Kira said in Issue with threading and opencv:

                                          150 times

                                          you can tell a breakpoint to be ignored for a certain amount of time

                                          0_1560506679424_0eaace31-88f0-4376-8417-f745494c6433-image.png


                                          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.

                                          1 Reply Last reply
                                          2

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