Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. General and Desktop
  4. Issue with threading and opencv
Forum Updated to NodeBB v4.3 + New Features

Issue with threading and opencv

Scheduled Pinned Locked Moved Solved General and Desktop
41 Posts 3 Posters 9.7k Views 1 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • KiraK Kira

    @J.Hilk : Yes i agree with you its unnessary to save the image. In future we will store it internally in queue. But this is an interesting use case. I wanted to know the problem exactly so that i can avoid it in near future.

    J.HilkJ Online
    J.HilkJ Online
    J.Hilk
    Moderators
    wrote on last edited by J.Hilk
    #4

    @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 ?


    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
    0
    • 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 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

                                          • Login

                                          • Login or register to search.
                                          • First post
                                            Last post
                                          0
                                          • Categories
                                          • Recent
                                          • Tags
                                          • Popular
                                          • Users
                                          • Groups
                                          • Search
                                          • Get Qt Extensions
                                          • Unsolved