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QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?

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  • mranger90M mranger90

    @R-P-H
    Did you change compilers as well ?

    R Offline
    R Offline
    R-P-H
    wrote on last edited by
    #8

    @mranger90 said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

    @R-P-H
    Did you change compilers as well ?

    I did add a x64 bit mingw compiler to Qt. However, I did not change the compiler of the project. It is still using the default mingw32 compiler.

    1 Reply Last reply
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    • R R-P-H

      @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

      @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

      This problem only occured after updating QT. Previously I was able to use imshow() inside of a console application without any errors.

      Assuming opencv uses the Qt backend then you have depended on an undocumented behavior. Fixing your code would be a place to start - use QApplication.

      QApplication only works with a GUI project.

      W Offline
      W Offline
      wrosecrans
      wrote on last edited by
      #9

      @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

      QApplication only works with a GUI project.

      If you want to show a GUI window, you are making a GUI project, surely?

      R 1 Reply Last reply
      0
      • W wrosecrans

        @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

        QApplication only works with a GUI project.

        If you want to show a GUI window, you are making a GUI project, surely?

        R Offline
        R Offline
        R-P-H
        wrote on last edited by
        #10

        @wrosecrans said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

        @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

        QApplication only works with a GUI project.

        If you want to show a GUI window, you are making a GUI project, surely?

        No, it's a console project. I just wanted a pop up window for test purposes. For example, to check if an image is loading correctly. As I said, it did work before. I can't say for sure what caused it to stop working.

        1 Reply Last reply
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        • R R-P-H

          @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

          @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

          This problem only occured after updating QT. Previously I was able to use imshow() inside of a console application without any errors.

          Assuming opencv uses the Qt backend then you have depended on an undocumented behavior. Fixing your code would be a place to start - use QApplication.

          QApplication only works with a GUI project.

          kshegunovK Offline
          kshegunovK Offline
          kshegunov
          Moderators
          wrote on last edited by
          #11

          @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

          QApplication only works with a GUI project.

          I'll bite. How are windows supposed to be created if you're not running X?

          Read and abide by the Qt Code of Conduct

          R 1 Reply Last reply
          0
          • kshegunovK kshegunov

            @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

            QApplication only works with a GUI project.

            I'll bite. How are windows supposed to be created if you're not running X?

            R Offline
            R Offline
            R-P-H
            wrote on last edited by
            #12

            @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

            @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

            QApplication only works with a GUI project.

            I'll bite. How are windows supposed to be created if you're not running X?

            Well Visual Studio has no problem doing it. I have made countless win32 console applications inside of Visual Studio (where the only output is a terminal window) and imshow() works perfectly.

            It's not like I'm trying to open the image inside a Qt GUI window. It's an OpenCV image window that should open separately. As I said, there is no question that it was working before.

            kshegunovK 1 Reply Last reply
            0
            • R R-P-H

              @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

              @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

              QApplication only works with a GUI project.

              I'll bite. How are windows supposed to be created if you're not running X?

              Well Visual Studio has no problem doing it. I have made countless win32 console applications inside of Visual Studio (where the only output is a terminal window) and imshow() works perfectly.

              It's not like I'm trying to open the image inside a Qt GUI window. It's an OpenCV image window that should open separately. As I said, there is no question that it was working before.

              kshegunovK Offline
              kshegunovK Offline
              kshegunov
              Moderators
              wrote on last edited by
              #13

              @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

              It's not like I'm trying to open the image inside a Qt GUI window.

              Well apparently you are.

              It's an OpenCV image window that should open separately.

              Which I suspect uses Qt as a backend and it rightfully complains.

              As I said, there is no question that it was working before.

              This doesn't mean anything if you don't follow the rules. QWidget goes with QApplication. To that end what's the problem of using QApplication and still use the console for input and output?

              Read and abide by the Qt Code of Conduct

              R 1 Reply Last reply
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              • kshegunovK kshegunov

                @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                It's not like I'm trying to open the image inside a Qt GUI window.

                Well apparently you are.

                It's an OpenCV image window that should open separately.

                Which I suspect uses Qt as a backend and it rightfully complains.

                As I said, there is no question that it was working before.

                This doesn't mean anything if you don't follow the rules. QWidget goes with QApplication. To that end what's the problem of using QApplication and still use the console for input and output?

                R Offline
                R Offline
                R-P-H
                wrote on last edited by
                #14

                @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                It's not like I'm trying to open the image inside a Qt GUI window.

                Well apparently you are.

                It's an OpenCV image window that should open separately.

                Which I suspect uses Qt as a backend and it rightfully complains.

                As I said, there is no question that it was working before.

                This doesn't mean anything if you don't follow the rules. QWidget goes with QApplication. To that end what's the problem of using QApplication and still use the console for input and output?

                The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                kshegunovK 1 Reply Last reply
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                • mranger90M Offline
                  mranger90M Offline
                  mranger90
                  wrote on last edited by mranger90
                  #15

                  I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

                  using namespace cv;
                  using namespace std;
                  
                  int main(int argc, char *argv[])
                  {
                      QCoreApplication a(argc, argv);
                  
                      Mat image;
                  
                      image = imread("c:\\tmp\\triangle_red.png", CV_LOAD_IMAGE_COLOR);
                      imshow("Frame", image);
                  
                      return a.exec();
                  }
                  
                  R 1 Reply Last reply
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                  • R R-P-H

                    @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                    @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                    It's not like I'm trying to open the image inside a Qt GUI window.

                    Well apparently you are.

                    It's an OpenCV image window that should open separately.

                    Which I suspect uses Qt as a backend and it rightfully complains.

                    As I said, there is no question that it was working before.

                    This doesn't mean anything if you don't follow the rules. QWidget goes with QApplication. To that end what's the problem of using QApplication and still use the console for input and output?

                    The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                    kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by
                    #16

                    @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                    The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                    I didn't say to change your project type. I just said to replace QCoreApplication with QApplication.

                    Read and abide by the Qt Code of Conduct

                    R 1 Reply Last reply
                    0
                    • mranger90M mranger90

                      I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

                      using namespace cv;
                      using namespace std;
                      
                      int main(int argc, char *argv[])
                      {
                          QCoreApplication a(argc, argv);
                      
                          Mat image;
                      
                          image = imread("c:\\tmp\\triangle_red.png", CV_LOAD_IMAGE_COLOR);
                          imshow("Frame", image);
                      
                          return a.exec();
                      }
                      
                      R Offline
                      R Offline
                      R-P-H
                      wrote on last edited by
                      #17

                      @mranger90 said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                      I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

                      using namespace cv;
                      using namespace std;
                      
                      int main(int argc, char *argv[])
                      {
                          QCoreApplication a(argc, argv);
                      
                          Mat image;
                      
                          image = imread("c:\\tmp\\triangle_red.png", CV_LOAD_IMAGE_COLOR);
                          imshow("Frame", image);
                      
                          return a.exec();
                      }
                      

                      Thanks for confirming. As I suspected, there is some other underlying problem on my system. Either from an update or from adding a 2nd compiler kit.

                      1 Reply Last reply
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                      • kshegunovK kshegunov

                        @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                        The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                        I didn't say to change your project type. I just said to replace QCoreApplication with QApplication.

                        R Offline
                        R Offline
                        R-P-H
                        wrote on last edited by
                        #18

                        @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                        @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                        The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                        I didn't say to change your project type. I just said to replace QCoreApplication with QApplication.

                        Sorry, I didn't realise you meant just changing that. Changing it gives the error that it's not found.

                        kshegunovK 1 Reply Last reply
                        0
                        • SGaistS Offline
                          SGaistS Offline
                          SGaist
                          Lifetime Qt Champion
                          wrote on last edited by
                          #19

                          You need to add QT += widgets to your .pro file to use QApplication.

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                          R 1 Reply Last reply
                          2
                          • R R-P-H

                            @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                            @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                            The traditional terminal window is suppressed when using a GUI project and I have no need for a GUI window of any kind which gets created by default.

                            I didn't say to change your project type. I just said to replace QCoreApplication with QApplication.

                            Sorry, I didn't realise you meant just changing that. Changing it gives the error that it's not found.

                            kshegunovK Offline
                            kshegunovK Offline
                            kshegunov
                            Moderators
                            wrote on last edited by kshegunov
                            #20

                            @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                            Changing it gives the error that it's not found.

                            Can you show the project file (*.pro)? I suspect you did not add QT = core gui widgets to it.

                            Read and abide by the Qt Code of Conduct

                            1 Reply Last reply
                            2
                            • R Offline
                              R Offline
                              R-P-H
                              wrote on last edited by
                              #21

                              I will check the .pro file again and post back. However, I never used to modify anything in it before (besides linking the OpenCV library) and it was working fine like that at some stage.

                              1 Reply Last reply
                              0
                              • SGaistS SGaist

                                You need to add QT += widgets to your .pro file to use QApplication.

                                R Offline
                                R Offline
                                R-P-H
                                wrote on last edited by
                                #22

                                @SGaist said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                You need to add QT += widgets to your .pro file to use QApplication.

                                @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                Changing it gives the error that it's not found.

                                Can you show the project file (*.pro)? I suspect you did not add QT = core gui widgets to it.

                                QT -= gui
                                
                                CONFIG += c++11 console
                                CONFIG -= app_bundle
                                
                                QT += widgets
                                
                                DEFINES += QT_DEPRECATED_WARNINGS
                                
                                SOURCES += \
                                        main.cpp
                                

                                If I add either QT += widgets or QT = core gui widgets to the .pro file and replace QCoreApplication with QApplication it works fine. It will not work with QCoreApplication however.

                                kshegunovK 1 Reply Last reply
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                                • R R-P-H

                                  @SGaist said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                  You need to add QT += widgets to your .pro file to use QApplication.

                                  @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                  @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                  Changing it gives the error that it's not found.

                                  Can you show the project file (*.pro)? I suspect you did not add QT = core gui widgets to it.

                                  QT -= gui
                                  
                                  CONFIG += c++11 console
                                  CONFIG -= app_bundle
                                  
                                  QT += widgets
                                  
                                  DEFINES += QT_DEPRECATED_WARNINGS
                                  
                                  SOURCES += \
                                          main.cpp
                                  

                                  If I add either QT += widgets or QT = core gui widgets to the .pro file and replace QCoreApplication with QApplication it works fine. It will not work with QCoreApplication however.

                                  kshegunovK Offline
                                  kshegunovK Offline
                                  kshegunov
                                  Moderators
                                  wrote on last edited by
                                  #23

                                  @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                  It will not work with QCoreApplication however.

                                  QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

                                  Read and abide by the Qt Code of Conduct

                                  R 1 Reply Last reply
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                                  • kshegunovK kshegunov

                                    @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                    It will not work with QCoreApplication however.

                                    QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

                                    R Offline
                                    R Offline
                                    R-P-H
                                    wrote on last edited by
                                    #24

                                    @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                    @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                    It will not work with QCoreApplication however.

                                    QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

                                    Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

                                    In terms of using QCoreApplication I was referring to @mranger90 post below:

                                    @mranger90 said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                    I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

                                    where he confirmed that it does work with QCoreApplication. Strange.

                                    sierdzioS 1 Reply Last reply
                                    0
                                    • R R-P-H

                                      @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                      @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                      It will not work with QCoreApplication however.

                                      QCoreApplication does not perform the needed steps to initialize the runtime for creating widgets. So yes it shouldn't be possible to create widgets with QCoreApplication; if it is, then you're using an undocumented behavior that's internal to the library, and it can stop working at any time without warning or crash. Warnings from the Qt library should be treated as errors in the user code.

                                      Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

                                      In terms of using QCoreApplication I was referring to @mranger90 post below:

                                      @mranger90 said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                      I've just confirmed that this will work on my system, Qt 5.10.0, MSVC 2017, OpenCV 3.1.0 (?), so it's not a matter of QCoreApplication vs QApplication.

                                      where he confirmed that it does work with QCoreApplication. Strange.

                                      sierdzioS Offline
                                      sierdzioS Offline
                                      sierdzio
                                      Moderators
                                      wrote on last edited by sierdzio
                                      #25

                                      @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                      Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

                                      You may want to rise this issue with OpenCV developers - looks like it is some change on their side. The documentation for imshow() does not mention any GUI config requirements.

                                      And if it worked before it may be a regression.

                                      (Z(:^

                                      kshegunovK 1 Reply Last reply
                                      0
                                      • sierdzioS sierdzio

                                        @R-P-H said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                        Alright thanks, at least I know how to get it to work now although I still find it strange that it was working before without having to use QApplication.

                                        You may want to rise this issue with OpenCV developers - looks like it is some change on their side. The documentation for imshow() does not mention any GUI config requirements.

                                        And if it worked before it may be a regression.

                                        kshegunovK Offline
                                        kshegunovK Offline
                                        kshegunov
                                        Moderators
                                        wrote on last edited by kshegunov
                                        #26

                                        @sierdzio said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                        The documentation for imshow() does not mention any GUI config requirements.

                                        It mentions that opencv will have a backend, and this is specified when building as well (--with-qt option), thus Qt requirements propagate through to opencv.

                                        And if it worked before it may be a regression.

                                        I doubt it. I continue to claim that creating a widget, which is what apparently imshow does, is not supported with QCoreApplication, never was and never will, regardless of whether it worked before, now or in the future.

                                        where he confirmed that it does work with QCoreApplication.

                                        Before applying the patch update 5.10.1.

                                        Read and abide by the Qt Code of Conduct

                                        sierdzioS 1 Reply Last reply
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                                        • kshegunovK kshegunov

                                          @sierdzio said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                          The documentation for imshow() does not mention any GUI config requirements.

                                          It mentions that opencv will have a backend, and this is specified when building as well (--with-qt option), thus Qt requirements propagate through to opencv.

                                          And if it worked before it may be a regression.

                                          I doubt it. I continue to claim that creating a widget, which is what apparently imshow does, is not supported with QCoreApplication, never was and never will, regardless of whether it worked before, now or in the future.

                                          where he confirmed that it does work with QCoreApplication.

                                          Before applying the patch update 5.10.1.

                                          sierdzioS Offline
                                          sierdzioS Offline
                                          sierdzio
                                          Moderators
                                          wrote on last edited by
                                          #27

                                          @kshegunov said in QWidget: Cannot create a QWidget without QApplication - OpenCV imshow() 2.4.13 ?:

                                          It mentions that opencv will have a backend, and this is specified when building as well (--with-qt option), thus Qt requirements propagate through to opencv.

                                          Is Qt the only supported backend? If yes, then indeed requirements do propagate. If no, then it may be the case that OP is using Qt for his console app, but OpenCV uses other framework to construct the window.

                                          Anyway, I don't know OpenCV so I'm only speculating.

                                          (Z(:^

                                          kshegunovK SGaistS 2 Replies Last reply
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