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[SOLVED] Use QScollArea & QLabel to display image

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  • sheeleyS Offline
    sheeleyS Offline
    sheeley
    wrote on last edited by sheeley
    #1

    Hello, everyone. I'm a freshman of Qt.
    Recently, I'm working on a frame grabber application. In order to display the image grabbed by the camera with vertical and horizontal scroll bar, I use QScollArea and QLabel. The code is as follow:
    https://gist.github.com/tianshilei1992/775234959c6bdbdd635b
    The grabber will call the callback function as long as it receives data. Perhaps due to the reason that the frame rate is too high, up to 400 fps, if I drag the scroll bar when the grabber is working, the application will crash.
    So how to fix this problem?

    I like programming :)

    1 Reply Last reply
    0
    • HamedH Offline
      HamedH Offline
      Hamed
      wrote on last edited by
      #2

      Hi
      Did you look at page below?
      http://doc.qt.io/qt-5/qtwidgets-widgets-imageviewer-example.html

      HamedBabaeyan@yahoo.com
      for more interesting stuff

      sheeleyS 1 Reply Last reply
      0
      • HamedH Hamed

        Hi
        Did you look at page below?
        http://doc.qt.io/qt-5/qtwidgets-widgets-imageviewer-example.html

        sheeleyS Offline
        sheeleyS Offline
        sheeley
        wrote on last edited by
        #3

        @Hamed Hi, thank you for your reply.
        I've read this. At present, the imageWindow, a QScrollArea class, could display the image correctly if I don't drag the scroll bar. When I drag the scroll bar, the application crashes. But if I click the arrow of scroll bar slowly, the application will not crash. So I guess it's probably because the fresh frequency is too high, when I drag the scroll bar, the application will re-draw the image, but the image has changed, so it crashed.

        I like programming :)

        mrjjM 1 Reply Last reply
        0
        • sheeleyS sheeley

          @Hamed Hi, thank you for your reply.
          I've read this. At present, the imageWindow, a QScrollArea class, could display the image correctly if I don't drag the scroll bar. When I drag the scroll bar, the application crashes. But if I click the arrow of scroll bar slowly, the application will not crash. So I guess it's probably because the fresh frequency is too high, when I drag the scroll bar, the application will re-draw the image, but the image has changed, so it crashed.

          mrjjM Offline
          mrjjM Offline
          mrjj
          Lifetime Qt Champion
          wrote on last edited by
          #4

          @sheeley
          Hi
          I think crashes when scrolling as the
          imageLabel->setPixmap(QPixmap::fromImage(bmp));
          Might have deleted the old pix and about to set the new one
          when you then scroll.
          Since something else calls the callback, it is like an other thread and it
          seems like a multithreading issue.

          sheeleyS 1 Reply Last reply
          1
          • mrjjM mrjj

            @sheeley
            Hi
            I think crashes when scrolling as the
            imageLabel->setPixmap(QPixmap::fromImage(bmp));
            Might have deleted the old pix and about to set the new one
            when you then scroll.
            Since something else calls the callback, it is like an other thread and it
            seems like a multithreading issue.

            sheeleyS Offline
            sheeleyS Offline
            sheeley
            wrote on last edited by
            #5

            @mrjj Hi.
            Yes, I guess so.
            Is there any solution for this issue? I have a software, made by the video capture card manufacture, which is also written by Qt, it could work well. So I guess there should be a solution. Perhaps asynchronous refresh?

            I like programming :)

            mrjjM 1 Reply Last reply
            0
            • sheeleyS sheeley

              @mrjj Hi.
              Yes, I guess so.
              Is there any solution for this issue? I have a software, made by the video capture card manufacture, which is also written by Qt, it could work well. So I guess there should be a solution. Perhaps asynchronous refresh?

              mrjjM Offline
              mrjjM Offline
              mrjj
              Lifetime Qt Champion
              wrote on last edited by
              #6

              @sheeley
              Would it be an option just to show it scaled to screen so no scroll needed?

              Yeah, some sort of asynchronous refresh, or scolling.
              like after pix is set, its ok to scroll.
              Not sure how to program it though as you would need to capture the scroll event and
              check that setPixmap is not in progress and then scroll or or queue the scroll request and
              do the scrolling after setPixmap. But if you start to use a mutex or similar construct, you might end up skipping some frames.

              mrjjM 1 Reply Last reply
              1
              • mrjjM mrjj

                @sheeley
                Would it be an option just to show it scaled to screen so no scroll needed?

                Yeah, some sort of asynchronous refresh, or scolling.
                like after pix is set, its ok to scroll.
                Not sure how to program it though as you would need to capture the scroll event and
                check that setPixmap is not in progress and then scroll or or queue the scroll request and
                do the scrolling after setPixmap. But if you start to use a mutex or similar construct, you might end up skipping some frames.

                mrjjM Offline
                mrjjM Offline
                mrjj
                Lifetime Qt Champion
                wrote on last edited by
                #7

                Hi
                I wonder if you do
                bool oldState = mageLabel->blockSignals(true);
                mageLabel->setPixmap(QPixmap::fromImage(bmp));
                mageLabel->blockSignals(oldState);
                Would help as to block any scrolling request while "in there".
                Not sure it uses signals for that though.
                Fast to test.

                sheeleyS 1 Reply Last reply
                1
                • mrjjM mrjj

                  Hi
                  I wonder if you do
                  bool oldState = mageLabel->blockSignals(true);
                  mageLabel->setPixmap(QPixmap::fromImage(bmp));
                  mageLabel->blockSignals(oldState);
                  Would help as to block any scrolling request while "in there".
                  Not sure it uses signals for that though.
                  Fast to test.

                  sheeleyS Offline
                  sheeleyS Offline
                  sheeley
                  wrote on last edited by
                  #8

                  @mrjj Thank you for your help. But it could not work... :(

                  I like programming :)

                  mrjjM 1 Reply Last reply
                  0
                  • sheeleyS sheeley

                    @mrjj Thank you for your help. But it could not work... :(

                    mrjjM Offline
                    mrjjM Offline
                    mrjj
                    Lifetime Qt Champion
                    wrote on last edited by mrjj
                    #9

                    @sheeley

                    I guess the paint event is not blocked
                    so it happens anyway.
                    Since the scrollbars tells scroll area to scroll and it then the label repaints itself.

                    If you build your own widget and draw the image in the paintevent, it would be
                    easier to make sure its not painting when being updated, I guess.

                    Do you really need to scroll the image at 400 fps ?
                    Its like preview of the stream?

                    sheeleyS 1 Reply Last reply
                    1
                    • mrjjM mrjj

                      @sheeley

                      I guess the paint event is not blocked
                      so it happens anyway.
                      Since the scrollbars tells scroll area to scroll and it then the label repaints itself.

                      If you build your own widget and draw the image in the paintevent, it would be
                      easier to make sure its not painting when being updated, I guess.

                      Do you really need to scroll the image at 400 fps ?
                      Its like preview of the stream?

                      sheeleyS Offline
                      sheeleyS Offline
                      sheeley
                      wrote on last edited by
                      #10

                      @mrjj Yes. This window is for preview of the stream. The stream is received from a high speed camera, its feature is high speed, up to 400 fps - -!
                      I will think about building my own widget. Could you please show me some examples, if convenient.
                      Thank you for your patience. :)

                      I like programming :)

                      mrjjM 1 Reply Last reply
                      0
                      • sheeleyS sheeley

                        @mrjj Yes. This window is for preview of the stream. The stream is received from a high speed camera, its feature is high speed, up to 400 fps - -!
                        I will think about building my own widget. Could you please show me some examples, if convenient.
                        Thank you for your patience. :)

                        mrjjM Offline
                        mrjjM Offline
                        mrjj
                        Lifetime Qt Champion
                        wrote on last edited by mrjj
                        #11

                        @sheeley
                        sketch code, fix to compile.
                        Then if you lock the image when you set it and unlock
                        when finished. and in paint check if ok to paint it.
                        Something like that.

                        class ImgPreview : public QWidget
                        {
                            Q_OBJECT
                            
                        public:
                            ImgPreview(QWidget *parent = 0) : QWidget(parent) {};
                        
                        protected:    
                            void paintEvent(QPaintEvent *event) {
                           QPainter p ( this );
                             if (ok_to_access_img)
                              p.drawImage(image);
                            }
                        
                        private:
                            void setImagePixel() {}
                            QImage image;
                            
                        };
                        
                        sheeleyS 2 Replies Last reply
                        1
                        • mrjjM mrjj

                          @sheeley
                          sketch code, fix to compile.
                          Then if you lock the image when you set it and unlock
                          when finished. and in paint check if ok to paint it.
                          Something like that.

                          class ImgPreview : public QWidget
                          {
                              Q_OBJECT
                              
                          public:
                              ImgPreview(QWidget *parent = 0) : QWidget(parent) {};
                          
                          protected:    
                              void paintEvent(QPaintEvent *event) {
                             QPainter p ( this );
                               if (ok_to_access_img)
                                p.drawImage(image);
                              }
                          
                          private:
                              void setImagePixel() {}
                              QImage image;
                              
                          };
                          
                          sheeleyS Offline
                          sheeleyS Offline
                          sheeley
                          wrote on last edited by
                          #12

                          @mrjj OK. Thank you. If any questions, I'll keep in touch with you later.

                          I like programming :)

                          1 Reply Last reply
                          0
                          • mrjjM mrjj

                            @sheeley
                            sketch code, fix to compile.
                            Then if you lock the image when you set it and unlock
                            when finished. and in paint check if ok to paint it.
                            Something like that.

                            class ImgPreview : public QWidget
                            {
                                Q_OBJECT
                                
                            public:
                                ImgPreview(QWidget *parent = 0) : QWidget(parent) {};
                            
                            protected:    
                                void paintEvent(QPaintEvent *event) {
                               QPainter p ( this );
                                 if (ok_to_access_img)
                                  p.drawImage(image);
                                }
                            
                            private:
                                void setImagePixel() {}
                                QImage image;
                                
                            };
                            
                            sheeleyS Offline
                            sheeleyS Offline
                            sheeley
                            wrote on last edited by
                            #13

                            @mrjj Hi, I found some issues.
                            If I overwrite the paint function, when I drag the scrollbars, the window will stay unchanged. This will lead to an un-continuous feeling, yes?

                            I like programming :)

                            mrjjM 1 Reply Last reply
                            0
                            • sheeleyS sheeley

                              @mrjj Hi, I found some issues.
                              If I overwrite the paint function, when I drag the scrollbars, the window will stay unchanged. This will lead to an un-continuous feeling, yes?

                              mrjjM Offline
                              mrjjM Offline
                              mrjj
                              Lifetime Qt Champion
                              wrote on last edited by mrjj
                              #14

                              @sheeley
                              hi
                              set you widget , minimum size to the image size to it will reflect the image. then
                              scrolling works like with the QLabel and you can control when its safe to repaint.

                              sheeleyS 1 Reply Last reply
                              1
                              • mrjjM mrjj

                                @sheeley
                                hi
                                set you widget , minimum size to the image size to it will reflect the image. then
                                scrolling works like with the QLabel and you can control when its safe to repaint.

                                sheeleyS Offline
                                sheeleyS Offline
                                sheeley
                                wrote on last edited by
                                #15

                                @mrjj Hi.
                                I'm very excited now. I fixed it with defining a new signal and slot to do the refreshing work. The code is as follow: https://gist.github.com/tianshilei1992/fd76520532127deb28de
                                Done! The application won't crash, anymore! :)
                                Thank you for your patience yesterday.

                                I like programming :)

                                mrjjM 1 Reply Last reply
                                1
                                • sheeleyS sheeley

                                  @mrjj Hi.
                                  I'm very excited now. I fixed it with defining a new signal and slot to do the refreshing work. The code is as follow: https://gist.github.com/tianshilei1992/fd76520532127deb28de
                                  Done! The application won't crash, anymore! :)
                                  Thank you for your patience yesterday.

                                  mrjjM Offline
                                  mrjjM Offline
                                  mrjj
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #16

                                  @sheeley
                                  Good work!
                                  Thank you for update.

                                  1 Reply Last reply
                                  0
                                  • P Offline
                                    P Offline
                                    phnhung98
                                    wrote on last edited by
                                    #17

                                    Hi,
                                    I followed your link but it is not available. I am interested in how you solved the problem. I am also trying to display video stream on QLabel with high framerate and large size of image. However, I suffer from screen tearing effect.
                                    Thanks.

                                    SGaistS 1 Reply Last reply
                                    0
                                    • P phnhung98

                                      Hi,
                                      I followed your link but it is not available. I am interested in how you solved the problem. I am also trying to display video stream on QLabel with high framerate and large size of image. However, I suffer from screen tearing effect.
                                      Thanks.

                                      SGaistS Offline
                                      SGaistS Offline
                                      SGaist
                                      Lifetime Qt Champion
                                      wrote on last edited by
                                      #18

                                      @phnhung98 said in [SOLVED] Use QScollArea & QLabel to display image:

                                      Hi,
                                      I followed your link but it is not available. I am interested in how you solved the problem. I am also trying to display video stream on QLabel with high framerate and large size of image. However, I suffer from screen tearing effect.
                                      Thanks.

                                      Hi and welcome to devnet,

                                      Please define high speed and large size.

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

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