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[SOLVED] Use QScollArea & QLabel to display image

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  • 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

                                    1 Reply Last reply
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