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Get Pixel-Values on Screen (KDE)

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beginnerkdedesktopwindow managerscreenshot
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  • C Online
    C Online
    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on 30 Dec 2022, 20:19 last edited by
    #7

    Since you did not show the whole CMakeLists.txt you did not tell us that you're also trying to link against KDE5 libs - this will not work you can't mix Qt5 and Qt6.

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    T 1 Reply Last reply 30 Dec 2022, 20:30
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    • C Christian Ehrlicher
      30 Dec 2022, 20:19

      Since you did not show the whole CMakeLists.txt you did not tell us that you're also trying to link against KDE5 libs - this will not work you can't mix Qt5 and Qt6.

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      tim-hilt
      wrote on 30 Dec 2022, 20:30 last edited by
      #8

      @Christian-Ehrlicher does that mean the idea of @wrosecrans won‘t work?

      C 1 Reply Last reply 30 Dec 2022, 20:33
      0
      • T tim-hilt
        30 Dec 2022, 20:30

        @Christian-Ehrlicher does that mean the idea of @wrosecrans won‘t work?

        C Online
        C Online
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on 30 Dec 2022, 20:33 last edited by
        #9

        @tim-hilt said in Get Pixel-Values on Screen (KDE):

        does that mean the idea of @wrosecrans won‘t work?

        It does if you use the qt-5 docs instead the qt-6 ones

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          tim-hilt
          wrote on 30 Dec 2022, 21:25 last edited by
          #10

          @Christian-Ehrlicher thanks. That did in fact work.

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            tim-hilt
            wrote on 30 Dec 2022, 22:05 last edited by tim-hilt
            #11

            @Christian-Ehrlicher @wrosecrans The original question is answered so I marked the thread as solved. However, allow me to ask one more question:

            My ultimate goal is to find the most prominent color in the first row of pixels in a window. My current, naive implementation looks like this:

              QScreen *screen = QGuiApplication::primaryScreen();
              auto window = screen->grabWindow(0).toImage(); // Convert to QImage
            
              // I have no way to debug, so I'm logging to journalctl
              syslog(LOG_NOTICE, "[Window Decoration] Width: %d, Height: %d",
                     window.width(), window.height());
            
              std::unordered_map<QRgb, unsigned int> cols{};
            
              for (int i = 0; i < window.width(); i++) {
                cols[window.pixel(i, 0)]++; // Count color-occurences
              }
            

            However the above code always logs, that the window is 0x0 pixels, so no pixel-value can actually be observed. Should I open another thread for this issue? Any idea what could have gone wrong? I added a 500ms delay before calling the above code, but that didn't change the behavior.

            Here you can see the code in context.

            M 1 Reply Last reply 30 Dec 2022, 22:35
            0
            • T tim-hilt
              30 Dec 2022, 22:05

              @Christian-Ehrlicher @wrosecrans The original question is answered so I marked the thread as solved. However, allow me to ask one more question:

              My ultimate goal is to find the most prominent color in the first row of pixels in a window. My current, naive implementation looks like this:

                QScreen *screen = QGuiApplication::primaryScreen();
                auto window = screen->grabWindow(0).toImage(); // Convert to QImage
              
                // I have no way to debug, so I'm logging to journalctl
                syslog(LOG_NOTICE, "[Window Decoration] Width: %d, Height: %d",
                       window.width(), window.height());
              
                std::unordered_map<QRgb, unsigned int> cols{};
              
                for (int i = 0; i < window.width(); i++) {
                  cols[window.pixel(i, 0)]++; // Count color-occurences
                }
              

              However the above code always logs, that the window is 0x0 pixels, so no pixel-value can actually be observed. Should I open another thread for this issue? Any idea what could have gone wrong? I added a 500ms delay before calling the above code, but that didn't change the behavior.

              Here you can see the code in context.

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              mpergand
              wrote on 30 Dec 2022, 22:35 last edited by mpergand
              #12

              @tim-hilt
              I'm using this kind of code:

                     QScreen* screen=qApp->primaryScreen();
                     screen=window()->windowHandle()->screen();
                     if(!screen) return;
                     QPoint pt=window()->geometry().topLeft();
                     auto pixmap= screen->grabWindow(this->winId(), pt.x(),pt.y(),300,height());
                     auto color = pixmap.toImage().pixelColor(2,2);
                     qDebug()<<hex<<color<<color.rgb();
              
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                tim-hilt
                wrote on 30 Dec 2022, 23:17 last edited by
                #13

                Hi @mpergand, thanks for the reply. It seems like you have an instance of QGuiApplication that you hold in the variable qApp. I unfortunately don't have any of that. I generally don't really understand your code. Why are you initializing screen with one value, if it's directly overwritten in the second line?

                I logged the screen-name that I get when using QGuiApplication::primaryScreen(). It prints eDP-1, which is my laptop-screen. So I correctly get the screen. The issue thus seems to happen when I use grabWindow. screen->grabWindow(0) should grab the pixels of the entire screen, but that doesn't seem to be the case for me.

                I also tried screen->grabWindow(QApplication::activeWindow()->winId()) to get the window-id of the currently active window, but that just crashes KDE.

                I'm unfortunately out of ideas for now. Any idea what else I could try?

                M 1 Reply Last reply 31 Dec 2022, 00:03
                0
                • T tim-hilt
                  30 Dec 2022, 23:17

                  Hi @mpergand, thanks for the reply. It seems like you have an instance of QGuiApplication that you hold in the variable qApp. I unfortunately don't have any of that. I generally don't really understand your code. Why are you initializing screen with one value, if it's directly overwritten in the second line?

                  I logged the screen-name that I get when using QGuiApplication::primaryScreen(). It prints eDP-1, which is my laptop-screen. So I correctly get the screen. The issue thus seems to happen when I use grabWindow. screen->grabWindow(0) should grab the pixels of the entire screen, but that doesn't seem to be the case for me.

                  I also tried screen->grabWindow(QApplication::activeWindow()->winId()) to get the window-id of the currently active window, but that just crashes KDE.

                  I'm unfortunately out of ideas for now. Any idea what else I could try?

                  M Offline
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                  mpergand
                  wrote on 31 Dec 2022, 00:03 last edited by
                  #14

                  @tim-hilt said in Get Pixel-Values on Screen (KDE):

                  Why are you initializing screen with one value, if it's directly overwritten in the second line?

                  You're right, that's old (test) code that I don't use because grab() doesn't work well:

                  • the colors of the grabbed image are ligther then the original
                  • there's a strange redraw bug on the original window after the grab operation (at least on mac)

                  Try with a portion of the screen:
                  auto pixmap= screen->grabWindow(winId,100,100,200,200);

                  In my opinion, the grab method is not really reliable.

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                    tim-hilt
                    wrote on 31 Dec 2022, 08:06 last edited by tim-hilt
                    #15

                    @mpergand the pixmap-size is still 0x0 pixels, even if I call grabWindow with coordinates. In my application I call .toImage() on the returned pixmap. However both the pixmap and the image are both of size 0x0 pixels. So it also doesn't get lost in the conversion-process from pixmap to image.

                    M 1 Reply Last reply 31 Dec 2022, 14:18
                    0
                    • T tim-hilt
                      31 Dec 2022, 08:06

                      @mpergand the pixmap-size is still 0x0 pixels, even if I call grabWindow with coordinates. In my application I call .toImage() on the returned pixmap. However both the pixmap and the image are both of size 0x0 pixels. So it also doesn't get lost in the conversion-process from pixmap to image.

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                      mpergand
                      wrote on 31 Dec 2022, 14:18 last edited by Christian Ehrlicher
                      #16

                      Test on KDE 21.04 in virtualbox.

                      int main(int argc, char *argv[])
                      {    
                       QApplication app(argc, argv);
                      
                        QWidget w1;
                        w1.show();
                        w1.resize(300,200);
                      
                        QTimer::singleShot(500, 0,[&w1]()
                           {
                            QScreen *screen = QGuiApplication::primaryScreen();
                            auto screen_img = screen->grabWindow(0).toImage();
                            qDebug()<<screen_img;
                            auto win_img= screen->grabWindow(w1.winId());
                            qDebug()<<win_img;
                           }
                       );
                       return app.exec();
                      }
                      

                      QImage(QSize(1891, 972),format=QImage::Format_RGB32,depth=32,devicePixelRatio=1,bytesPerLine=7564,sizeInBytes=7352208)
                      QPixmap(QSize(300, 200),depth=32,devicePixelRatio=1,cacheKey=0x700000001)

                      Work as expected.

                      T 1 Reply Last reply 31 Dec 2022, 15:31
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                      • M mpergand
                        31 Dec 2022, 14:18

                        Test on KDE 21.04 in virtualbox.

                        int main(int argc, char *argv[])
                        {    
                         QApplication app(argc, argv);
                        
                          QWidget w1;
                          w1.show();
                          w1.resize(300,200);
                        
                          QTimer::singleShot(500, 0,[&w1]()
                             {
                              QScreen *screen = QGuiApplication::primaryScreen();
                              auto screen_img = screen->grabWindow(0).toImage();
                              qDebug()<<screen_img;
                              auto win_img= screen->grabWindow(w1.winId());
                              qDebug()<<win_img;
                             }
                         );
                         return app.exec();
                        }
                        

                        QImage(QSize(1891, 972),format=QImage::Format_RGB32,depth=32,devicePixelRatio=1,bytesPerLine=7564,sizeInBytes=7352208)
                        QPixmap(QSize(300, 200),depth=32,devicePixelRatio=1,cacheKey=0x700000001)

                        Work as expected.

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                        tim-hilt
                        wrote on 31 Dec 2022, 15:31 last edited by tim-hilt
                        #17

                        @mpergand can you provide a CMakeLists.txt or the compile-command you used?

                        and just to make that clear: I’m using the same code distilled to

                        QScreen *screen = QGuiApplication::primaryScreen();
                        auto screen_img = screen->grabWindow(0).toImage();
                        

                        So no app.exec or something like that. That shouldn’t matter however, since screen->name did resolve to the expected screen name.

                        If you have the capacity, you could try running my code (linked above). It‘s actually a window-decoration for KDE! The Readme contains installation-instructions and I added logs that you can read through journalctl.

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                          tim-hilt
                          wrote on 1 Jan 2023, 19:57 last edited by tim-hilt 1 Jan 2023, 19:58
                          #18

                          Some more news about this:

                          1. I used qDebug to log the returned object for QScreen::grabWindow. It prints QPixmap(null)! So the grabWindow-function doesn't seem to work on my machine. Versions are printed below.
                          2. I can observe the same behavior when compiling and running the Screenshot-Example that you guys were referring to earlier. When I execute the compiled binary and try to take a screenshot, this is the output I'm getting:
                          $ ./screenshot
                          QPixmap::scaled: Pixmap is a null pixmap
                          QPixmap::scaled: Pixmap is a null pixmap
                          QPixmap::scaled: Pixmap is a null pixmap
                          

                          Versions:

                          qt5-base: 5.15.7+kde+r177-1
                          kwin_wayland --version: kwin 5.26.4
                          plasmashell --version: plasmashell 5.26.4
                          uname -a: Linux t14s 6.1.1-arch1-1 #1 SMP PREEMPT_DYNAMIC Wed, 21 Dec 2022 22:27:55 +0000 x86_64 GNU/Linux
                          

                          @Christian-Ehrlicher can you give me your guess as to where this issue should be further pursued? Is it likely due to Arch Linux, KDE or Qt?

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                            tim-hilt
                            wrote on 1 Jan 2023, 21:09 last edited by
                            #19

                            I have since found out that grabWindow doesn’t work under Wayland. So I either have to find another way or resort to X11 again.

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                            31 Dec 2022, 14:18

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