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Should I use QDialog widget to draw line?

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  • B Bonnie

    Just as the error told you: QGraphicsScene is not a QPaintDevice, you cannot use QPainter like that.
    There's even not a paintEvent in QGraphicsScene, only QGraphicsView have that.
    The example draws on a non-graphics QWidget, but you are not.
    If you want to draw like this, then don't use QGraphicsView / QGraphicsScene, just inherit a QWidget.

    L Offline
    L Offline
    lansing
    wrote on last edited by
    #7

    @Bonnie

    Sorry I got mixed up with the suggestion here to always use QGraphicsScene to draw. I will use QGraphicsView to see how it goes.

    B 1 Reply Last reply
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    • L lansing

      @Bonnie

      Sorry I got mixed up with the suggestion here to always use QGraphicsScene to draw. I will use QGraphicsView to see how it goes.

      B Offline
      B Offline
      Bonnie
      wrote on last edited by Bonnie
      #8

      @lansing
      Just pick from those two methods (QGraphicsScene + QGraphicsView / QWidget + QPainter), they are totally different.
      You shouldn't "paint" by yourself if you use QGraphicsScene + QGraphicsView.
      Use QGraphicsScene::addLine to add the line, the painting part will be finished by themselves.

      L 1 Reply Last reply
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      • B Bonnie

        @lansing
        Just pick from those two methods (QGraphicsScene + QGraphicsView / QWidget + QPainter), they are totally different.
        You shouldn't "paint" by yourself if you use QGraphicsScene + QGraphicsView.
        Use QGraphicsScene::addLine to add the line, the painting part will be finished by themselves.

        L Offline
        L Offline
        lansing
        wrote on last edited by
        #9

        @Bonnie

        Oh, I am looking to draw like in photoshop. So I should be using QWidget + QPainter?

        B 1 Reply Last reply
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        • L lansing

          @Bonnie

          Oh, I am looking to draw like in photoshop. So I should be using QWidget + QPainter?

          B Offline
          B Offline
          Bonnie
          wrote on last edited by Bonnie
          #10

          @lansing
          Depending on how complicated it will be.
          If you only want to draw like "press, drag, release, done", then you may use QWidget + QPainter.
          But you need to save the lines you draw as member variables (the Scribble example save them to a QImage).
          If you want move or do other transform operations to the drawn lines, you need to use QGraphicsScene + QGraphicsView.
          You may also look at this example: Diagram Scene Example
          It have some line drawing thing in a QGraphicsScene (but in this example the line will be deleted when releasing the mouse)

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          • L Offline
            L Offline
            lansing
            wrote on last edited by
            #11

            I have followed the scribble example to use QWidget and QPainter to draw line. I have used QPixmap to hold the line drawing instead of QImage from the example. The drawing works fine until I centered the QPixmap in paintEvent(), my drawing wasn't centered along with the QPixmap.

            void Canvas::mousePressEvent(QMouseEvent *a_pEvent)
            {
                if (a_pEvent->button() == Qt::LeftButton) {
                    m_drawing = true;
                    m_lastPoint =  a_pEvent->pos();
                }
            }
            
            void Canvas::mouseMoveEvent(QMouseEvent *a_pEvent)
            {
                if ((a_pEvent->buttons() & Qt::LeftButton) && m_drawing) {
                    drawPointTo(a_pEvent->pos());        
                }
            }
            
            void Canvas::paintEvent(QPaintEvent *event)
            {
                QPainter painter(this);
                /* center image */
                painter.drawPixmap((rect().bottomRight() - m_drawingPixmap.rect().bottomRight()) / 2, m_drawingPixmap); 
                painter.end();
            }
            
            void Canvas::drawPointTo(const QPointF &a_endPoint)
            {
                QPainter painter(&m_drawingPixmap);
                painter.setPen(QPen(Qt::black, 2));
                painter.drawLine(m_lastPoint, a_endPoint);
                update();
                m_lastPoint = a_endPoint;
            }
            
            
            B 1 Reply Last reply
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            • L lansing

              I have followed the scribble example to use QWidget and QPainter to draw line. I have used QPixmap to hold the line drawing instead of QImage from the example. The drawing works fine until I centered the QPixmap in paintEvent(), my drawing wasn't centered along with the QPixmap.

              void Canvas::mousePressEvent(QMouseEvent *a_pEvent)
              {
                  if (a_pEvent->button() == Qt::LeftButton) {
                      m_drawing = true;
                      m_lastPoint =  a_pEvent->pos();
                  }
              }
              
              void Canvas::mouseMoveEvent(QMouseEvent *a_pEvent)
              {
                  if ((a_pEvent->buttons() & Qt::LeftButton) && m_drawing) {
                      drawPointTo(a_pEvent->pos());        
                  }
              }
              
              void Canvas::paintEvent(QPaintEvent *event)
              {
                  QPainter painter(this);
                  /* center image */
                  painter.drawPixmap((rect().bottomRight() - m_drawingPixmap.rect().bottomRight()) / 2, m_drawingPixmap); 
                  painter.end();
              }
              
              void Canvas::drawPointTo(const QPointF &a_endPoint)
              {
                  QPainter painter(&m_drawingPixmap);
                  painter.setPen(QPen(Qt::black, 2));
                  painter.drawLine(m_lastPoint, a_endPoint);
                  update();
                  m_lastPoint = a_endPoint;
              }
              
              
              B Offline
              B Offline
              Bonnie
              wrote on last edited by Bonnie
              #12

              @lansing
              I think that's not hard to understand.
              If your pixmap's top left isn't the same with the widget, then they have different coordinate system.
              You need to map your points' coordinates from the widget to the pixmap when saving.

              L 1 Reply Last reply
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              • B Bonnie

                @lansing
                I think that's not hard to understand.
                If your pixmap's top left isn't the same with the widget, then they have different coordinate system.
                You need to map your points' coordinates from the widget to the pixmap when saving.

                L Offline
                L Offline
                lansing
                wrote on last edited by
                #13

                @Bonnie

                How do I do that? I did a few trials and errors but still couldn't get it right.

                So I ended up subclassing a QLabel class and added it to the canvas, and then set the pixmap and move all the paintEvent, mouse event functions to this label class.

                Do I not need to do this?

                B 1 Reply Last reply
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                • L lansing

                  @Bonnie

                  How do I do that? I did a few trials and errors but still couldn't get it right.

                  So I ended up subclassing a QLabel class and added it to the canvas, and then set the pixmap and move all the paintEvent, mouse event functions to this label class.

                  Do I not need to do this?

                  B Offline
                  B Offline
                  Bonnie
                  wrote on last edited by
                  #14

                  @lansing
                  If you feel the mapping difficult I think it is no problem to put a centered "painting QWidget" inside your canvas.
                  But what's wrong with just subclass a QWidget and add that to the canvas?
                  Do you have a reason that you must use a QLabel?

                  L 1 Reply Last reply
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                  • B Bonnie

                    @lansing
                    If you feel the mapping difficult I think it is no problem to put a centered "painting QWidget" inside your canvas.
                    But what's wrong with just subclass a QWidget and add that to the canvas?
                    Do you have a reason that you must use a QLabel?

                    L Offline
                    L Offline
                    lansing
                    wrote on last edited by
                    #15

                    @Bonnie

                    The QLabel is the "centered Qwidget" you talked of. Because it can hold a QPixmap for the line drawing, have all the mouse event functions and paintEvent, as well as able to retrieve the position.

                    canvas

                    Canvas::Canvas(QWidget *a_pParent): QWidget(a_pParent),  
                    {
                        m_drawingLabel = new DrawingLabel(this);
                        QHBoxLayout *layout = new QHBoxLayout();
                        layout->addWidget(m_drawingLabel );
                        setLayout(layout);
                    }
                    
                    void Canvas::drawFrame(const QPixmap &a_drawingPixmap)
                    {
                        // center the Qlabel when loading in the pixmap
                        int x = (rect().width() - a_drawingPixmap.width()) / 2 ;
                        int y = (rect().height() - a_drawingPixmap.height()) /2;
                        m_drawingLabel ->setGeometry (x, y, a_drawingPixmap.width(), a_drawingPixmap.height());
                    
                        m_drawingLabel ->setDrawingPixmap(a_drawingPixmap);
                    }
                    

                    qlabel

                    void DrawingLabel::paintEvent(QPaintEvent *event)
                    {
                        QPainter painter(this);
                        painter.drawPixmap(0,0,m_drawingPixmap);
                        painter.end();
                    }
                    
                    void DrawingLabel::setDrawingPixmap(const QPixmap & a_pixmap)
                    {
                        m_drawingPixmap= a_pixmap;
                        update();
                    }
                    
                    void DrawingLabel::mousePressEvent(QMouseEvent *a_pEvent)
                    {
                        if (a_pEvent->button() == Qt::LeftButton) {
                            m_drawing = true;
                            m_lastPoint = a_pEvent->pos();
                        }
                    }
                    ...
                    mousePressEvent()
                    mouseReleaseEvent()
                    drawPointTo()
                    
                    B 1 Reply Last reply
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                    • L lansing

                      @Bonnie

                      The QLabel is the "centered Qwidget" you talked of. Because it can hold a QPixmap for the line drawing, have all the mouse event functions and paintEvent, as well as able to retrieve the position.

                      canvas

                      Canvas::Canvas(QWidget *a_pParent): QWidget(a_pParent),  
                      {
                          m_drawingLabel = new DrawingLabel(this);
                          QHBoxLayout *layout = new QHBoxLayout();
                          layout->addWidget(m_drawingLabel );
                          setLayout(layout);
                      }
                      
                      void Canvas::drawFrame(const QPixmap &a_drawingPixmap)
                      {
                          // center the Qlabel when loading in the pixmap
                          int x = (rect().width() - a_drawingPixmap.width()) / 2 ;
                          int y = (rect().height() - a_drawingPixmap.height()) /2;
                          m_drawingLabel ->setGeometry (x, y, a_drawingPixmap.width(), a_drawingPixmap.height());
                      
                          m_drawingLabel ->setDrawingPixmap(a_drawingPixmap);
                      }
                      

                      qlabel

                      void DrawingLabel::paintEvent(QPaintEvent *event)
                      {
                          QPainter painter(this);
                          painter.drawPixmap(0,0,m_drawingPixmap);
                          painter.end();
                      }
                      
                      void DrawingLabel::setDrawingPixmap(const QPixmap & a_pixmap)
                      {
                          m_drawingPixmap= a_pixmap;
                          update();
                      }
                      
                      void DrawingLabel::mousePressEvent(QMouseEvent *a_pEvent)
                      {
                          if (a_pEvent->button() == Qt::LeftButton) {
                              m_drawing = true;
                              m_lastPoint = a_pEvent->pos();
                          }
                      }
                      ...
                      mousePressEvent()
                      mouseReleaseEvent()
                      drawPointTo()
                      
                      B Offline
                      B Offline
                      Bonnie
                      wrote on last edited by Bonnie
                      #16

                      @lansing
                      I mean I don't think it is necessary to use a QLabel as the centered widget since you don't seem to use any QLabel functions.
                      Why can't you just subclass a QWidget as the "DrawingLabel"?

                      And also, why are you setting the geometry while you are using a layout?
                      That don't make any sense.

                      L 1 Reply Last reply
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                      • B Bonnie

                        @lansing
                        I mean I don't think it is necessary to use a QLabel as the centered widget since you don't seem to use any QLabel functions.
                        Why can't you just subclass a QWidget as the "DrawingLabel"?

                        And also, why are you setting the geometry while you are using a layout?
                        That don't make any sense.

                        L Offline
                        L Offline
                        lansing
                        wrote on last edited by lansing
                        #17

                        @Bonnie

                        I can change to that, I just need some wrapper that can do all the things I mentioned above.

                        The layout was used to add the QLabel to the canvas.

                        B T 2 Replies Last reply
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                        • L lansing

                          @Bonnie

                          I can change to that, I just need some wrapper that can do all the things I mentioned above.

                          The layout was used to add the QLabel to the canvas.

                          B Offline
                          B Offline
                          Bonnie
                          wrote on last edited by Bonnie
                          #18

                          @lansing
                          You either use a layout to keep the label centered, or set the geometry by yourself and don't use any layout.
                          Set geometry to a widget managed by a layout is meaningless, the layout may reset its geometry at any time.
                          You can use the layout like that:

                          layout->addWidget(m_drawingLabel, Qt::AlignCenter);
                          
                          void Canvas::drawFrame(const QPixmap &a_drawingPixmap)
                          {
                              m_drawingLabel->setFixedSize(a_drawingPixmap.size());
                              m_drawingLabel->setDrawingPixmap(a_drawingPixmap);
                          }
                          
                          1 Reply Last reply
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                          • L lansing

                            @Bonnie

                            I can change to that, I just need some wrapper that can do all the things I mentioned above.

                            The layout was used to add the QLabel to the canvas.

                            T Offline
                            T Offline
                            Thomas Stein
                            wrote on last edited by
                            #19
                            This post is deleted!
                            1 Reply Last reply
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                            • L Offline
                              L Offline
                              lansing
                              wrote on last edited by
                              #20

                              I'm not spreading myself too thin right now, I'll stick to one method first until it worked well before I try other methods.

                              So I got it working under normal condition. I have changed the canvas to subclass a QScrollArea because I need to zoom in. I did the zoom function in the wheelEvent(). However, the problem now is with the QPixmap.scaled function, the more I zoomed in, the more laggy the program got.

                              void Canvas::wheelEvent(QWheelEvent *a_pEvent)
                              {
                                  QPoint delta = a_pEvent->angleDelta();
                                  // zooming
                                  if (delta.y() > 0) {
                                      m_zoomRatio += 0.6;
                                  } else {
                                      m_zoomRatio -= 0.6;
                                  }
                                  m_drawingLabel->setFixedSize(m_drawingPixmap.size() * m_zoomRatio);
                              
                                  QSize zoomSize = m_drawingPixmap.size() * m_zoomRatio;
                                  m_drawingLabel->setDrawingPixmap(m_drawingPixmap.scaled(zoomSize));
                              }
                              
                              void Canvas::drawFrame(const QPixmap &a_drawingPixmap)
                              {
                                  m_drawingPixmap = a_drawingPixmap; // save a copy for zooming
                                  
                                  m_drawingLabel->setFixedSize(a_drawingPixmap.size());
                                  m_drawingLabel->setDrawingPixmap(a_drawingPixmap);
                              }
                              
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                              • B Offline
                                B Offline
                                Bonnie
                                wrote on last edited by
                                #21

                                Oh, if you need QScrollArea + zoom, I'm afraid QGraphics* classes are more suitable for that.
                                For current method, I don't know if this will take any affect, but you can try not scaling the pixmap, but the painter.
                                Change your paintEvent to

                                void DrawingLabel::paintEvent(QPaintEvent *event)
                                {
                                        QPainter painter(this);
                                        painter.scale(m_zoomRatio, m_zoomRatio);
                                        painter.drawPixmap(0,0,m_drawingPixmap);
                                }
                                

                                And in your wheelEvent, dont' create and set new pixmap, just set the zoomed fixed size.

                                L 1 Reply Last reply
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                                • B Bonnie

                                  Oh, if you need QScrollArea + zoom, I'm afraid QGraphics* classes are more suitable for that.
                                  For current method, I don't know if this will take any affect, but you can try not scaling the pixmap, but the painter.
                                  Change your paintEvent to

                                  void DrawingLabel::paintEvent(QPaintEvent *event)
                                  {
                                          QPainter painter(this);
                                          painter.scale(m_zoomRatio, m_zoomRatio);
                                          painter.drawPixmap(0,0,m_drawingPixmap);
                                  }
                                  

                                  And in your wheelEvent, dont' create and set new pixmap, just set the zoomed fixed size.

                                  L Offline
                                  L Offline
                                  lansing
                                  wrote on last edited by lansing
                                  #22

                                  @Bonnie

                                  Hi, setting the scale factor inside paintEvent() solved the lagging issue. But now I have problem mapping to the scaled position on the drawing. I tried multiplying the x, y with the same zoom ratio, but they still doesn't line up when I tried to draw. What did I do wrong?

                                  void DrawingLabel::mousePressEvent(QMouseEvent *a_pEvent)
                                  {
                                      if (a_pEvent->button() == Qt::LeftButton) {
                                          m_drawing = true;
                                  
                                          double x = double(a_pEvent->pos().x()) * m_zoomRatio;
                                          double y = double(a_pEvent->pos().y()) * m_zoomRatio;  
                                          m_lastPoint = QPointF(x, y);
                                      }
                                  }
                                  
                                  void DrawingLabel::mouseMoveEvent(QMouseEvent *a_pEvent)
                                  {
                                      if ((a_pEvent->buttons() & Qt::LeftButton) && m_drawing) {
                                          double x = double(a_pEvent->pos().x()) * m_zoomRatio;
                                          double y = double(a_pEvent->pos().y()) * m_zoomRatio;
                                          QPointF zoomPos(x,y);
                                  
                                          drawPointTo(zoomPos);
                                      } else {
                                          QWidget::mouseMoveEvent(a_pEvent);
                                      }
                                  }
                                  
                                  void DrawingLabel::drawPointTo(const QPointF &a_endPoint)
                                  {
                                      QPainter painter(&m_drawingPixmap);
                                      painter.setPen(QPen(Qt::black, 2));
                                  
                                      painter.drawLine(m_lastPoint, a_endPoint);
                                      update();
                                      m_lastPoint = a_endPoint;
                                  }
                                  

                                  Also a cosmetic problem, the drew line was also scaled with the pixmap, making it look pixelated when zoomed in, how do I make its look independent from the zoom?

                                  B 1 Reply Last reply
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                                  • L lansing

                                    @Bonnie

                                    Hi, setting the scale factor inside paintEvent() solved the lagging issue. But now I have problem mapping to the scaled position on the drawing. I tried multiplying the x, y with the same zoom ratio, but they still doesn't line up when I tried to draw. What did I do wrong?

                                    void DrawingLabel::mousePressEvent(QMouseEvent *a_pEvent)
                                    {
                                        if (a_pEvent->button() == Qt::LeftButton) {
                                            m_drawing = true;
                                    
                                            double x = double(a_pEvent->pos().x()) * m_zoomRatio;
                                            double y = double(a_pEvent->pos().y()) * m_zoomRatio;  
                                            m_lastPoint = QPointF(x, y);
                                        }
                                    }
                                    
                                    void DrawingLabel::mouseMoveEvent(QMouseEvent *a_pEvent)
                                    {
                                        if ((a_pEvent->buttons() & Qt::LeftButton) && m_drawing) {
                                            double x = double(a_pEvent->pos().x()) * m_zoomRatio;
                                            double y = double(a_pEvent->pos().y()) * m_zoomRatio;
                                            QPointF zoomPos(x,y);
                                    
                                            drawPointTo(zoomPos);
                                        } else {
                                            QWidget::mouseMoveEvent(a_pEvent);
                                        }
                                    }
                                    
                                    void DrawingLabel::drawPointTo(const QPointF &a_endPoint)
                                    {
                                        QPainter painter(&m_drawingPixmap);
                                        painter.setPen(QPen(Qt::black, 2));
                                    
                                        painter.drawLine(m_lastPoint, a_endPoint);
                                        update();
                                        m_lastPoint = a_endPoint;
                                    }
                                    

                                    Also a cosmetic problem, the drew line was also scaled with the pixmap, making it look pixelated when zoomed in, how do I make its look independent from the zoom?

                                    B Offline
                                    B Offline
                                    Bonnie
                                    wrote on last edited by Bonnie
                                    #23

                                    @lansing
                                    Well, I think you don't need to map the points in the mouse events.
                                    Just set the painter of the pixmap to be scaled reversely.
                                    And in order to keep the line width, set the pen width accordingly.

                                        QPainter painter(&m_drawingPixmap);
                                        painter.scale(1/m_zoomRatio, 1/m_zoomRatio);
                                        painter.setPen(QPen(Qt::black, 2/m_zoomRatio));
                                        painter.drawLine(lastPoint, endPoint);
                                    

                                    But I feel it look not "smooth"...

                                    If this line width issue can't be solved, then you should use back your old way, but try to optimize the wheelEvent part.
                                    I think the lagging issue is caused by scaling be called rapidly in a short time (from several wheel events).

                                    L 1 Reply Last reply
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                                    • B Bonnie

                                      @lansing
                                      Well, I think you don't need to map the points in the mouse events.
                                      Just set the painter of the pixmap to be scaled reversely.
                                      And in order to keep the line width, set the pen width accordingly.

                                          QPainter painter(&m_drawingPixmap);
                                          painter.scale(1/m_zoomRatio, 1/m_zoomRatio);
                                          painter.setPen(QPen(Qt::black, 2/m_zoomRatio));
                                          painter.drawLine(lastPoint, endPoint);
                                      

                                      But I feel it look not "smooth"...

                                      If this line width issue can't be solved, then you should use back your old way, but try to optimize the wheelEvent part.
                                      I think the lagging issue is caused by scaling be called rapidly in a short time (from several wheel events).

                                      L Offline
                                      L Offline
                                      lansing
                                      wrote on last edited by
                                      #24

                                      @Bonnie

                                      Hi yes this doesn't work well, the drew line get softer and softer with more zoom, and it gets bigger when zoomed out. I have to revert back to the wheelEvent controlled zoom.

                                      Oh and I just observed that the lagging issue are also presented on Photoshop too. When the zoom factor get to about 7x, zoom will get less responsive. So I'm all good now, if even Photoshop is having the same problem, that's nothing more I can do about it. I'll call this a day.

                                      Thanks for the help.

                                      1 Reply Last reply
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