Can I Get hWnd or DC from QPainter in QGraphicsPixmapItem's paint() when I just need coding in Windows?



  • Can you help to let me get hWnd or DC from QPainter in QGraphicsPixmapItem's paint()?
    I am just coding in Windows. I need to add some features which need hWnd and DC to draw in my object.
    Should I need to change the src of QPainter and re-compile QT5?


  • Moderators

    in your paintEvent() handler you also get a QWidget parameter.
    On that call QWidget::effectiveWinId() and you will get the HWND.



  • Thanks. I do get the hWnd or DC in paint() of subclass of QGraphicsPixmapItem, but what I draw is covered, I cannot see it, how to draw my object with others those using QPainter?



  • The only chance I can see what I draw is when I maximize the app, it flashed in about 0.1 sec then covered by the backgroud of the scene.



  • Can you give me some advice?



  • are you calling QGraphicsPixmapItem::paint(...) (the base class implementation) somewhere? anyway, if you need to have the default painting plus some extra ones just call it at the beginning of your paint(...) implementation

    Cheers!



  • no, I am not calling the base class's paint(), and the draw using QWidget::effectiveWinId() is at the beginning of my paint().
    I guess the problem is caused by the painting mechanism of the Qt scene. All the paints of scene and its object using QPainter are drawn in a memory DC then drawn to the window of QWidget::effectiveWinId()? Can you tell me the truth?



  • I have solved the problem by myself:
    In Qt5, QPaintEngineEx is subclass of QPaintEngine, and QRasterPaintEngine is subclass of QPaintEngineEx, so I can:
    QPaintEngine PE = painter->paintEngine();
    QRasterPaintEngine * pRasterPE = dynamic_cast<QRasterPaintEngine
    > (PE);
    HDC rasperDC = pRasterPE->getDC();

    drawing in rasperDC is not covered by the backgroud of the scene.



  • Hi,

    I have just tested and effectiveWinId() is ok; it returns the HWND of that QWidget from paint event which is the same! as the effectiveWinId() of the graphics view used

    @WId id = widget->effectiveWinId();
    WId id1 = scene()->views().at(0)->effectiveWinId();

    HDC dc = ::GetDC(id);
    ::LineTo(dc,500,500);
    ::ReleaseDC(id,dc);@

    this code in paint event just works ...

    be carefully as I said the device context belongs to the QGraphicsView(inheriting QWidget) used by the scene!

    Cheers!


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