"QPaintEngine* MyQWidget::paintEngine() const" Never called!

st.germain

Hi,
I try to do an offscreen "display" without OpenGL.
I try to do something similar to this:
https://parich.us/blog/2019/07/02/qt-widgets-vulkan.html

Unfortunately, my paintEngine overload is never called.

For now, I only have
.hpp:

class MyQWidget: public QWidget
{
public:
    QPixmap m_oTheBackendPixmap;
    QPaintEngine *paintEngine() const override;
};

.cpp:

QPaintEngine* MyQWidget::paintEngine() const
{
    qDebug() << "MyQWidget::paintEngine()";
    return m_oTheBackendPixmap.paintEngine();
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    mainWindowWidget = new MyQWidget; 
    this->setCentralWidget(mainWindowWidget);
...

But my main windows are drawn as before, and I never see my debug message "MyQWidget::paintEngine()"

Someone can help me to see what I miss?

Thanks,

Stéphane Germain

st.germain

Don't look for it anymore, I got it.
For info, far in QWidget doc, we can read:

    //If you are using a custom paint engine without Qt's backingstore,
    //  Qt::WA_PaintOnScreen must be set. Otherwise
    //  QWidget::paintEngine() will never be called;
    //  the backingstore will be used instead.
    setAttribute(Qt::WA_PaintOnScreen);

It was that!

Thanks
Stéphane

SGaist

Hi and welcome to devnet,

What version of Qt are you using ?
What OS are you running ?