Qt Directx rendering - c++

Question

I'm rendering in Qt widget by DirectX.
I disabled qt paint engine
QPaintEngine *paintEngine() const { return NULL; }
but sometimes I want to enable qt paint and disable directX rendering.
I'm disabling DirectX rendering, but how can I enable Qt paintEngine?

Answer 1

The possible solutions include:
Use a QWindow, and then switch rendering between [raster][0] and DirectX.
Use a DirectX-rendered QWindow or QWidget, and render the optional content to a QImage. You can use that image as a texture and easily overlay it over the DirectX-rendered content.
A quick workaround with little code changes would be to re-create the widget each time the rendering mode changes. This could be implemented very cleanly by having the state of the widget reside in a PIMPL. See the example below.
class MyWidgetPrivate {
public:
bool qtRendering;
// your data members etc.
MyWidgetPrivate(bool qtRendering) :
qtRendering(qtRendering)
{}
};
class MyWidget : public QWidget {
Q_OBJECT
Q_DECLARE_PRIVATE(MyWidget)
QScopedPointer<MyWidgetPrivate> const d_ptr;
QPaintEngine *paintEngine() const {
Q_D(const MyWidget);
return d->qtRendering ? QWidget::paintEngine() : nullptr;
}
MyWidget(QScopedPointer<MyWidgetPrivate> & data, QWidget * parent, bool qtRendering) :
QWidget(parent),
d_ptr(data.take())
{
d_ptr->qtRendering = qtRendering;
}
public:
MyWidget(QWidget * parent, bool qtRendering) :
QWidget(parent),
d_ptr(new MyWidgetPrivate(qtRendering))
{}
void setQtRendering(bool qtRendering) {
if (qtRendering == d_ptr->qtRendering) return;
auto geom = geometry();
auto parent = this->parentWidget();
auto & d = const_cast<QScopedPointer<MyWidgetPrivate>&>(d_ptr);
QScopedPointer<MyWidgetPrivate> pimpl(d.take());
this->~MyWidget(); // destroy in place
new (this) MyWidget(pimpl, parent, qtRendering); // reconstruct in place
setGeometry(geom);
}
};

Answer 2

Use
QWidget::setAttribute(Qt::WA_OpaquePaintEvent, false);
to reenable Qt background drawing on your widget.