清除QWidget的透明背景

Question

I have a widget as on overlay for another widget. The transparency works fine, as long as I don't clear the background of the overlay.

But I have to clear the widget to acutalize the displayed "effects". I tried to solve the inital problem (the background getting the default color) like described in 20848594 , but except changing the color to black it had no effect...

Has anyone an idea why the widget which should be transparent does not display the underlaying widget(s)?

---

Here the code:

SudokuMarkerOverlayWidget::SudokuMarkerOverlayWidget(QWidget* parent, uint const fieldSize) : QWidget(parent)
{
    // Translucent should be the correct one
    setAttribute(Qt::WA_TranslucentBackground);
    //setAttribute(Qt::WA_NoSystemBackground);
    setAttribute(Qt::WA_TransparentForMouseEvents);

    ...
}

void SudokuMarkerOverlayWidget::paintEvent(QPaintEvent*)
{
    QPainter painter(this);
    painter.setRenderHint( QPainter::Antialiasing );

    // Tried this too, no difference
    // painter.setCompositionMode(QPainter::CompositionMode_Source);
    // painter.fillRect( this->rect(), Qt::transparent );
    painter.setCompositionMode(QPainter::CompositionMode_Clear);
    painter.eraseRect( this->rect() );
    painter.setCompositionMode(QPainter::CompositionMode_SourceOver);

    ...
}

EDIT: Just noticed, using CompositionMode Source instead of SourceOver for my semi-transparent painting (gradient seen in first image) also causes them to be a red-to-black-gradient instead of red-to-transparent.

That means every transparency except the inital by WA_TranslucentBackground or WA_NoSystemBackground isn't working.

Answer 1

Widgets in Qt can be either a 'native' or 'alien' type . In one case they're a separate operating system window. In some operating systems a transparent window isn't supported.

You might want to consider using QML if you're after fancy visual effects. That's what it was designed for.