Qt::WA_TranslucentBackground available everywhere?

Question

I have a custom widget to emulate balloon tips. To be displayed properly, the widget depends on the QWidget attribute Qt::WA_TranslucentBackground. My application should work on all major platforms (Windows XP, Windows 7, Linux, Mac), so I worry a bit: Is this attribute available on all major platforms? If not, can I query if it is? testAttribute() doesn't do that. It only returns whether the attribute has been set, not whether setting it has an effect.

Answer 1

For Linux you should check whether compositing is enabled:

bool QX11Info::isCompositingManagerRunning() [static]

e.g.

#ifdef Q_WS_X11
    if(QX11Info::isCompositingManagerRunning())
        setAttribute(Qt::WA_TranslucentBackground);
#endif

This question is old, but this might help someone.

Answer 2

This should work with the only exception of Linux over X11 when this is configured not to support ARGB. Refer to the QWidget documentation:

Creating Translucent Windows

Since Qt 4.5, it has been possible to create windows with translucent regions on window systems that support compositing.

To enable this feature in a top-level widget, set its Qt::WA_TranslucentBackground attribute with setAttribute() and ensure that its background is painted with non-opaque colors in the regions you want to be partially transparent.

Platform notes:

X11: This feature relies on the use of an X server that supports ARGB visuals and a compositing window manager.

Windows: The widget needs to have the Qt::FramelessWindowHint window flag set for the translucency to work.

Consider reading also the paragraph titled "Transparency and Double Buffering", might be interesting.