[Interest] QPixmap and threads

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Fri May 5 10:25:45 CEST 2017

Il 05/05/2017 09:05, maitai ha scritto:
> But still, I have hundreds of other QPixmaps used in threads, with no
> problem at all (creating, painting, etc). All is working fine for years,
> without any warning or so from Qt.

Well, not having problems doesn't imply that what you were doing was 

> Should I change also all these
> QPixmaps to QImages (with a conversion cost at the end of the threads)?

Yes. Note that the conversion cost may be extremely small: QPixmap these 
days is simply holding a QImage in the "right format" for display. If 
your QImage is already in that format, only a cheap refcount increase is 
needed, not a full format conversion.

> If QPixmaps are forbidden outside the main GUI thread, why is it not
> written in the documentation?

It is implied by the lack of "This class is reentrant" note in QPixmap 
documentation. It's also documented that way in other places, like


See also slides 27+

> https://conf.qtcon.org/system/attachments/104/original/multithreading-with-qt.pdf?1473018682


Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (UK) Ltd., a KDAB Group company | Tel: UK +44-1625-809908
KDAB - Qt, C++ and OpenGL Experts

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