[Interest] QPrintPreviewWidget

Dan Allen dan at jerber.co.uk
Tue Apr 4 19:47:01 CEST 2017

No problem. Thanks Roland.

On 04/04/17 18:32, Roland Hughes wrote:
> Not ignoring your question Bill, but this response answered it better.
> There is always __something__ which gets in the way. Ultimately you 
> end up needing an object which can only exist in the primary thread 
> due to the tight coupling of that thread to the underlying windowing 
> system and that object being "GUI" instead of data.
> On 04/04/2017 12:00 PM, interest-request at qt-project.org wrote:
>> I do not think that is an issue.
>> I think you can paint to a QImage in a non-GUI thread, because
>> a QIMage is a non-GUI object, unlike a QPixmap which is (or at least
>> was historically) a "GUI" object.
>> The term GUI (in this conversation and in my understanding)
>> is generally when you interact with the underlying window system to
>> create and use Windows which appear on the screen.
>> Yes, the QPainter class lives in the QtGUI module, but I do not
>> believe it's use is restricted to the main thread. On Linux and
>> in the past, interaction with the X11 server was limited to the main
>> thread. That is why use of QPixmap was limited to the main thread.
>> That is why use of on screen Windows was limited to the main thread.
>>   From the QPixmap doc:
>>       "...pixel data in a pixmap is internal and is managed
>>        by the underlying window system."
>> So (at least in the past) when you draw on a QPixmap, instructions
>> are being sent to the "underlying window system" and (in the past)
>> that was across a network connection to the X11 server.
>> How any of this has changed with the introduction
>> of QPA (Qt Platform Abstraction) I do not know.

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