Qt-interest Archive, March 2007
Re: Semi- transparent QWidget
Message 1 in thread
Thanks Volker,
Really good explained, thanks again. You message make me understand the
mechanism behind new Qt 4. backingstore" - "shining" principles. I'll try to
make it as scenario 1 from your message it's simple seams most logical/
reliable and I trust Qt approach / I can not say the same for some OS and
companies as M$/
Cheers,
Julian
"Volker Hilsheimer" <unwatched@xxxxxxx> wrote in message
news:es3vpe$reb$1@xxxxxxxxxxxxxxxxxxxxx
>> I succeed to make it. It works perfect.
>>
>> My problem was / and still it is/ that I can not draw semi transparent
>> rectangles over Qwiget which / here is the key moment/ playing Video .
>> What I do is adding the MPlayer into a Widget and want to add semi
>> transparent description as " What is this in Qt -Demo".
>>
>> I succeed to make a fully transparent QImage and to draw with QPainter
>> over it and everything works fine. The problem is when I draw a semi
>> transparent thing it comes as fully opaque. That is situation only over
>> MPlayer running into Widget otherwise works perfectly.
>>
>> Do you have any idea how I can make it?
>
>
> Qt's support for semi transparent widget really is a feature of Qt; it's
> not a feature of the windowing system that Qt just happened to start using
> in version 4.2.
>
> For this to work, all widgets in the widget hierarchy have to cooperate.
> I.e. all of them have to paint themselves into Qt's "backingstore", so
> that Qt can correctly compose the look of the complete window. Any widget
> that does not render into the backingstore will not, and cannot, shine
> through other widgets that are on top, or of widgets below shine through.
>
> This means that any QWidget that has the PaintOnScreen flag set (i.e.
> QGLWidget), and any window that is not a Qt widget, cannot participate in
> the semi-transparency of Qt, unless you jump through several hopps to make
> it work. I.e. you could just not show the MPlayer at all, but instead have
> it provide the frames that you then paint into the QWidget using a
> paintEvent implementation. However, this is naturally going to be very
> slow and very heavy on the CPU (and I don't know enough about MPlayer to
> say if you can do this in the first place).
>
> Alternatively, you can use toplevel widgets and modify their opacity. Qt
> doesn't know what is "behind" a toplevel windows - only the windowing
> system knows. So the windowing system, if it supports the notion of semi
> transparent toplevel windows, can take care of the composition. But this
> might again fail (or at least cause drastic performance decrease) in some
> cases, i.e. your OpenGL rendering might start to go through the windowing
> system rather than directly blt to the framebuffer.
>
>
> Volker
>
>
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