[Qt-interest] Trying To Understand Transparent Background

[Cheung Yik Pang]

Francisco Ares wrote:
> On Mon, Oct 5, 2009 at 5:05 AM, Girish Ramakrishnan 
> <girish at forwardbias.in <mailto:girish at forwardbias.in>> wrote:
>
>     Cheung Yik Pang wrote:
>     > Girish Ramakrishnan wrote:
>     >> Cheung Yik Pang wrote:
>     >>
>     >>> Girish Ramakrishnan wrote:
>     >>>
>     >>>> Cheung Yik Pang wrote:
>     >>>>
>     >>>>
>     >>>>> Hi all,
>     >>>>>
>     >>>>> I have a questions about using transparent color as background
>     >>>>> color of
>     >>>>> widgets.
>     >>>>>
>     >>>>> My little test program basically move a blue widget around in a
>     >>>>> transparent widget for every 5 seconds.  Both widgets had
>     >>>>> autoFillBackground set to true.
>     >>>>>
>     >>>>> My expectation is that when the blue widget moved from area A to
>     >>>>> area B
>     >>>>> in the parent widget, area A will be cleared with
>     transparent color
>     >>>>> and
>     >>>>> area B will be blue.  But what I get is both area A and area
>     B became
>     >>>>> blue.
>     >>>>>
>     >>>>> I have tried the new 4.5.3 and the 4.6 git head on Windows.
>      Both have
>     >>>>> the same result.
>     >>>>>
>     >>>> What you are seeing is expected behavior. Maybe you can tell
>     us what
>     >>>> are
>     >>>> you trying to achieve? Are you trying to make your window
>     transparent
>     >>>> i.e one can see through your window? If so, this is not the
>     correct
>     >>>> approach - see WA_TranslucentWindow.
>     >>>>
>     >>>> Girish
>     >>>>
>     >>> Thank you for replying.
>     >>>
>     >>> And yes, I am trying to have a transparent window.  Frankly, I
>     am a
>     >>> little bit confused.
>     >>>
>     >>> Qt::WA_TranslucentBackground implies WA_NoSystemBackground,
>     which means
>     >>> I need to fill the background with Qt::transparent in every
>     >>> paintEvent(), right?  But how is it different from
>     >>> setAutoFillBackground(true) with QPalette::Window set to
>     >>> Qt::transparent?
>     >>>
>     >>>
>     >>
>     >> That's correct. Here's how I can explain the situation in the
>     X11 world.
>     >> With WA_TranslucentBackground, you ask Qt to choose ARGB
>     visuals for
>     >> your application window (I think in the Windows world it's called a
>     >> 'transparent' layer or something). Without ARGB visuals and a
>     >> compositing windowing manager, top level transparency does not
>     work.
>     >>
>     >> Now, you can argue that you want setting a Qt::transparent
>     background to
>     >> automatically imply WA_TranslucentBackground but the fact is
>     that it
>     >> isn't that way :) Qt simply hasn't been special cased for
>     tlw/windows
>     >> having a autofillbackground with a non-opaque color. You can
>     report a
>     >> bug, if you like.
>     >>
>     >
>     > Ah, so WA_TranslucentBackground affects windows management,
>     instead of
>     > the widget itself.  That explains a lot.  Thanks :)
>     >
>     >
>     >>
>     >>> And let's forget about the transparent window thing for a
>     minute.  Why
>     >>> does Qt refuse to fill the background if QPalette::Window is
>     >>> Qt::tranpsarent, but not Qt::black?
>     >>>
>     >>>
>     >>
>     >> As I said above, autoFillBackground with non-opaque colors
>     doesn't work
>     >> on tlw/windows. The consequence is that in the absence of argb
>     visuals,
>     >> your background is basically garbage since your app doesn't
>     clear the
>     >> existing background after expose.
>     >>
>     >
>     > So I take this as: If the native window manager is not aware of the
>     > alpha channel of the widget, whatever native call fillRect() maps to
>     > will take no effect for non-opaque colors.  Right?
>     >
>
>     Almost right. It does have an effect, you just end up painting a
>     transparent color over garbage :) You can verify this is painting with
>     some transparent color (you should cause I haven't :)).
>
>     Girish
>     _______________________________________________
>     Qt-interest mailing list
>     Qt-interest at trolltech.com <mailto:Qt-interest at trolltech.com>
>     http://lists.trolltech.com/mailman/listinfo/qt-interest
>
>
>
> There's an article in Qt Quarterly #30, along with a ZIPed sample 
> code, that may help: http://doc.trolltech.com/qq/

Thanks for the reference.  It is nice to know how to make transparent 
windows work.  Yet I also want to know how Qt deals with transparency, 
so that I will not make similar mistakes later.  That is why I keep 
posting follow-ups in this thread.  Please bear with me :)

Regards,
Cheung

>
> Francisco
>
> -- 
> "If you have an apple and I have an apple and we exchange apples then 
> you and I will still each have one apple. But if you have an idea and 
> I have one idea and we exchange these ideas, then each of us will have 
> two ideas." - George Bernard Shaw
> ------------------------------------------------------------------------
>
> _______________________________________________
> Qt-interest mailing list
> Qt-interest at trolltech.com
> http://lists.trolltech.com/mailman/listinfo/qt-interest
>