[Qt-interest] Trying To Understand Transparent Background

[Girish Ramakrishnan]

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.

> 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.

Girish