[PyQt] Need to draw rotating transparant image... without flickering!

[David Boddie]

On Sun, 30 Dec 2007 15:27:56 -0800 (PST), Wim wrote:

> This is what I would like to do (simplified): I have 2 images (dome.png,
> telescope.png) which are 100x100px, transparant and are meant to be painted
> on top of each other, each rotated under a certain angle. These 2 angles
> can vary with time, and I would like the painting on the screen to be
> updated with these new angles, say at a frequency of once per second.
>
> My solution (simplified):

[...]

>     def paintEvent(self, ev):
>         p = QPainter()
>         p.begin(self.buffer)
>         p.translate(50,50)
>
>         p.save()
>         p.rotate(self.telescopeAngle)
>         p.drawImage(QRect(-205,-205,410,410),self.telescopeimage)
>         p.restore()
>        
>         p.save()
>         p.rotate(self.domeAngle)
>         p.drawImage(QRect(-205,-205,410,410),self.domeimage)
>         p.restore()
>
>         p.flush()
>         p.end()
>         bitBlt(self, 0, 0, self.buffer)
>
> It works, but the problem is that it flickers! The current displayed
> painting is "removed" at the first drawImage, and it takes until the bitBlt
> before it is painted again!

It's been a while since I looked at this kind of problem. Have you tried
experimenting with the updateEnabled property?

  http://doc.trolltech.com/3.3/qwidget.html#updatesEnabled-prop

You could also try setting the WNoAutoErase flag on the widget:

  http://doc.trolltech.com/3.3/qt.html#WidgetFlags-enum

The (C++) Qt 3 book contains an example of using bitBlt() to avoid flicker,
and there's an old Qt Quarterly article that might be relevant to you - if
you're comfortable with C++, though it should be readable even if you're
not:

  http://doc.trolltech.com/qq/qq06-flicker-free.html

David