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Qt-interest Archive, March 2002
Need for speed vs. QApplication::processEvents()

• Subject: Need for speed vs. QApplication::processEvents()
• From: mh <crapsite@xxxxxxx>
• Date: Tue, 5 Mar 2002 11:22:40 +0100
• To: qt-interest@xxxxxxxxxxxxx

Hi,
in my app I have a couple of imagefilters (blur,rotate and the like).
Since those operations may take a while, I'm using the following approach:

<pseudo code>
bool SomeFilter::applyFilter(QImage* image)
{
  int progress;//range 0 - 100
  while(processing_image)
  {
    //do lots of stuff here
    if(user_stopped_the_show)
      return false;
    if((progress % 10) == 0)
    {
      emit signalProgress(progress);
      qApp->processEvents();
    }
  }
  return true;
}

signalProgress(int) is connected to a slot in a QMainWidget derived class, 
which updates a QProgressWidget. There's also a stop button to abort the 
operation. This works rather well, however, the performance drops enormously 
just because of this qApp->processEvents() call. If I let applyFilter() 
block, it's more than three times faster. Does someone know a faster 
(non-blocking) method or has some ideas how to speed this up?
I'm using Qt2.3.1 (no thread support).

Thanks
Michael