[Qt-interest] MainWindow::paintEvent() constantly called

[Oliver Demetz]

Hi Mike,
I followed your thread, and was interested in the "webcam-part" of it.
Did you write the webcam interface yourself, or is it a library you're 
using? Especially, I am searching for a portable implementation that 
works on various platforms.

Regards,
Oliver

Alexandre Courbot schrieb:
>> The painEvent() is called constantly, if I don't prevent it.
>> The side effects are that the image on screen is constantly updated and the
>> CPU is overworked. This is not what I want.
>> I want an image update every 1 or two seconds.
>>
>> void MainWindow::Setup()
>> {
>>   pView = ui->ViewLable;
>>   pCamera = new CCamera(CSize(CameraImageWidth, CameraImageHeight));
>>   pQImage = NULL;
>>   GrabTimer = new QTimer(this);
>>   connect(GrabTimer, SIGNAL(timeout()), this, SLOT(update()));
>>   GrabTimer->start(2000);
>> }
>>
>> // If I don't do the "Doneit" nonsense, this function will be called
>> repeatedly, immediately after it exits.
>> //
>> // If I disable the timer and remove the "Doneit" code
>> // the ViewLable ('pView' as it's called here) is constantly updated with
>> camera output
>> // i.e. there is a constant flow of live frames to the ViewLable
>>
>> void MainWindow::paintEvent(QPaintEvent *)    // how do I use QPaintEvent?
>> {
>>   static bool Doneit = false;
>>
>>   if(!Doneit)
>>   {
>>       if(pCamera->IsValid())
>>       {
>>           MyImage *p = pCamera->GetImage();
>>           delete pQImage;
>>           pQImage = new QImage(p->GetPixelAddress(0, 0), p->GetWidth(),
>> p->GetHeight(), QImage::Format_RGB888);
>>           QPixmap newPixMap = QPixmap::fromImage(*pQImage);
>>           pView->setScaledContents(true);
>>           pView->setPixmap(newPixMap);
>>           Doneit = true;
>>       }
>>   }
>>   else
>>       Doneit = false;
>> }
> 
> I guess your problem is that you trigger a paint event by yourself
> within the paintEvent method. I'm not exactly sure what's wrong
> though. What happens if you comment the setPixmap call out in
> paintEvent? (not sure about it, but maybe that method calls update()
> on its parent to schedule another paint event?)
> 
> Maybe a cleaner way to do that would be to have a custom slot called
> by the timer (instead of update()) and to do everything in that slot.
> You GUI will then be updated appropriately. In any case, you should
> try to avoid overloading paintEvent as much as possible since it's the
> best way to have this kind of problems.
> 
>> I just realized that I hit reply to your email and sent it to your gmail. Should
>> I have sent it to qt-interest at trolltech.com?
> 
> Actually, yes. Please use "reply to all" for mailing-list messages.
> 
>> Sorry, first time user.
> 
> No problem, you're welcome.
> 
> Alex.
> 
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