QPainter ouside of paintEvent with a pixmap

I'm porting a Qt 3.3.2 application to Qt 4.1.1

In Qt 3 you can create a QPainter p(&pixmap) and paint to the pixmap anywhere in the code.

In Qt 4 it says that all painting must take place inside the paintEvent.

Why is that the following will work in Qt 4 outside of paintEvent

Code:

---

void someFunc()
{
  Pixmap pix(size());
  pix.fill(this, 0, 0);
  QPainter p(&pix);   // no error
  // paint away
}

---

but, the following will not.

Code:

---

// here ptrPix was filled by another function
void someFunc()
{
  QPainter p(ptrPix);   // QPainter::begin(), paintdevice returned engine == 0, type: 2
  // paint away
}

---

am i missing a concept here?

I will move all of my painting to the paintEvent so that I can take advantage of the double buffering built in to Qt 4, I just want to know what is going on with the above statements.

Thanks.

Quote:

---

From Qt docs:
Unless a widget has the Qt::WA_PaintOutsidePaintEvent attribute set. A QPainter can only be used on a widget inside a paintEvent() or a function called by a paintEvent().

---

This is about painting on a widget, not on a pixmap.

The first example creates a QPixmap of the widgets size and fills it with the widgets background color or pixmap. Then you can paint on the pixmap with that QPainter.

The second one assigns a prefilled pixmap pointer to a QPainter and then paints on the pixmap.

They seem to be doing the same thing, which neither is painting to a widget.

So, I'm still confused why one would work and one would not.

Please explain more if I'm missing a point.

Do you use threads?

Yes 5 of them in this application.

Do you modify QPixmaps in non-GUI thread? If yes, then you will have to switch to QImages.

After trying some more things I'm confused as to why the following don't work in Qt 4.1.1
This is an application not using threads.

Code:

---

void MyType::paintEvent(QPaintEvent*)
{
  QPainter painter;
  someFunc(&painter);
 
  painter.begin(this);
  painter.drawPixmap(QPoint(0, 0), *pmap);
  painter.end();
}
 
void MyType::someFunc(QPainter *painter)
{
  painter->begin(pmap);
  painter->setPen(dark);
  painter->drawLine(0, 0, 50, 50);
  // .. do something useful
  painter->end();
}

---

How do you create that pmap pixmap?

Code:

---

class MyType : public QWidget
{
  public:
    // ... methods
 
  private:
    QPixmap* pmap;
    QImage* image;
    QColor* backgroundColor;
};
 
void MyType::createImage()
{
  image = new QImage(50, 50, QImage::Format_RGB32);
  image->fill(backgroundColor->rgb());
 
  for (int i=0; i<50; i++)
  {
    QRgb rgb = qRgb(i, i+25, i+50);
    for (int x=0; x<50; ++x)
      image->setPixel(x, i, rgb);
  }
 
  convertImage();
}
 
void MyType::convertImage()
{
  pmap = new QPixmap();
  pmap->fromImage(*image, Qt::PreferDither);
  update();
}

---

Hope this is enough sudo code for you. ;)

QPixmap::fromImage() is static.

It should be something like:

Code:

---

void MyType::convertImage()
{
  pmap = new QPixmap( QPixmap::fromImage(*image, Qt::PreferDither) );
  update();
}

---

Good catch, now it makes sense. Since static member functions have no "this" pointer my usage was wrong. That fixed it.

Thanks again.