Using QPainter to draw on a QPixmap and save it

canellas

Based on a example (that I can not find again) that show the use of a QPainter and `QPdfWriterP , I am trying to create a PNG file like this:

void MainWindow::on_pushButton_5_clicked() {
  QPixmap paintDevice;
  QPainter painter(&paintDevice);

  int x0{10};
  int y0{10};

  const int width{200};
  const int height{200};
  const int cols{5};
  const int rows{3};

  QPen line_pen(Qt::black);
  line_pen.setWidth(5);
  painter.setPen(line_pen);
   
  // calls to drawLine, setPen e setFont among other methods of  `QPainter`

  const QString fileName("/var/tmp/mydoc.png");
  paintDevice.save(fileName, "PNG");
}

But the file /var/tmp/mydoc.png is not generated, and the messages QPainter::setFont: Painter not active and QPainter::setPen: Painter not active are displayed.

I tried to find an example that showed QPainter and QPixmap used this way, but I failed to find.

Does anyone have an example or reference showing how to do this?

TIA

JonB

@canellas Start by checking return result of QPixmap::save(). I suppose it's false.

Is paintDevice still a null QPixmap? I think it is, depending on what code you removed. Hence earlier
@Christian-Ehrlicher said in Using QPainter to draw on a QPixmap and save it:

Your QPixmap is null. You should pass a valid QPixmap to the painter.

I think you have to pass a QPixmap big enough for your painting.

Then maybe the save() will do something. It wants something valid to save.

Christian Ehrlicher

Your QPixmap is null. You should pass a valid QPixmap to the painter.

canellas

@Christian-Ehrlicher

I understood QPixmap default constructor created a valid object, that could be used by QPainter as a "drawing surface".

Would you be kind to explain how should I do it?

JonB

@canellas
You have to call bool QPainter::begin(QPaintDevice *device).

Apart from that, so far as I see in code shown you never draw into painter so you're not going to see much. All you do is set a pen and never craw anything. I don't know about not saving the file, check that line is reached and check /var/tmp files are not getting deleted.

canellas

@JonB

The code that actually draws was removed to reduce the size of the code.

The line that saves the file is executed, and the files at var/tmp are not deleted by the OS automatically.

JonB

This post is deleted!

JonB

@canellas Start by checking return result of QPixmap::save(). I suppose it's false.

Is paintDevice still a null QPixmap? I think it is, depending on what code you removed. Hence earlier
@Christian-Ehrlicher said in Using QPainter to draw on a QPixmap and save it:

Your QPixmap is null. You should pass a valid QPixmap to the painter.

I think you have to pass a QPixmap big enough for your painting.

Then maybe the save() will do something. It wants something valid to save.

canellas

@JonB

Yes, you are correct.

Unlike QPdfWriter, QPixmap must be big enough for the image that is to be drawn.

Thanks!

Christian Ehrlicher

This post is deleted!