How to print to PNG format with Qt4

Question

I'm struggling to print an image to the PNG format using Qt4. The code below has default settings of either PDF or PS, but no way to choose PNG:

void DetectorView::printToFile()
{

// A basic printing function
QPrinter printer;
QPrintDialog dialog(&printer, this);
if (dialog.exec()==QDialog::Accepted) {
    QPainter painter(&printer);
    this->render(&painter);
    std::cout << "INFO [DetectorView::printToFile] Wrote file " << std::endl;
}
else {
    std::cout << "INFO [DetectorView::printToFile] Cancelling printer " << std::endl;
}
}

Any help would be appreciated!

Answer 1

Using this link: Rendering QWidget to QImage loses alpha-channel , you can render your widget to a QImage .

Then, using QImageWriter , you can save it to a png:

// render QWidget to QImage:
QImage bitmap(this->size(), QImage::Format_ARGB32);
bitmap.fill(Qt::transparent);
QPainter painter(&bitmap);
this->render(&painter, QPoint(), QRegion(), QWidget::DrawChildren);

// save QImage to png file:
QImageWriter writer("file.png", "png");
writer.write(bitmap);

Note: links provided are for Qt5, but this should work with Qt4.

Answer 2

Using jpo38's answer, I expanded to get the behaviour I wanted:

void DetectorView::printToFile()
{
    QString default_name = "myImage.png";
    QImage bitmap(this->size(), QImage::Format_ARGB32);
    QPainter painter(&bitmap);
    this->render(&painter,QPoint(),QRegion(),QWidget::DrawChildren);
    QString filename = QFileDialog::getSaveFileName(this, tr("Save File"),QDir::homePath()+"/"+default_name,tr("Image Files (*.png *.jpg *.bmp)"));
    QImageWriter writer(filename, "png");
    writer.write(bitmap);
    std::cout << "INFO [DetectorView::printToFile] Wrote image to file" << std::endl;
}

Note the QFileDialog which is needed to create the interactive window.