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!

Does this really need to make use of `QPrintDialog`? Why not just render to a `QImage` and then use [`QImage::save`](http://doc.qt.io/qt-5/qimage.html#save)? — G.M., Jul 19 '16 at 09:19

score 1 · Accepted Answer · edited May 23 '17 at 12:22

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.

edited May 23 '17 at 12:22

Community

1
1

answered Jul 19 '16 at 09:38

jpo38

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This does work, but not really the behaviour I was looking for. There needs to be an interactive GUI where you can change file names and locations. Still, thanks! It helped me get my solution which I'll post below – jwsmithers Jul 19 '16 at 11:15
That's really a detail, as you proposed, just use `QFileDialog::getSaveFileName` to pickup file name. – jpo38 Jul 19 '16 at 11:46

score 0 · Answer 2 · answered Jul 19 '16 at 11:18

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.

How to print to PNG format with Qt4

2 Answers2