Printing QTextDocument in different thread distorts image.

adutzu89

Hello,
I am trying to print with QTextDocument and I have an issue with image aspect ratio when I do the printing using QtConcurrent.

The image is stretched when printing in different thread, through QtConcurrent::run, but prints correctly when in main thread.

At first I was loading a html string in the document and when I saw this issue I tried inserting an image dirrectly through QTextCursor but I got the same result.

I tried using a scaled QImage object and it keeps the image aspect ratio, but it loses a lot of quality, even with Qt::SmoothTransformation.

Here is the printing method:

void AbstractEcoForm::printDocument() {
    std::unique_ptr<QTextDocument> printDoc(new QTextDocument);
    std::unique_ptr<QPrinter> printer(new QPrinter);
    printDoc->setPageSize(QSizeF(printer->pageRect().size()));
    setDocumentData(printDoc.get());
    printer->setPrinterName(mCurrentPrinter);
    QString numeDoc = QStringLiteral("Test-print");
    printer->setDocName(numeDoc);
    QPrinterInfo printerInfo(*printer);
    if (printerInfo.isNull()) {
        emit printerError("Nu este setata nici o imprimanta!");
        return;
    }
    if (!printer->isValid()) {
        emit printerError("Imprimanta setata nu este valida!");
        return;
    }
    printDoc->print(printer.get());
    emit printSuccess();
}

Here is the initial html string for the image :

<img width=\"300\" src=\"qrc:/assets/logo.png\">

This is my first try with QTextCursor, using QTextImageFormat:

QTextImageFormat imageFormat;
imageFormat.setName("qrc:/assets/logo.png");
imageFormat.setWidth(300);
cursor.insertImage(imageFormat);

This is my first try with QTextCursor, using QTextImageFormat:

QImage logo(":/assets/logo.png");
cursor.insertImage(logo.scaledToWidth(300, Qt::SmoothTransformation));

I am looking for a smooth way to print, without making the application freeze when multiple images are loaded.
Was printing in an QEventLoop, previously, using web engine but that print method returns a boolean value, QTextDocument doesn't return anything.

Now I am a bit stuck; any help is much appreciated.

Environment: Qt 5.12.6, MSVC 2017 32bit, Windows 10 64bit.

gde23

Just a guess, but when I had some weird problems with scaling it was a timing problem.
It could be solved by adding small delays using QTimer::Singelshot() to trigger the actual painting of the widgets. So since it depends on the thread this might be some similar case.

adutzu89

@gde23 hello, thanks for your answer tried to do a QTimer::singleshot , but with no succes.
Maybe I did it wrong, could you exmplify a bit what you meant?

gde23

What I would try is something like this, but as I said its just a guess...

void AbstractEcoForm::printDocument() {
    std::unique_ptr<QTextDocument> printDoc(new QTextDocument);
    std::unique_ptr<QPrinter> printer(new QPrinter);
    printDoc->setPageSize(QSizeF(printer->pageRect().size()));
    setDocumentData(printDoc.get());
    printer->setPrinterName(mCurrentPrinter);
    QString numeDoc = QStringLiteral("Test-print");
    printer->setDocName(numeDoc);
    QPrinterInfo printerInfo(*printer);
    if (printerInfo.isNull()) {
        emit printerError("Nu este setata nici o imprimanta!");
        return;
    }
    if (!printer->isValid()) {
        emit printerError("Imprimanta setata nu este valida!");
        return;
    }
    
    QTimer::SingleShot(10, this, SLOT(printTheDocument());

where printTheDocument() is a slot that calls the

printDoc->print(printer.get());

of your original code

adutzu89

@gde23 said in Printing QTextDocument in different thread distorts image.:

What I would try is something like this, but as I said its just a guess...

void AbstractEcoForm::printDocument() {
    std::unique_ptr<QTextDocument> printDoc(new QTextDocument);
    std::unique_ptr<QPrinter> printer(new QPrinter);
    printDoc->setPageSize(QSizeF(printer->pageRect().size()));
    setDocumentData(printDoc.get());
    printer->setPrinterName(mCurrentPrinter);
    QString numeDoc = QStringLiteral("Test-print");
    printer->setDocName(numeDoc);
    QPrinterInfo printerInfo(*printer);
    if (printerInfo.isNull()) {
        emit printerError("Nu este setata nici o imprimanta!");
        return;
    }
    if (!printer->isValid()) {
        emit printerError("Imprimanta setata nu este valida!");
        return;
    }
    
    QTimer::SingleShot(10, this, SLOT(printTheDocument());

where printTheDocument() is a slot that calls the

printDoc->print(printer.get());

of your original code

Holy Qt licensing, it works, I was calling the singleshot inside method where i was adding the elements.
Thanks a lot!

adutzu89

@gde23 said in Printing QTextDocument in different thread distorts image.:

What I would try is something like this, but as I said its just a guess...

void AbstractEcoForm::printDocument() {
    std::unique_ptr<QTextDocument> printDoc(new QTextDocument);
    std::unique_ptr<QPrinter> printer(new QPrinter);
    printDoc->setPageSize(QSizeF(printer->pageRect().size()));
    setDocumentData(printDoc.get());
    printer->setPrinterName(mCurrentPrinter);
    QString numeDoc = QStringLiteral("Test-print");
    printer->setDocName(numeDoc);
    QPrinterInfo printerInfo(*printer);
    if (printerInfo.isNull()) {
        emit printerError("Nu este setata nici o imprimanta!");
        return;
    }
    if (!printer->isValid()) {
        emit printerError("Imprimanta setata nu este valida!");
        return;
    }
    
    QTimer::SingleShot(10, this, SLOT(printTheDocument());

where printTheDocument() is a slot that calls the

printDoc->print(printer.get());

of your original code

And I'm back :D, seems while the code somehow works and the app doesn't crash I get the following warning:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QTextDocument(0x10f7a800), parent's thread is QThread(0x143b22b8), current thread is QThread(0x12e10d0)

Edit:: I gave up on QtConcurrent and went with QueuedConnection connection.
Thank you.