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Printing QTextDocument in different thread distorts image.



  • 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.



  • 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.



  • @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?



  • 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



  • @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!



  • @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.


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