draw Image to QPrinter in QThread

[qt-nubie]

with the code below, i cannot print my image (not shown up). how to solve this? thank in advance.


#include <QPrintDialog>
#include <QPrinter>
#include <QPainter>
#include <QThread>
#include <QDebug>

#include "documentprinter.h"
#include "document.h"

DocumentPrinter::DocumentPrinter(Document *document, QObject *parent) :
QObject(parent),
m_printer(0),
m_document(document)
{
}

void DocumentPrinter::setDocument(Document *document)
{
m_document = document;
}

bool DocumentPrinter::exec()
{
QThread printerThread;
DocumentPrinterWorker docPrinterWorker;
docPrinterWorker.moveToThread(&printerThread);
connect(this, SIGNAL(printStarted(QPrinter*,Document*,int,int)),
&docPrinterWorker, SLOT(printDocument(QPrinter*,Document*,int,int)));

QPrintDialog dialog((QWidget *)parent());
dialog.setMinMax(1, m_document->numPages());

if (dialog.exec()) {
m_printer = dialog.printer();
printerThread.start();
if (dialog.printRange() == QAbstractPrintDialog::AllPages)
emit printStarted(m_printer, m_document, 1, m_document->numPages());
else if (dialog.printRange() == QAbstractPrintDialog::PageRange)
emit printStarted(m_printer, m_document, dialog.fromPage(), dialog.toPage());
printerThread.quit();
printerThread.wait();
}

return true;
}

void DocumentPrinterWorker::printDocument(QPrinter *printer, Document *document, int fromPage, int toPage)
{
if (!printer && !document)
return;

printer->setResolution(300);
printer->setPageMargins(0,0,0,0,QPrinter::Point);

QPainter painter(printer);
for (int i=fromPage; i<=toPage; i++) {
QImage image("./images/logo.png");
painter.drawImage(0,0, image);
document->render(&painter, i, QRectF(0, 0, printer->width(), printer->height()));
if (i < toPage)
printer->newPage();
}
painter.end();
}

sorry, i'm not good at english.

[wysota]

Why are you trying to print from within a thread?

[qt-nubie]

when i print a lot of page, does it freeze the gui?

[wysota]

No, why would it do that?

[qt-nubie]

render function i implement also load pdf using muPDF and render it to QPainter pointer that pass through it. is it blocking the main thread? if no, then i miss understood something. :D

[wysota]

Yes, it's blocking the main thread but nobody said you have to do that in one big loop working for 10 minutes.

Read this: Keeping the GUI Responsive

[qt-nubie]

Thank you for the advise... :D

btw about that QImage, it's not possible to to use drawImage not in main thread? there is a warning say about "QPixmap: It is not safe to use pixmaps outside the GUI thread", but i'm using QImage not QPixmap.

[wysota]

It is possible to draw on images in non-gui thread. But pixmaps are a completely different issue. It's possible the library you are using is drawing on pixmaps. Also accessing the same object (QPrinter instance) from more than one thread is a straight way to trouble. Using threads here is out of the question, at least for printing itself. You can try rendering the document in an external thread (e.g. using QtConcurrent) and once you have that, send everything to the printer.

[qt-nubie]

ok, got it....