QPdfWriter and Qpainter Position

Jun_

I am using QPdfWriter to output pdf files and Qpainter to draw. The dpi affects the coordinate system of Qpainter, but I don't know how the coordinate system of Qpainter is converted.

	QPdfWriter *pdfWriter = new QPdfWriter(&pdfFile);             
	pdfWriter->setPageSize(QPagedPaintDevice::A4);              
	pdfWriter->setResolution(QPrinter::ScreenResolution);           
	pdfWriter->setPageMargins(QMarginsF(0, 0, 0, 0));
        QTextOption option(Qt::AlignCenter);                           
      	option.setWrapMode(QTextOption::WordWrap);       
        QFont font("Arial", 12);
        QPainter *pdfPainter = new QPainter(pdfWriter); 
        pdfPainter->setFont(font);
        
        pdfPainter->drawText(QRect(500, 500, 2500, 500), "test", option);

The text should be drawn at x:500 y:500, but when I open the pdf file it is not at that position. How is this conversion done? How can I ensure that it is drawn at the correct position?

Bonnie

@Jun_ Actually I don't quite understand what is "correctly at 40,40". When setting 96dpi, drawing at 40,40 just means drawing at 0.42inch, 0.42inch.

pdfWriter->setResolution(QPrinter::ScreenResolution);

I don't know why are you keep writing like ⬆this, as I said this is invalid, it equals

pdfWriter->setResolution(0);

Anyway after calling that, pdfWriter->resolution() tells me the current resolution is 1200.
For 1200dpi, to draw at the same position as 96dpi, every value should * 12.5.
So 500,500 is the same position as 40,40 on 96dpi.

pdfWriter->setResolution(96);
pdfPainter->drawText(QPoint(40, 40), "test");

pdfWriter->setResolution(1200);
pdfPainter->drawText(QPoint(500, 500), "test");

Bonnie

pdfWriter->setResolution(QPrinter::ScreenResolution);

This is wrong and invalid (because QPrinter::ScreenResolution = 0), so it will keep the default resolution which is 1200(dpi).
So for A4 size (8.26inch x 11.69inch), that will be about 9912 x 14028.

QTextOption option(Qt::AlignCenter);

You draw the text in the center of QRect(500, 500, 2500, 500), so the text won't be at x:500 y:500.
If you want to test the coordinate, draw a rect by

pdfPainter->drawRect(QRect(500, 500, 2500, 500));

Jun_

@Bonnie Thanks for your reply.
What I mean is how to convert x, y, width, and height under the influence of different DPI.
exp:
pdfWriter->setResolution(96);
QTextOption option(Qt::AlignLeft);
pdfPainter->drawText(QRect(40, 40, 2500, 500), "test", option);
Can be drawn correctly at 40,40

pdfWriter->setResolution(QPrinter::ScreenResolution);
QTextOption option(Qt::AlignLeft);
pdfPainter->drawText(QRect(x, y, width, height), "test", option);
x, y How to convert to achieve the same position effect as 40

Bonnie

@Jun_ Actually I don't quite understand what is "correctly at 40,40". When setting 96dpi, drawing at 40,40 just means drawing at 0.42inch, 0.42inch.

pdfWriter->setResolution(QPrinter::ScreenResolution);

I don't know why are you keep writing like ⬆this, as I said this is invalid, it equals

pdfWriter->setResolution(0);

Anyway after calling that, pdfWriter->resolution() tells me the current resolution is 1200.
For 1200dpi, to draw at the same position as 96dpi, every value should * 12.5.
So 500,500 is the same position as 40,40 on 96dpi.

pdfWriter->setResolution(96);
pdfPainter->drawText(QPoint(40, 40), "test");

pdfWriter->setResolution(1200);
pdfPainter->drawText(QPoint(500, 500), "test");

Jun_

@Bonnie thanks a lot It is indeed a 12.5-fold relationship.