Printing with a coordinate system given in millimeters

[wysota]

If you scale the canvas in milimeters, the font size will also have to be given in milimeters but scaled with a factor matching the one between the window and the viewport (in other words you have to calculate how many "pixels" or "points" fit into one "milimeter").

[_Jack_]

HI, I have a quite similar problem, I want to print a square 20 * 20 mm and I'm using this code:

QPrinter printer;
 printer.setPaperSize(QPrinter::A4);
 printer.setOrientation (QPrinter::Portrait);
 
 QPainter painter;
 int LogicalWidth  = 210;
 int LogicalHeight = 297;
 painter.setWindow(0, 0,LogicalWidth, LogicalHeight);
 int PhysicalWidth  = printer.width();
 int PhysicalHeight = printer.height();
 painter.setViewport(0, 0, PhysicalWidth, PhysicalHeight);
 painter.begin(&printer);
 painter.drawRect(50,50,20,20);
 painter.end();

QPrinter printer;
 printer.setPaperSize(QPrinter::A4);
 printer.setOrientation (QPrinter::Portrait);

 QPainter painter;
 int LogicalWidth  = 210;
 int LogicalHeight = 297;
 painter.setWindow(0, 0,LogicalWidth, LogicalHeight);
 int PhysicalWidth  = printer.width();
 int PhysicalHeight = printer.height();
 painter.setViewport(0, 0, PhysicalWidth, PhysicalHeight);
 painter.begin(&printer);
 painter.drawRect(50,50,20,20);
 painter.end();

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the square I obtain is quite 5*5 mm , but i expect a square 20 * 20 mm...where is my mistake?
Thankyou in advance.

[doberkofler]

This is strange as your example looks correct to me. Maybe "wysota" has an idea.

[_Jack_]

Thanks for reply.
I resolved my problem in this way: taking in account printer resolution (hoping this is a general solution) :

QTransform TPiXEL = QTransform::fromScale(
painter.device()->physicalDpiX() / 25.400,
painter.device()->physicalDpiY() / 25.400);
painter.setWorldTransform(TPiXEL, false);

Thanks.

[doberkofler]

Are you now using setWorldTransform together with setWindow/setViewport or instead of the method suggested by wysota?