Qt QPainter in millimetres instead of inches - c++

Question

I have a QPrinter that prints A4 either directly to a physical printer or a PDF. Now I'd like to use QPainter to draw in millimetres, but the current coordinate system seems to be the width and height of an A4 in inches times the resolution of the printer.
8.26 inch x 1200 res = 9912
11.69 inch x 1200 res = 14028
I have tried the following but text just ended up huge.
auto page = printer.pageRect(QPrinter::Unit::Millimeter);
painter.setWindow(QRect(0, 0, page.width(), page.height()));
How do I change this so my QPainter can draw to 210 x 297 mm instead of the above system?
This is on Windows 10 and with Qt 5.10.

Answer 1

I tested this method on X11 (ubuntu linux) PDF print, using ScreenResolution printer mode:
painter.begin(printer);
int log_w = 210;
int log_h = 297;
painter.setWindow(0, 0, log_w, log_h);
int phys_w = printer->width();
int phys_h = printer->height();
painter.setViewport(0, 0, phys_w, phys_h);
Basically, set your logical size in mm using the painter window, and give the painter's viewport the printer's physical size.
This line should print a rectangle around the page with a border of 10 mm:
painter.drawRect(10, 10, log_w - 20, log_h -20);
Text should work accordingly. This code should print the word Ok at the top left corner of the rectangle:
QFont font = painter.font();
font.setPointSize(10); //1 cm height
painter.setFont(font);
painter.drawText(10, 20, "Ok");
painter.end();
Using HighResolution printer mode, font size must be set using
font.setPixelSize(10); //1 cm height
and a QPen must be set to the painter:
QPen pen(Qt::black);
pen.setWidthF(0.2);
painter.setPen(pen);
painter.drawRect(10, 10, log_w - 20, log_h - 20);
About loss of device dependency using setPixelSize, I'm aware that here is stated:
It is possible to set the height of characters shown on the screen to
a specified number of pixels with setPixelSize(); however using
setPointSize() has a similar effect and provides device independence.
but I think it refers to screen only, given that here is stated:
When rendering text on a QPrinter device, it is important to realize
that the size of text, when specified in points, is independent of the
resolution specified for the device itself. Therefore, it may be
useful to specify the font size in pixels when combining text with
graphics to ensure that their relative sizes are what you expect.

Answer 2

I think that you are looking for the QTransform class, according to the official doc:
The QTransform class specifies 2D transformations of a coordinate
system. A transformation specifies how to translate, scale, shear,
rotate or project the coordinate system, and is typically used when
rendering graphics.
You can initialise your custom transform class:
QTransform transform = QTransform::fromScale(painter.device()->physicalDpiX() / scale, painter.device()->physicalDpiY() / scale);
A think that this could be helpfull, the number of dots per militmeter:
const int dot_per_millimeter = qRound(qApp->primaryScreen()->physicalDotsPerInch() / 25.40);
Customise then your scale & apply it using a QPainter:
QPainter painter(parent);
painter.setWorldTransform(transform, false);

Answer 3

Your approach is correct. Here is an example of how to set up a printer/painter pair. I don't fiddle around with the transformation matrix since it's sufficient to specify a window/viewport pair. I don't even specify the viewport explicitly, since it is automatically set to the metrics of the paint device (in this case the QPrinter object).
#include <QPrinter>
#include <QPainter>
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
QPrinter printer(QPrinter::PrinterResolution);
printer.setOrientation(QPrinter::Portrait);
printer.setPageSize(QPageSize(QPageSize::A4));
printer.setResolution(300 /*dpi*/);
printer.setOutputFormat(QPrinter::PdfFormat);
printer.setOutputFileName("ellipse.pdf");
QPainter painter(&printer);
auto page = printer.pageRect(QPrinter::Unit::Millimeter);
painter.setWindow(page.toRect());
// Draw a 5mm thick ellipse across the whole page.
painter.setPen(QPen(Qt::black, 5.0));
painter.drawEllipse(0, 0, 210, 297);
return 0;
}
It is hard to tell what goes wrong in your case without seeing the rest of the code