Setting custom paper size with QPrinter doesn't print correctly - qt

Question

I need to be able to print from qt (the rendered contents of a QGraphicsScene, or a QImage), to scale, on normal printer, pdf, but also on custom printers, including roll fed.
It seems that anything that works for standard printers fails on custom printers, and the reverse.
I have made the printing work as expected on custom printers now (going back and forth between what works in the different printers).
I set the custom size required, and the preferred orientation, based on length/width ratio.
I open a print dialog (and even check the supply- the paper is set to desired size, and the orientation is set as expected)
print:
On custom printer, I get the correct size, and if the supply is smaller, the print clips as needed. margins are set correctly too.
On Pdf, I get a document of custom size as requested, printed correctly - but the orientation is not respected !!! (even though print dialog showed it correctly) - see image
On HP printer, I get a white page - nothing gets printed.
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QPrinter>
#include <QPrintDialog>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
// some scene to print - show rectangle for easy measure
QGraphicsScene* s = new QGraphicsScene();
s->setSceneRect(-500, -500, 1500, 1500);
s->setBackgroundBrush(Qt::red);
QGraphicsView* view = new QGraphicsView();
view->setScene(s);
view->show();
qreal in = 72;
QRectF canvasRect(-0.1*in, -0.1*in, 6*in, 4*in);
qreal margins = 0.1*in;
QRectF actualCanvasRect = canvasRect.adjusted(margins,margins,-margins,-margins);
// this is to show actual scene
QGraphicsRectItem* contourItem = new QGraphicsRectItem(actualCanvasRect);
contourItem->setBrush(Qt::blue);
s->addItem(contourItem);
// an item partially on canvas (so I can check margins)
QGraphicsRectItem* item = new QGraphicsRectItem(-.5*in, -in, 2*in, 3*in);
item->setBrush(Qt::yellow);
s->addItem(item);
// actual printing:
// print the scene, to scale, using user margins, on given printer
QPrinter printer;
QPrinter::Orientation orient = (actualCanvasRect.width() >
actualCanvasRect.height() ?
QPrinter::Landscape : QPrinter::Portrait);
printer.setOrientation(orient);
printer.setPaperSize(canvasRect.size(), QPrinter::Point);
printer.setPageMargins(margins, margins, margins, margins, QPrinter::Point);
QPrintDialog printDialog(&printer);
if (printDialog.exec() != QDialog::Accepted)
{
qDebug("dialog canceled");
return 1;
}
QPainter painter;
if (! painter.begin(&printer))
{
qDebug("failed to open printer");
return 1;
}
// render the contents, clipped to printer page size, and scaled from point to device pixel
QRectF source = actualCanvasRect;
// convert target rect to DevicePixel and clip to page
QRectF page = printer.pageRect(QPrinter::DevicePixel);
qreal scale = printer.resolution()/in;
QRectF target = QRectF(page.topLeft(), source.size() * scale);
target &= page; // clip target rect to page
// clip source rect to page - without this, if printer paper is smaller I get unwanted scaling
source &= printer.pageRect(QPrinter::Point);
s->render(&painter, target, source);
painter.end();
return app.exec();
}
I don't understand why pdf creates a portrait page even though I clearly requested landscape (without changing the print dialog: see image). (the width and height are reversed, yet correct - Document Properties shows 4x6, and the page attempts to print correctly and to scale)
More important, I don't understand why a typical laser jet printer prints nothing - blank page - or sometimes for a simple canvas it scales to fit.
BUT if I change any properties in the print dialog from HP, anything irrelevant (like paper source, or paper type.... ), it prints correctly.
What am I doing wrong ?
(using Qt 4.7 and 5.5, must work on 4.7 - Windows, have yet to try Linux)