Setting pen on QPainter using print engine may result in distorted painting

Consider the following example:

int main(int argc, char *argv[]) {
QApplication app(argc, argv);

QPrinter printer;
QPrintDialog printDialog(&printer, 0);

if(printDialog.exec() == QDialog::Accepted)

{ QPainter painter(&printer); painter.setMatrix(QMatrix(0.000153778, 0.0, 0.0, -0.000153778, 0136.581, 01236.72)); painter.setPen(Qt::black); painter.setBrush(QBrush(Qt::red)); painter.drawRect(4099900, 5099900, 300000, 300000); painter.setPen(Qt::blue); painter.drawLine(-6001000, 5099900, 4799900, 5099900); painter.end(); }

return 0;
}

The expected result is that the blue line is aligned with the top of the rectangle (visually at the bottom since the matrix is scaled with a negative factor), as the two have the same y-coordinate.

The actual result is that the blue line is painted a little bit above the rectangle (visually under the rectangle because of the matrix scaling). If the blue pen is not set, the line is aligned correctly with the rectangle. The line should be painted the same regardless of whether or not the pen is set on it.

The difference between setting the pen and not setting the pen is that setting the pen causes an incorrect matrix to be written to the pdf file. If a pen is not set, the matrix is not written to the file. The problem seems to be caused by qt_real_to_string() in qpdf.cpp.

The written matrix has a too low precision and results in the problem. Here are the events in the wrong program:

1. The rectangle is written out using the following matrix:
QTransform(
11=0.000153778
12=0
13=0
21=0
22=-0.000153778
23=0
31=136.581
32=1236.72
33=1)

This results in the coordinates:
MoveTo: QPointF(767.055, 452.468)
LineTo: QPointF(813.189, 452.468)
LineTo: QPointF(813.189, 406.334)
LineTo: QPointF(767.055, 406.334)
LineTo: QPointF(767.055, 452.468)

2. The pen is set and the matrix is written to the pdf file. But instead of the above matrix the following one is written:

11 = 0.000153
12 = 0
13 = 0
21 = 0
22 = -0.000153
23 = 0
31 = 136.580
32 = 1236.72
33 = 1

One sees that the values 11 and 22 have been truncated. Additionally value 31 is wrong.

3. The line is written in its own coordinates and the PDF-Viewer has to perform the matrix transformation.

Altogether the problem lies in qt_real_to_string() in qpdf.cpp. This function truncates values and performs wrong rounding.