Qt5 breaks rendering to pdf on Windows?

[davidb]

When I render a scene to a QPrinter set to PdfFormat, my QGraphicsItem subclass is rendered wrong on two counts:

1. opacity appears to be 1.0, no matter what I set it to.
2. With a QGradient, when I use setColorAt() to set the color to Qt::transparent, it uses black instead.

Rendering the scene to a QGraphicsView works correctly. But my users need to output to pdf. Rendering to a pdf does not have these issues in Qt 4.8, so it seems like a regression in Qt 5 or I'm missing something I should be doing in Qt 5.

I'm using Qt 5.6.1-1 on Windows 8.1 and Windows 10. Below is a sscce to illustrate the issues:
Main.cpp

#include <QtWidgets/QApplication>
#include <QtPrintSupport/QPrinter>
#include <QtWidgets/QGraphicsItem>
#include <QtWidgets/QGraphicsScene>
#include <QtWidgets/QFileDialog>
#include <QtGui/QPainter>

const qreal kGradientWidth = 25.0;

// This class demonstrates the issues of rendering to pdf when opacity < 1 and gradient to transparent.
class MyGraphicsItem : public QGraphicsItem {
public:
  explicit MyGraphicsItem() {
    base_rect_ = QRectF(QPointF(150, 100), QPointF(250, 140));
  }

  virtual QRectF boundingRect() const {
    QRectF bounds = base_rect_.adjusted(-kGradientWidth,
                                        -kGradientWidth,
                                        kGradientWidth,
                                        kGradientWidth);
    return bounds;
  }

  virtual void paint(QPainter *painter,
                     const QStyleOptionGraphicsItem * /*option*/,
                     QWidget * /*widget*/)  {
    painter->save();

    qreal opacity = 0.3;
    painter->setOpacity(opacity); // seems to ignore the opacity setting!
    painter->setPen(Qt::NoPen);

    QColor item_color("#ffffff");  // white

    QRectF rect_to_paint =
        base_rect_.adjusted(0.0,
                            -kGradientWidth,
                            0.0,
                            kGradientWidth);
    QLinearGradient gradient(rect_to_paint.topLeft(), rect_to_paint.bottomLeft());
    qreal padding_percent = kGradientWidth / rect_to_paint.height();
    gradient.setColorAt(0.0, Qt::red);  // works fine
    gradient.setColorAt(padding_percent, item_color);
    gradient.setColorAt(1.0 - padding_percent, item_color);
    gradient.setColorAt(1.0, Qt::transparent);  // uses black instead of transparent!

    painter->setBrush(gradient);
    painter->drawRect(rect_to_paint);

    painter->restore();
  }

private:
  QRectF base_rect_;
};

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

  // Create the scene with my graphics item to illustrate the problems:
  //  1. opacity of .6 becomes opacity of 1 when rendered to pdf.
  //  2. gradient to transparent becomes gradient to black when rendered to pdf.
  QGraphicsScene *scene = new QGraphicsScene(0, 0, 400, 400);
  scene->addRect(30, 15, 300, 300, QPen(), QBrush(Qt::green));
  MyGraphicsItem *my_item = new MyGraphicsItem;
  scene->addItem(my_item);

  // Where to save the pdf?
  QString filter("PDF Document (*.pdf)");
  QString folder;
  folder = "Y:\\Downloads";
  QString pdf_file = QFileDialog::getSaveFileName(NULL,
                                                  "save pdf",
                                                  folder,
                                                  filter);
  if (!pdf_file.isEmpty()) {
    // Save the pdf.
    QPrinter *printer = new QPrinter();
    printer->setOutputFileName(pdf_file);
    printer->setOutputFormat(QPrinter::PdfFormat);

    QPainter painter(printer);
    scene->render(&painter);
  }

  return 0;
}

.pro:

QT += core gui widgets printsupport
#CONFIG += c++11

TARGET = win_pdf_contrast_sscce
CONFIG += console
CONFIG -= app_bundle

TEMPLATE = app

SOURCES += main.cpp

What are my options for outputting my QGraphicsScene to a .pdf from a Qt 5.6.x app in Windows, in such a way that the opacity and gradient issues are resolved? Is there a workaround, a patch, or something I'm missing...?

Thanks,
David

[SGaist]

Hi,

The printing system has changed in Qt 5 so you may have unearthed something. You should go to the bug report system to see if it's something known. If not please consider opening a new report providing your soccer.

[davidb]

I couldn't find any bugs that matched this, so I created this one.

As a matter of procedure: should I paste the sample code into the bug report also, or is it better to just reference this topic as I did?

[SGaist]

Thanks for the report !

The real best thing to do is to provide a zip file with the .pro and code files in it directly in the report. That way the developer taking a look at the bug can simply download, uncompress and start building. It's a level less of indirection and if for any reason the forum goes down, the bug can still be replicated.

[davidb]

ok, thanks for the guidance @SGaist - I have uploaded a .zip file in the bug report.

[davidb]

I created a fix for this issue and pasted the code into the bug report.

[SGaist]

Patches can not be taken like that from the bug tracker. Please create a submission through gerrit.