QPrinter to PDF does not respect composition modes

bordaigorl

I am using a QGraphicsView where I subclassed some QGraphicsItem to use composition mode "Darken". The QGraphicsView displays perfectly when rendering on screen, but when I render to a QPrinter to export to PDF the composition modes are completely ignored.

Below is a MWE using PyQt5.

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtPrintSupport import *


class QGraphicsRectItemD(QGraphicsRectItem):

  def paint(self, painter, sty, w):
    painter.setCompositionMode(QPainter.CompositionMode_Darken)
    QGraphicsRectItem.paint(self, painter, sty, w)


class QGraphicsPathItemD(QGraphicsPathItem):

  def paint(self, painter, sty, w):
    painter.setCompositionMode(QPainter.CompositionMode_Darken)
    QGraphicsPathItem.paint(self, painter, sty, w)


class Test(QGraphicsView):

  def __init__(self):
    QGraphicsView.__init__(self)

    self.scene = QGraphicsScene()
    self.setScene(self.scene)
    r = self.scene.addRect(0, 0, 1404, 1872)
    # r.setBrush(Qt.green)
    r.setFlag(QGraphicsItem.ItemClipsChildrenToShape)

    pen = QPen()
    pen.setWidth(50)
    pen.setCapStyle(Qt.RoundCap)
    pen.setJoinStyle(Qt.RoundJoin)
    pen.setColor(Qt.black)

    path = QPainterPath(QPointF(100, 100))
    path.lineTo(300, 300)
    path.lineTo(300, 1000)
    path.lineTo(700, 700)
    self.pathItem = pathItem = QGraphicsPathItem(path, r)
    pathItem.setPen(pen)

    # pen = QPen()
    pen.setWidth(150)
    pen.setCapStyle(Qt.RoundCap)
    pen.setJoinStyle(Qt.RoundJoin)
    pen.setColor(Qt.red)

    path = QPainterPath(QPointF(200, 200))
    path.lineTo(300, 300)
    path.lineTo(300, 1000)
    path.lineTo(700, 700)
    self.pathItem2 = pathItem2 = QGraphicsPathItemD(path, r)
    pathItem2.setPen(pen)

  def resizeEvent(self, event):
    self.fitInView(self.sceneRect(), Qt.KeepAspectRatio)

  def mouseDoubleClickEvent(self, event):
    print("PRINTING")
    printer = QPrinter(QPrinter.HighResolution)
    printer.setOutputFormat(QPrinter.PdfFormat)
    # printer.setPageSize(QPrinter.A4)
    printer.setOutputFileName("test.pdf")
    printer.setPaperSize(QSizeF(1404, 1872), QPrinter.Millimeter)
    printer.setPageMargins(0, 0, 0, 0, QPrinter.Millimeter)
    p = QPainter()
    p.begin(printer)
    self.scene.render(p)
    p.end()


if __name__ == '__main__':
  app = QApplication([])
  print(QT_VERSION_STR)
  print(PYQT_VERSION_STR)
  viewer = Test()
  viewer.show()
  app.exec_()

Is this a bug? A known limitation? I could not find anything in the docs.
In principle PDFs support blend modes so I don't see why this would be particularly difficult to preserve.

ChrisW67

The PDF Format outputs a V1.4 PDF file by default. It seems this PDF version supports a Darken blend mode that applies when rendering independent PDF objects. The image in the output PDF appears to me to be a single blob (zlib compressed PDF commands I think); i.e. one object.
QPdfWriter also produces the same result.

Bug? Possibly just a primitive implementation.

If you "print" to a QImage you get a result more like the screen. Perhaps you can then put that in a PDF?

bordaigorl

Thanks for looking into this!
One of the goals of my tool was to obtain vector output so rendering to a raster image is not a solution.
I could try rendering only the objects that need the Darken composition mode on a image overlay but that is not the same thing...