Graph Printing

The Graph Printing example demonstrates how to print or export to PDF 2D and 3D graphs.

Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, see Qt Creator: Tutorial: Build and run.

GraphPrinter class

The printing functionality is implemented in the GraphPrinter class. The class exposes these functions:

• The generatePDF function, which works as follows.
  • Sets up the output PDF file.

    The function instantiates QPdfWriter with a "graph.pdf" file pointed in the specified folder. The function also specifies the options for the exported PDF file: title, resolution, page size, and margins.

     QFile file = QFile(path.toLocalFile());
    
     {
         QPdfWriter writer(file.fileName());
         writer.setResolution(90);
         writer.setTitle(u"Graph"_s);
         writer.setPageSize(QPageSize(image.size()));
         writer.setPageMargins(QMarginsF(0, 0, 0, 0));
         writer.newPage();
         paintImage(image, &writer);
     }
    

  • Sets up image processing.

    The static helper paintImage function draws the image captured from QML onto an instance of the QPaintDevice class:

    The function creates a QPainter referring to the device.

    To ensure the graph is printed correctly, it is scaled to the painter's viewport size with the original aspect ratio.

    The painter's rendering hint is set to lossless image rendering. After this, the function draws the image to the PDF file.

     static void paintImage(const QImage &image, QPaintDevice *paintDevice)
     {
         QPainter painter(paintDevice);
         const QImage finalImage = image.scaled(painter.viewport().size(), Qt::KeepAspectRatio);
         painter.setRenderHint(QPainter::LosslessImageRendering);
         painter.drawImage(finalImage.rect(), finalImage);
     }
    

    Since QPdfWriter and QPrinter inherit QPaintDevice, the helper function can be called from the generatePDF function and the print function explained below.

  The function returns a status message to be displayed in the application's message dialog, including the full path to the exported file.

• The getPrinters function returns a list of available printers.

   QStringList GraphPrinter::getPrinters()
   {
       return QPrinterInfo::availablePrinterNames();
   }
  

• The print function, which works like the generatePDF function, but creates a QPainter referring a QPrinter instance:

   QString GraphPrinter::print(const QImage &image, const QString &printerName)
   {
       QPrinterInfo printInfo = QPrinterInfo::printerInfo(printerName);
       if (printInfo.isNull())
           return tr("%1 is not a valid printer").arg(printerName);
  
       QPrinter printer(printInfo, QPrinter::HighResolution);
       printer.setOutputFormat(QPrinter::NativeFormat);
  
       paintImage(image, &printer);
  
       return tr("Printed to %1").arg(printerName);
   }
  

  The function returns a status message to be displayed in the application's message dialog.

Application setup

In addition to the application setup code, the main.cpp file contains code that creates a new instance of the GraphPrinter class and makes it reachable from the QML code.

 GraphPrinter graphPrinter;
 viewer.rootContext()->setContextProperty(u"graphPrinter"_s, &graphPrinter);

Setting up the layout and image capture

The 2D and 3D graphs are laid out in a Stacklayout. Users can navigate it with a TabBar.

 TabBar {
     id: tabBar
     anchors.left: parent.left
     anchors.right: parent.right

     TabButton {
         text: "2D Graph"
         implicitHeight: 48
         icon.source: checked ? "flatten_square_fill.svg" : "flatten.svg"
         icon.height: 36
         icon.width: 36
     }

     TabButton {
         text: "3D Graph"
         implicitHeight: 48
         icon.source: checked ? "box_left_fill.svg" : "box_left.svg"
         icon.height: 36
         icon.width: 36
     }
 }
 Frame {
     id: tabFrame
     anchors.left: parent.left
     anchors.right: parent.right
     anchors.top: tabBar.bottom
     anchors.bottom: parent.bottom

     StackLayout {
         id: stackLayout

         anchors.fill: parent
         currentIndex: tabBar.currentIndex

         Graph2D {
             id: linegraph
         }

         Graph3D {
             id: bargraph
         }
     }
 }

The FileDialog component is used to choose the PDF file to be written. This component has no visual representation in the application layout, but its API is accessible from the current QML file.

 FileDialog {
     id: dialog
     currentFolder: StandardPaths.writableLocation(StandardPaths.PicturesLocation)
     nameFilters: ["PDF files (*.pdf)"]
     defaultSuffix: "pdf"

     fileMode: FileDialog.SaveFile

A custom printing dialog is created for selecting a printer, and is triggered with the button. The Dialog retrieves the list of available printers and displays them in a list view.

 Dialog {
     id: printerDialog
     anchors.centerIn: parent
     contentHeight: printerListView.height
     contentWidth: printerListView.width

     title: qsTr("Available Printers")
     modal: true

     onOpened: {
         printerModel.clear()
         var printers = graphPrinter.getPrinters()
         printers.forEach((x, i) => printerModel.append({
                                                            "name": x
                                                        }))
     }
 ...
 contentItem: Rectangle {
     id: printerItem
     height: printerListView.height
     width: printerListView.width
     color: mainView.item.theme.plotAreaBackgroundColor

     ListView {
         id: printerListView
         height: 100
         width: 200
         clip: true

         model: printerModel
         delegate: printerDelegate
         highlight: Rectangle {
             color: mainView.item.theme.grid.subColor
         }
     }
 }

The button triggers the PDF export.

When either PDF export or printing is triggered, the following code is executed:

• Capture an image using the grabToImage method. The current graph is the Stacklayout's item at the current index.
• In the grabToImage parameters, we specify the callback as the generatePDF or print function in the GraphPrinter class.

  PDF export:

   onAccepted: {
       mainView.prepareForPrint()
  
       mainView.item.grabToImage(function (result) {
           message.title = "Save PDF"
           message.text = "PDF saved to " + graphPrinter.generatePDF(
                       dialog.selectedFile, result.image)
           message.open()
       }, mainView.outputsize)
   }
  

  Print:

   onAccepted: {
       var selectedPrinter = printerModel.get(printerListView.currentIndex)
       mainView.prepareForPrint()
       mainView.item.grabToImage(function (result) {
           message.title = "Print"
           message.text = graphPrinter.print(result.image,
                                             selectedPrinter.name)
           message.open()
       }, mainView.outputsize)
   }
  
   onClosed: {
       mainView.cleanAfterPrint()
   }
  

  For the size, the code makes the image render at 4x the actual resolution. For 3D graphs, the item must also be expanded for the duration of printing:

   function prepareForPrint() {
       if (stackLayout.currentIndex === 1) {
           var newsize = Qt.size(bargraph.width * 4, bargraph.height * 4)
  
           // check that we do not exceed maximum texture size
           if (newsize.width * Screen.devicePixelRatio > graphPrinter.maxTextureSize() ) {
               // scale to 25% under max texture size to be on the safe side; some GPUs seem
               // to glitch when using the abosulute max
               var ratio = (newsize.width * Screen.devicePixelRatio * 1.25)
                       / graphPrinter.maxTextureSize()
               newsize.width /= ratio
               newsize.height /= ratio
           }
           outputsize.width = Math.round(newsize.width)
           outputsize.height = Math.round(newsize.height)
  
           // resize the bar graph to match the PDF output size
           item.width = outputsize.width
           item.height = outputsize.height
       } else {
           outputsize = Qt.size(linegraph.width * 4, linegraph.height * 4)
       }
   }
  
   function cleanAfterPrint() {
       if (stackLayout.currentIndex === 1) {
           // resize the bar graph back to the actual visual size
           item.width = stackLayout.width
           item.height = stackLayout.height
       }
   }
  

Example project @ code.qt.io