10. Map Rendering and Printing

Hint

The code snippets on this page need the following imports:

import os

from qgis.core import (
    QgsGeometry,
    QgsMapSettings,
    QgsPrintLayout,
    QgsMapSettings,
    QgsMapRendererParallelJob,
    QgsLayoutItemLabel,
    QgsLayoutItemLegend,
    QgsLayoutItemMap,
    QgsLayoutItemPolygon,
    QgsLayoutItemScaleBar,
    QgsLayoutExporter,
    QgsLayoutItem,
    QgsLayoutPoint,
    QgsLayoutSize,
    QgsUnitTypes,
    QgsProject,
    QgsFillSymbol,
    QgsAbstractValidityCheck,
    check,
)

from qgis.PyQt.QtGui import (
    QPolygonF,
    QColor,
)

from qgis.PyQt.QtCore import (
    QPointF,
    QRectF,
    QSize,
)

There are generally two approaches when input data should be rendered as a map: either do it quick way using QgsMapRendererJob or produce more fine-tuned output by composing the map with the QgsLayout class.

10.1. Simple Rendering

The rendering is done creating a QgsMapSettings object to define the rendering settings, and then constructing a QgsMapRendererJob with those settings. The latter is then used to create the resulting image.

Here’s an example:

image_location = os.path.join(QgsProject.instance().homePath(), "render.png")

vlayer = iface.activeLayer()
settings = QgsMapSettings()
settings.setLayers([vlayer])
settings.setBackgroundColor(QColor(255, 255, 255))
settings.setOutputSize(QSize(800, 600))
settings.setExtent(vlayer.extent())

render = QgsMapRendererParallelJob(settings)

def finished():
    img = render.renderedImage()
    # save the image; e.g. img.save("/Users/myuser/render.png","png")
    img.save(image_location, "png")

render.finished.connect(finished)

# Start the rendering
render.start()

# The following loop is not normally required, we
# are using it here because this is a standalone example.
from qgis.PyQt.QtCore import QEventLoop
loop = QEventLoop()
render.finished.connect(loop.quit)
loop.exec_()

10.2. Rendering layers with different CRS

If you have more than one layer and they have a different CRS, the simple example above will probably not work: to get the right values from the extent calculations you have to explicitly set the destination CRS

layers = [iface.activeLayer()]
settings = QgsMapSettings()
settings.setLayers(layers)
settings.setDestinationCrs(layers[0].crs())

10.3. Output using print layout

Print layout is a very handy tool if you would like to do a more sophisticated output than the simple rendering shown above. It is possible to create complex map layouts consisting of map views, labels, legend, tables and other elements that are usually present on paper maps. The layouts can be then exported to PDF, SVG, raster images or directly printed on a printer.

The layout consists of a bunch of classes. They all belong to the core library. QGIS application has a convenient GUI for placement of the elements, though it is not available in the GUI library. If you are not familiar with Qt Graphics View framework, then you are encouraged to check the documentation now, because the layout is based on it.

The central class of the layout is the QgsLayout class, which is derived from the Qt QGraphicsScene class. Let us create an instance of it:

project = QgsProject.instance()
layout = QgsPrintLayout(project)
layout.initializeDefaults()

This initializes the layout with some default settings, specifically by adding an empty A4 page to the layout. You can create layouts without calling the initializeDefaults() method, but you’ll need to take care of adding pages to the layout yourself.

The previous code creates a “temporary” layout that is not visible in the GUI. It can be handy to e.g. quickly add some items and export without modifying the project itself nor expose these changes to the user. If you want the layout to be saved/restored along with the project and available in the layout manager, then add:

layout.setName("MyLayout")
project.layoutManager().addLayout(layout)

Now we can add various elements (map, label, …) to the layout. All these objects are represented by classes that inherit from the base QgsLayoutItem class.

Here’s a description of some of the main layout items that can be added to a layout.

• map — Here we create a map of a custom size and render the current map canvas

  map = QgsLayoutItemMap(layout)
  # Set map item position and size (by default, it is a 0 width/0 height item placed at 0,0)
  map.attemptMove(QgsLayoutPoint(5,5, QgsUnitTypes.LayoutMillimeters))
  map.attemptResize(QgsLayoutSize(200,200, QgsUnitTypes.LayoutMillimeters))
  # Provide an extent to render
  map.zoomToExtent(iface.mapCanvas().extent())
  layout.addLayoutItem(map)
  

• label — allows displaying labels. It is possible to modify its font, color, alignment and margin

  label = QgsLayoutItemLabel(layout)
  label.setText("Hello world")
  label.adjustSizeToText()
  layout.addLayoutItem(label)
  

• legend

  legend = QgsLayoutItemLegend(layout)
  legend.setLinkedMap(map) # map is an instance of QgsLayoutItemMap
  layout.addLayoutItem(legend)
  

• scale bar

  item = QgsLayoutItemScaleBar(layout)
  item.setStyle('Numeric') # optionally modify the style
  item.setLinkedMap(map) # map is an instance of QgsLayoutItemMap
  item.applyDefaultSize()
  layout.addLayoutItem(item)
  

• nodes based shape

  polygon = QPolygonF()
  polygon.append(QPointF(0.0, 0.0))
  polygon.append(QPointF(100.0, 0.0))
  polygon.append(QPointF(200.0, 100.0))
  polygon.append(QPointF(100.0, 200.0))
  
  polygonItem = QgsLayoutItemPolygon(polygon, layout)
  layout.addLayoutItem(polygonItem)
  
  props = {}
  props["color"] = "green"
  props["style"] = "solid"
  props["style_border"] = "solid"
  props["color_border"] = "black"
  props["width_border"] = "10.0"
  props["joinstyle"] = "miter"
  
  symbol = QgsFillSymbol.createSimple(props)
  polygonItem.setSymbol(symbol)
  

Once an item is added to the layout, it can be moved and resized:

item.attemptMove(QgsLayoutPoint(1.4, 1.8, QgsUnitTypes.LayoutCentimeters))
item.attemptResize(QgsLayoutSize(2.8, 2.2, QgsUnitTypes.LayoutCentimeters))

A frame is drawn around each item by default. You can remove it as follows:

# for a composer label
label.setFrameEnabled(False)

Besides creating the layout items by hand, QGIS has support for layout templates which are essentially compositions with all their items saved to a .qpt file (with XML syntax).

Once the composition is ready (the layout items have been created and added to the composition), we can proceed to produce a raster and/or vector output.

10.3.1. Checking layout validity

A layout is a made of a set of interconnected items and it can happen that these connections are broken during modifications (a legend connected to a removed map, an image item with missing source file,…) or you may want to apply custom constraints to the layout items. The QgsAbstractValidityCheck helps you achieve this.

A basic check looks like:

@check.register(type=QgsAbstractValidityCheck.TypeLayoutCheck)
def my_layout_check(context, feedback):
  results = ...
  return results

Here’s a check which throws a warning whenever a layout map item is set to the web mercator projection:

@check.register(type=QgsAbstractValidityCheck.TypeLayoutCheck)
def layout_map_crs_choice_check(context, feedback):
  layout = context.layout
  results = []
  for i in layout.items():
    if isinstance(i, QgsLayoutItemMap) and i.crs().authid() == 'EPSG:3857':
      res = QgsValidityCheckResult()
      res.type = QgsValidityCheckResult.Warning
      res.title = 'Map projection is misleading'
      res.detailedDescription = 'The projection for the map item {} is set to <i>Web Mercator (EPSG:3857)</i> which misrepresents areas and shapes. Consider using an appropriate local projection instead.'.format(i.displayName())
      results.append(res)

  return results

And here’s a more complex example, which throws a warning if any layout map items are set to a CRS which is only valid outside of the extent shown in that map item:

@check.register(type=QgsAbstractValidityCheck.TypeLayoutCheck)
def layout_map_crs_area_check(context, feedback):
    layout = context.layout
    results = []
    for i in layout.items():
        if isinstance(i, QgsLayoutItemMap):
            bounds = i.crs().bounds()
            ct = QgsCoordinateTransform(QgsCoordinateReferenceSystem('EPSG:4326'), i.crs(), QgsProject.instance())
            bounds_crs = ct.transformBoundingBox(bounds)

            if not bounds_crs.contains(i.extent()):
                res = QgsValidityCheckResult()
                res.type = QgsValidityCheckResult.Warning
                res.title = 'Map projection is incorrect'
                res.detailedDescription = 'The projection for the map item {} is set to \'{}\', which is not valid for the area displayed within the map.'.format(i.displayName(), i.crs().authid())
                results.append(res)

    return results

10.3.2. Exporting the layout

To export a layout, the QgsLayoutExporter class must be used.

base_path = os.path.join(QgsProject.instance().homePath())
pdf_path = os.path.join(base_path, "output.pdf")

exporter = QgsLayoutExporter(layout)
exporter.exportToPdf(pdf_path, QgsLayoutExporter.PdfExportSettings())

Use exportToSvg() or exportToImage() in case you want to export to respectively an SVG or image file instead of a PDF file.

10.3.3. Exporting a layout atlas

If you want to export all pages from a layout that has the atlas option configured and enabled, you need to use the atlas() method in the exporter (QgsLayoutExporter) with small adjustments. In the following example, the pages are exported to PNG images:

exporter.exportToImage(layout.atlas(), base_path, 'png', QgsLayoutExporter.ImageExportSettings())

Notice that the outputs will be saved in the base path folder, using the output filename expression configured on atlas.