.. _composer:

Map Rendering and Printing
==========================

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


Simple Rendering
----------------

Render some layers using :class:`QgsMapRenderer` - create destination paint device (``QImage``, ``QPainter`` etc.), set up layer set, extent, output size and do the rendering::

  # create image
  img = QImage(QSize(800,600), QImage.Format_ARGB32_Premultiplied)

  # set image's background color
  color = QColor(255,255,255)
  img.fill(color.rgb())

  # create painter
  p = QPainter()
  p.begin(img)
  p.setRenderHint(QPainter.Antialiasing)

  render = QgsMapRender()

  # set layer set
  lst = [ layer.getLayerID() ]  # add ID of every layer
  render.setLayerSet(lst)

  # set extent
  rect = QgsRect(render.fullExtent())
  rect.scale(1.1)
  render.setExtent(rect)

  # set output size
  render.setOutputSize(img.size(), img.logicalDpiX())

  # do the rendering
  render.render(p)

  p.end()

  # save image
  img.save("render.png","png")


Output using Map Composer
-------------------------

Map composer is a very handy tool if you would like to do a more sophisticated output than the simple rendering shown above.
Using the composer 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, raster images or directly printed
on a printer.

The composer 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 <http://doc.qt.nokia.com/stable/graphicsview.html>`_, then you are
encouraged to check the documentation now, because the composer is based on it.

The central class of the composer is :class:`QgsComposition` which is derived from :class:`QGraphicsScene`. Let us create
one::

  mapRenderer = iface.mapCanvas().mapRenderer()
  c = QgsComposition(mapRenderer)
  c.setPlotStyle(QgsComposition.Print)

Note that the composition takes an instance of :class:`QgsMapRenderer`. In the code we expect we are running within QGIS
application and thus use the map renderer from map canvas. The composition uses various parameters from the map renderer,
most importantly the default set of map layers and the current extent. When using composer in a standalone application,
you can create your own map renderer instance the same way as shown in the section above and pass it to the composition.

It is possible to add various elements (map, label, ...) to the composition - these elements have to be descendants
of :class:`QgsComposerItem` class. Currently supported items are:

* map - this item tells the libraries where to put the map itself. Here we create a map and stretch it over the whole paper size::
  
    x, y = 0, 0
    w, h = c.paperWidth(), c.paperHeight()
    composerMap = QgsComposerMap(c, x,y,w,h)
    c.addItem(composerMap)

* label - allows displaying labels. It is possible to modify its font, color, alignment and margin.
  ::

    composerLabel = QgsComposerLabel(c)
    composerLabel.setText("Hello world")
    composerLabel.adjustSizeToText()
    c.addItem(composerLabel)

* legend
  ::

    legend = QgsComposerLegend(c)
    legend.model().setLayerSet(mapRenderer.layerSet())
    c.addItem(legend)

* scale bar
  ::

    item = QgsComposerScaleBar(c)
    item.setStyle('Numeric') # optionally modify the style
    item.setComposerMap(composerMap)
    item.applyDefaultSize()
    c.addItem(item)

* arrow
* picture
* shape
* table

By default the newly created composer items have zero position (top left corner of the page) and zero size.
The position and size are always measured in millimeters::

  # set label 1cm from the top and 2cm from the left of the page
  composerLabel.setItemPosition(20,10)
  # set both label's position and size (width 10cm, height 3cm)
  composerLabel.setItemPosition(20,10, 100, 30)

A frame is drawn around each item by default. How to remove the frame::

  composerLabel.setFrame(False)


Besides creating the composer items by hand, QGIS has support for composer templates which are essentially compositions
with all their items saved to a .qpt file (with XML syntax). Unfortunately this functionality is not yet available in the API.

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

The default output settings for composition are page size A4 and resolution 300 DPI. You can change them if necessary. The paper
size is specified in millimeters::

  c.setPaperSize(width, height)
  c.setPrintResolution(dpi)


Output to a raster image
~~~~~~~~~~~~~~~~~~~~~~~~

The following code fragment shows how to render a composition to a raster image::

  dpi = c.printResolution()
  dpmm = dpi / 25.4
  width = int(dpmm * c.paperWidth())
  height = int(dpmm * c.paperHeight())

  # create output image and initialize it
  image = QImage(QSize(width, height), QImage.Format_ARGB32)
  image.setDotsPerMeterX(dpmm * 1000)
  image.setDotsPerMeterY(dpmm * 1000)
  image.fill(0)

  # render the composition
  imagePainter = QPainter(image)
  sourceArea = QRectF(0, 0, c.paperWidth(), c.paperHeight())
  targetArea = QRectF(0, 0, width, height)
  c.render(imagePainter, targetArea, sourceArea)
  imagePainter.end()

  image.save("out.png", "png")


Output to PDF
~~~~~~~~~~~~~

The following code fragment renders a composition to a PDF file::

  printer = QPrinter()
  printer.setOutputFormat(QPrinter.PdfFormat)
  printer.setOutputFileName("out.pdf")
  printer.setPaperSize(QSizeF(c.paperWidth(), c.paperHeight()), QPrinter.Millimeter)
  printer.setFullPage(True)
  printer.setColorMode(QPrinter.Color)
  printer.setResolution(c.printResolution())
  
  pdfPainter = QPainter(printer)
  paperRectMM = printer.pageRect(QPrinter.Millimeter)
  paperRectPixel = printer.pageRect(QPrinter.DevicePixel)
  c.render(pdfPainter, paperRectPixel, paperRectMM)
  pdfPainter.end()