How to render layer into png in a standalone application?

Question

I would like to use standalone appliction to apply .qml style to a layer and use QgsMapRenderer() to save it as image on a hard disk. The are a lot of questions like mine at this site and it seems that most of the issues emerge from settings for QgsApplication(). Unfortunately it seems that I wasn't able to setup QgsApplication() in a right way. My code is pretty standart rendering code from PyQGIS Cookbook that was used as a template in other questions like mine. It produces transparent .png inage with no layer rendered. Though this code works (without initialisation of QgsApplication()) if I ran it line by line in QGIS console (QGIS 2.4).

So I have following QgsApplication() settings (on Alt Linux).

Application state:
QGIS_PREFIX_PATH env var:       
Prefix:     /usr
Plugin Path:        /usr/lib64/qgis
Package Data Path:  /usr/share/qgis
Active Theme Name:  
Active Theme Path:  :/images/themes//
Default Theme Path: :/images/themes/default/
SVG Search Paths:   /usr/share/qgis/svg/
User DB Path:   /usr/share/qgis/resources/qgis.db 

The code itself:

from qgis.core import * 
from qgis.utils import *
from qgis.gui import *
from PyQt4.QtCore import QSize
from PyQt4.QtGui import QPainter, QImage, QColor

QgsApplication.setPrefixPath('/usr', True)
QgsApplication.initQgis()
print QgsApplication.showSettings()


lyr = QgsVectorLayer('test.shp', 'points', 'ogr')
lyr_id = lyr.id()
QgsMapLayerRegistry.instance().addMapLayer(lyr)
lyr.loadNamedStyle( 'test.qml')


# 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)

layers = QgsMapLayerRegistry.instance().mapLayers()
lst = layers.keys()
print 'lst', lst
renderer = QgsMapRenderer()
renderer.setLayerSet(lst)

extent = renderer.fullExtent()
rect = QgsRectangle(extent)
# print rect.toString()
rect.scale(1.1)
renderer.setExtent(rect)

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

# do the rendering
renderer.render(p)
p.end()

# save image
img.save('test.png', 'png')

QgsApplication.exitQgis()

Answer 1

I got it to work on my GNU/Linux machine by replacing this code block from your script:

QgsApplication.setPrefixPath('/usr', True)
QgsApplication.initQgis()

by this one:

qgisApp = QgsApplication([], True)
QgsApplication.setPrefixPath('/usr', True)
QgsApplication.initQgis()

The first line of this code block initializes the QgsApplication class, making it clear (second parameter set to True) that we want a GUI-enabled application, not only a console one. After all, the script relies on some Qt4 GUI classes.

I tested it using QGIS 2.14.1. Hope this solves your question.

By the way, there's no need to import anything from qgis.utils nor qgis.gui.

Answer 2

I have done a similar stuff, mine does work. Check it out. It is for 2.14 qgis. If you are locked you should probably check http://docs.qgis.org/2.2/en/docs/pyqgis_developer_cookbook/raster.html (see the version number on the link)

from PyQt4.QtCore import QSize
from PyQt4.QtGui import QPainter, QImage, QColor
from PyQt4.QtCore import QFileInfo

from qgis.core import *
import qgis.utils



app = QgsApplication([],True)

# supply path to where is your qgis installed
QgsApplication.setPrefixPath("/usr/", True)

# load providers
QgsApplication.initQgis()

fileName = '/home/pegg/Downloads/ECMWF_ATP_160_10UV_015.tiff'
dst_filename = '/home/pegg/Downloads/test.png'


fileInfo = QFileInfo(fileName)
baseName = fileInfo.baseName()
rlayer = QgsRasterLayer(fileName, baseName)
if not rlayer.isValid():
  print "Layer failed to load!"




#rlayer.setDrawingStyle('MultiBandColor') #make sure
QgsMapLayerRegistry.instance().addMapLayer(rlayer)

# create image
w=rlayer.width()
h=rlayer.height()
img = QImage(QSize(w, h), 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 = QgsMapRenderer()

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

# set extent
rect = QgsRectangle(render.fullExtent())
rect.scale(1.0)

render.setExtent(rect)

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

# do the rendering
render.render(p)

p.end()

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




QgsApplication.exitQgis()