QGIS standalone application for map rendering: bad extent

Question

I made a simple standalone QGIS app for rendering a set of layers and export it as a PDF file.

When i use iface.mapCanvas().mapRenderer() (inside QGIS console) for rendering, my output is OK.
If i want to create my own QgsMapRenderer (inside standalone app), the extent is displaced like on the picture:

Here is my code:

from qgis.core import *
from qgis.gui import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *
import config

app = QgsApplication([], True)
QgsApplication.setPrefixPath(config.qgis_path, True)
QgsApplication.initQgis()

mlr = QgsMapLayerRegistry.instance()
crs = QgsCoordinateReferenceSystem(3857)

# creating a single layer
layer = QgsVectorLayer(config.data_dir + 'ZASRO.shp', 'ZASRO', 'ogr')
if not layer.isValid():
    print "layer failed to load !"
layer.setCrs(crs)
mlr.addMapLayer(layer)

layerSet = [layer.id()]

# map renderer
renderer = QgsMapRenderer()
renderer.setDestinationCrs(crs)
renderer.setLayerSet(layerSet)
renderer.setExtent(layer.extent())

# composition
comp = QgsComposition(renderer)
comp.setPlotStyle(QgsComposition.Print)
comp.setPaperSize(420.0, 297.0)
comp.setPrintResolution(300)

# composer map
composerMap = QgsComposerMap(comp, 0, 0, 420.0, 297.0)
composerMap.setNewExtent(renderer.fullExtent())
comp.addComposerMap(composerMap)

# pdf rendering
comp.exportAsPDF(config.out_dir + "out.pdf")

QgsApplication.exitQgis()

This i due to the fullExtent() function returning some bad values.
I guess my QgsMapRenderer is not valid for some reason ?

Answer 1

OK I solved the problem by setting the renderer output size:

renderer.setOutputUnits(QgsMapRenderer.Millimeters)
renderer.setOutputSize(QSizeF(width, height), dpi)

And use zoomToExtent() instead of setNewExtent() on your QgsMapComposer

#composerMap.setNewExtent(extent)
composerMap.zoomToExtent(extent)