[Qgis-developer] Rendering and Labeling Shapefile with PyQGIS

[Zied Habtoul]

Hello i use this function to  take a shapefile, label it based on the
specified attribute name, and render an image. but i get a white image.
i use Qgis 2.12 .



def quick_export(self):
    file = QgsVectorLayer("path to shapefile", "BMAS", "ogr")
    QgsMapLayerRegistry.instance().addMapLayer(file)


#Adjust layer Settings
#Code sample from
http://gis.stackexchange.com/questions/77870/how-to-label-vector-features-programmatically
    palyr = QgsPalLayerSettings()
    palyr.enabled = True
    palyr.fieldName = 'Sum_emplpa'
    palyr.placement= QgsPalLayerSettings.OverPoint
    palyr.setDataDefinedProperty(QgsPalLayerSettings.Size,True,True,'8','')
    palyr.writeToLayer(file)

    if file.isValid():
     print "File is valid."


    mapRenderer = QgsMapRenderer()

    rect = file.extent()
    mapRenderer.setExtent(rect)
    mapRenderer.setLabelingEngine(QgsPalLabeling())

    lst = [file.id()]
    mapRenderer.setLayerSet(lst)

    mapRenderer.setLayerSet( lst )
    c = QgsComposition(mapRenderer)
    c.setPlotStyle(QgsComposition.Print)
    x, y = 0, 0
    w, h = c.paperWidth(), c.paperHeight()
    composerMap = QgsComposerMap(c, x,y,w,h)
    c.addItem(composerMap)
    composerLabel = QgsComposerLabel(c)

    composerLabel.adjustSizeToText()
    c.addItem(composerLabel)
    composerLabel.setItemPosition(20,10)
    composerLabel.setItemPosition(20,10, 100, 30)

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

#set image sizing
    dpi = c.printResolution()
    dpmm = dpi / 25.4
    width = int(dpmm * c.paperWidth())
    height = int(dpmm * c.paperHeight())
    img = QImage(QSize(width, height), QImage.Format_ARGB32)
    img.setDotsPerMeterX(dpmm * 1000)
    img.setDotsPerMeterY(dpmm * 1000)
    img.fill(0)
    imagePainter = QPainter(img)
    sourceArea = QRectF(0, 0, c.paperWidth(), c.paperHeight())
    targetArea = QRectF(0, 0, width, height)

#renders image
    c.render(imagePainter, targetArea, sourceArea)
    imagePainter.end()
    img.save("path to save the image", "png")
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