Screenshot every layer from a geoJSON object, failing to zoom to extents of each layer correctly

Question

What I've been trying to do is I have a ton of geoJSON files (a few thousand) and I need to load them, change the colors of the Polygons and background to white and black, then export them as an image. Like the one below...

Though I've had a lot of trouble, I now can load all the geoJSON layers (though some of the geoJSON layers have nothing in them...so it loads up as Unknown in QGIS). I add the geojson layer to a LAYERS list that I use later, add the symbols, set the extent to that layer, and repeat.

Then, to screenshot each image, I iterate through the LAYERS list, set my extent to each layer, and screenshot it.

However, the screenshots are not correctly viewing the layer.

My code is below.

I'm not very familiar with QGIS and PyQGIS

from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.core import *
import os
import sys
from time import sleep

NUMBER_OF_GEOJSON_FILES = 580
GEOJSON_DIR = "C:\\path\\to\\geojson"
IMAGE_DIR = "C:\\path\\to\\destination"
FILENAME = "filename_format_img"

#Set canvas color to black
iface.mapCanvas().setCanvasColor(QColor.fromRgb(0,0,0))

print GEOJSON_DIR
print IMAGE_DIR
print FILENAME

LAYERS = []

def loadGeoJSON():
    i = 570
    while i <= NUMBER_OF_GEOJSON_FILES:
        #Load geojson
        print(GEOJSON_DIR + FILENAME + str(i) + ".geojson")
        layer = iface.addVectorLayer(GEOJSON_DIR + FILENAME + str(i) + ".geojson", "layer", "ogr")
        LAYERS.append(layer)

        #Set symbol colors, if no symbols do nothing
        try:
            symbols = layer.rendererV2().symbols()
            symbol = symbols[0]
            symbol.setColor(QColor.fromRgb(255,255,255))
        except AttributeError:
            print "No symbols"

        #Zoom to layer
        iface.mapCanvas().setExtent(layer.extent())
        i = i + 1

def exportImages():
    i = 1
    for j in LAYERS:
        sleep(3)
        #Zoom to layer
        iface.mapCanvas().setExtent(j.extent())
        # Take screenshot
        print("Screen Shotting:  " + IMAGE_DIR + FILENAME + str(i) + ".png")
        iface.mapCanvas().saveAsImage(IMAGE_DIR + FILENAME + str(i) + ".png")
        i = i + 1

loadGeoJSON()
exportImages()

Answer 1

After setting the current layer extent, you would need to refresh the map display

iface.mapCanvas().refresh()

If you want to do a particular action when the layer is empty, you can count the features

layer.featureCount()

The function would look like:

def exportImages():
    i = 1
    for j in LAYERS:
        sleep(3)

        if j.featureCount() == 0:
          #do something with empty layer... (skip, copy a blank image etc)
        else:
          #Zoom to layer
          iface.mapCanvas().setExtent(j.extent())
          iface.mapCanvas().refresh()
          # Take screenshot
          print("Screen Shotting:  " + IMAGE_DIR + FILENAME + str(i) + ".png")
          iface.mapCanvas().saveAsImage(IMAGE_DIR + FILENAME + str(i) + ".png")
        i = i + 1