A common requirement in GIS is to apply a processing tool to a number of files or apply a process for a number of features in one file to another file.
Much of these operations are embarrassingly parallel in that the results of the calculations in no way influence any other operation in the loop. Not only that but often the input files are each distinct.
A classic case in point is the tiling out of shape files against files containing polygons to clip them against.
Here is a (tested) classical procedural method to achieve this in a python script for QGIS. (fyi the output of temporary memory files to real files more than halved the time to process my test files)
import processing import os input_file="/path/to/input_file.shp" clip_polygons_file="/path/to/polygon_file.shp" output_folder="/tmp/test/" input_layer = QgsVectorLayer(input_file, "input file", "ogr") QgsMapLayerRegistry.instance().addMapLayer(input_layer) tile_layer = QgsVectorLayer(clip_polygons_file, "clip_polys", "ogr") QgsMapLayerRegistry.instance().addMapLayer(tile_layer) tile_layer_dp=input_layer.dataProvider() EPSG_code=int(tile_layer_dp.crs().authid().split(":")) tile_no=0 clipping_polygons = tile_layer.getFeatures() for clipping_polygon in clipping_polygons: print "Tile no: "+str(tile_no) tile_no+=1 geom = clipping_polygon.geometry() clip_layer=QgsVectorLayer("Polygon?crs=epsg:"+str(EPSG_code)+\ "&field=id:integer&index=yes","clip_polygon", "memory") clip_layer_dp = clip_layer.dataProvider() clip_layer.startEditing() clip_layer_feature = QgsFeature() clip_layer_feature.setGeometry(geom) (res, outFeats) = clip_layer_dp.addFeatures([clip_layer_feature]) clip_layer.commitChanges() clip_file = os.path.join(output_folder,"tile_"+str(tile_no)+".shp") write_error = QgsVectorFileWriter.writeAsVectorFormat(clip_layer, \ clip_file, "system", \ QgsCoordinateReferenceSystem(EPSG_code), "ESRI Shapefile") QgsMapLayerRegistry.instance().addMapLayer(clip_layer) output_file = os.path.join(output_folder,str(tile_no)+".shp") processing.runalg("qgis:clip", input_file, clip_file, output_file) QgsMapLayerRegistry.instance().removeMapLayer(clip_layer.id())
This would be fine except that my input file is 2GB and the polygon clipping file contains 400+ polygons. The resulting process takes over a week on my quad core machine. All the while three cores are just idling.
The solution I have in my head is to export the process out to script files and run them asynchronously using gnu parallel for example. However it seems a shame to have to drop out of QGIS into an OS specific solution rather than use something native to QGIS python. So my question is:
Can I parallelise embarrassingly parallel geographic operations natively inside python QGIS?
If not, then perhaps someone already has the code to send this sort of work off to asynchronous shell scripts?