PyQGIS Splitting – How to Split Feature When Intersecting with Another Layer in PyQGIS

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I have a buffer layer (green polygon) which I want to split to two polygons whenever it crosses a barrier (blue line). I have been trying to use "splitGeometry" method, but I just can't get it to work. My code so far is this :

while ldbuffprovider.nextFeature(feat):
  while barprovider.nextFeature(feat2):
    if feat.geometry().intersects(feat2.geometry()):
        intersection = feat.geometry().intersection(feat2.geometry())
        result, newGeometries, topoTestPoints=feat.geometry().splitGeometry(intersection.asPolyline(),True)

Which returns 1 for result( error) and an empty list for newGeometries.
Any help is greatly appreciated.

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Best Answer

You can use the reshapeGeometry function of the QgsGeometry object for this, which cuts a polygon along its intersection with a line.

The following will intersect the buffer polygons with the lines, and add the split polygon features to a memory layer (QGIS 2.0 syntax):

# Get the dataProvider objects for the layers called 'line' and 'buffer'
linepr = QgsMapLayerRegistry.instance().mapLayersByName('line')[0].dataProvider()
bufferpr = QgsMapLayerRegistry.instance().mapLayersByName('buffer')[0].dataProvider()

# Create a memory layer to store the result
resultl = QgsVectorLayer("Polygon", "result", "memory")
resultpr = resultl.dataProvider()
QgsMapLayerRegistry.instance().addMapLayer(resultl)


for feature in bufferpr.getFeatures():
  # Save the original geometry
  geometry = QgsGeometry.fromPolygon(feature.geometry().asPolygon())
  for line in linepr.getFeatures():
    # Intersect the polygon with the line. If they intersect, the feature will contain one half of the split
    t = feature.geometry().reshapeGeometry(line.geometry().asPolyline())
    if (t==0):
      # Create a new feature to hold the other half of the split
      diff = QgsFeature()
      # Calculate the difference between the original geometry and the first half of the split
      diff.setGeometry( geometry.difference(feature.geometry()))
      # Add the two halves of the split to the memory layer
      resultpr.addFeatures([feature])
      resultpr.addFeatures([diff])

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