I have a line layer and a polygon layer in QGIS:

Before mask

I would like to style the part of the line layer outside the polygon using one style and the part inside using a different style:

After mask

I don't want to create a derivative data set, ex. clip the line layer and style the two parts.

This is a simple case but in my QGIS project I have +30 layers so I think any layer blending would disturb the underlying layers.

Is it possible to do something like this?

I don't want to show the polygon layer, it is just here to visualise what I would like to do.

  • 1
    Nice method! I think that should be posted as an answer instead of an edit to the question :)
    – Joseph
    Commented Jul 13, 2017 at 15:21
  • @Joseph, it's done!
    – Chau
    Commented Jul 14, 2017 at 6:44

2 Answers 2


Not a perfect solution but you could make use of the Geometry Generator which adds a visualised line to represent the intersection. You could then set this to overlap the original line feature.

Add a new symbol layer by clicking the plus sign and select the Geometry generator as symbol layer type. Set the geoemtry type to LineString / MultiLineString and use the following expression:

intersection($geometry, geometry(get_feature( 'polygonLayer','fieldName','value'))) 

You would need to add details about your specific polygon where:

  • polygonLayer is the name of your polygon layer
  • fieldName is the name of the field
  • value is the feature value of your specific polygon

Style properties

Note that to colour the visual line, you may need to do it from the Draw effects property:

Draw effects properties

This was the result (note that the visual line did not overlap the original line completely so I modified the offset slightly):


And without the polygon:

Result without polygon


If you want this to be applied for each line feature intersecting a polygon feature, go to the Function Editor and use the following function (change the name of polygon example_2 to match the name of your polygon layer):

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def func(feature, parent):
    polygon_layer = QgsMapLayerRegistry.instance().mapLayersByName( "polygon example_2" )[0]
    feat_list = []
    geoms = QgsGeometry.fromWkt('GEOMETRYCOLLECTION()')
    for polygon_feat in polygon_layer.getFeatures():
        if feature.geometry().intersects(polygon_feat.geometry()):
            intersection = feature.geometry().intersection(polygon_feat.geometry())
    for x in feat_list:
        geoms = geoms.combine(x)
    return geoms

Function editor

Click Load then go to the Expression tab and type in func(). Hopefully the result should look like the following (using the same style properties mentioned above):

Final result

  • I actually looked at that but stopped when I discovered, that get_feature requires field name and value. I just have a polygon layer and would like to use all features on that layer to mask out. Is that possible?
    – Chau
    Commented Jul 10, 2017 at 13:51
  • @Chau - Edited post to include a possible method :)
    – Joseph
    Commented Jul 10, 2017 at 14:53
  • 1
    Another option would be to dissolve the polygon layer.
    – csk
    Commented Jul 10, 2017 at 19:07
  • 1
    @Joseph - When using a Geometry Generator is the method func called for every feature on the layer where it is used for styling? So if my line layer has 3 features, then func is called 3 times and drawing the same result 3 times?
    – Chau
    Commented Jul 12, 2017 at 8:08
  • 1
    @Chau - I think you were right, the code iterated through each feature multiple times. Edited the post so that func should now only be called per each line feature and will only draw the result once (which seems to be the case as shown by the vertex markers inside the polygons, before this was hidden underneath which I missed). Thanks for pointing this out :)
    – Joseph
    Commented Jul 12, 2017 at 9:14

Extending on Joseph's answer I came up with this function. It accounts for different coordinate systems and I needed to lookup into two masking layers, thus it handles that too. Further more I wanted to be able to either mask the lines inside the polygons or the lines outside the polygons.

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface

@qgsfunction(args='auto', group='Custom')
def mask_line_with_polygon(mask_type, line_layer_name, polygon_layer_name_1, polygon_layer_name_2, feature, parent):
    line_layer = QgsMapLayerRegistry.instance().mapLayersByName( line_layer_name )[0]

    # This is the geometry outside the polygon mask.
    outside = QgsGeometry(feature.geometry())

    polygon_layer_names = [polygon_layer_name_1, polygon_layer_name_2]
    line_feature_extent = outside.boundingBox()

    geoms = QgsGeometry.fromWkt('MultiLineString()')

    for polygon_layer_name in polygon_layer_names:
        if polygon_layer_name is None or len(polygon_layer_name) == 0:

        # If the line and the polygon layers have different projections, handle them here.
        polygon_layer = QgsMapLayerRegistry.instance().mapLayersByName(polygon_layer_name)[0]
        trs = QgsCoordinateTransform(line_layer.crs(), polygon_layer.crs())
        polygon_extent = trs.transform(line_feature_extent)
        trs = QgsCoordinateTransform(polygon_layer.crs(), line_layer.crs())

        # Go through the features in the polygon layer, but only those within the line feature bounding box.
        for feature in polygon_layer.getFeatures(QgsFeatureRequest().setFilterRect(polygon_extent)):
            polygon_geometry = QgsGeometry(feature.geometry())

            # Transform the polygon to line space.

            if outside.intersects(polygon_geometry):
                if mask_type.lower() == 'outside':
                    inside = outside.intersection(polygon_geometry)

                    if inside.isMultipart():
                        for x in inside.asMultiPolyline():

                outside = outside.difference(polygon_geometry)

    if mask_type.lower() == 'inside':
        if outside.isMultipart():
            for x in outside.asMultiPolyline():

    return geoms

This exercise has shown me that QGIS isn't too fond of working with large data sets and this algorithm with QGIS crashing on my way too often. I suspect that the QGIS renderer doesn't like to render Geometry Generators which are time consuming.


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