vector - Styling specific layer by using polygon mask in QGIS?

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

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:

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.

Answer

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

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

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:

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Edit:

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())

            feat_list.append(intersection)
    for x in feat_list:
        geoms = geoms.combine(x)
    return geoms

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):