Filtering features outside features of another layer using QGIS geometry generator

Question

I am trying to filter features in one layer outside features in another layer using geometry generator.

I have a buildings layer and an area layer and now I want to only show the buildings outside the areas.

The buildings layer is a polygon layer and the area layer is also a polygon layer consisting of several areas (features).

Since it is a many feature area layer I think I need to use an aggregate function e.g. collect() and/or aggregate() from the QGIS expression builder.

I trying the following expression on the geometry generator on the buildings layer...

difference( 
    $geometry, collect(geometry
                            (get_feature('layer_id','attribute','value')
                                )
                            )
                       )

...which ends up in an syntax error message.

Answer 1

Try something like this:

if(disjoint($geometry,aggregate('area','collect',$geometry)) = 1,$geometry,NULL)

Explanation:

• aggregate('area','collect',$geometry) creates one single MultiPolygon out of your area layer
• disjoint($geometry,MultiPolygonFromAreaLayer) then checks if the current building is outside the MultiPolygon or inside. It will return 1 if its outside and 0 if its inside. If disjoint() does not meet your criteria, you can use intersects(), within(), contains(), etc instead.
• The if() statement then evaluates wheter the current geometry should be displayed ($geometry) or not (NULL). Depending on your usecase you can vary this.