Changing line style while crossing specific multipolygon using QGIS

Question

I'm new to QGIS and trying to change river style while crossing a lake. Something like this with one line and multipolygon:

Currently, I have two styles on one line:

It is one single line with "waterway"='stream' tag and one polygon with "natural"='water' tag. Have data from OSM file.

Is it possible to check if the line crosses a polygon and automatically changes the style?

I want to apply this rule to all lines with "waterway"='stream' tag.

Answer 1

Based on this awesome answer something like this should work:

1. Create a Geometry Generator with type LineString where you want to get the difference of your current geometry and the aggregated geometry of your polygon layer (only for elements that intersect with lines - see other answer for a very good explanation). This will give you the lines without the parts where they overlap the water polygons

difference(
    collect($geometry), 
    aggregate(
        'waterway_polygons',
        'collect',
        $geometry,
        intersects(
            $geometry,
            geometry(@parent)
        )
    )
)

2. Then you create a second Geometry Generator where you do the same but this time we are not calculating the difference but the intersection with the polygons, which will give you all the parts where you want to display only markers. Here you can use a Marker Line for styling.

intersection(
    collect($geometry), 
    aggregate(
        'waterway_polygons',
        'collect',
        $geometry,
        intersects(
            $geometry,
            geometry(@parent)
        )
    )
)

Answer 2

I use QuickOSM plugin to download all data with the Canvas Extent setting:

After it is finished I rename the polygon layer to polygons and the line layer to lines:

Then set lines layer symbology to rule-based and created one rule ("waterway"='stream'):

For the polygons layer I set another rule ("natural"='water'):

Then create two geometry generators under the "waterway"='stream' rule:

For the first geometry generator (blue arrows) I use this code:

intersection(
     aggregate('lines', 'collect',$geometry, "waterway"='stream'),
     aggregate(
         'polygons',
         'collect',
         $geometry,
         "natural"='water'
     )
)

And for the second geometry generator (blue line) I use this:

difference(
     aggregate('lines', 'collect',$geometry, "waterway"='stream'),
     aggregate(
         'polygons',
         'collect',
         $geometry,
         "natural"='water'
     )
)

And it works: