Styling shared boundaries in QGIS?

Question

I have 2 polygon layers, 1 showing roads and another showing parking restrictions on those roads (no parking, residents only, etc). I'm trying to style the layer according to the markings that would appear on the actual street (double yellow lines for no parking and white dashed line for residents only).

I've generated styles using a negative buffer in the geometry generator. But I'm not sure how to show only the section of polygon outline that are next to the curb (i.e. the shared outline of the road and restriction layers). This is as far as I have got:

But I'd like this as the end result (created with MS paint):

I need to achieve this without actually altering the geometry or creating any new layers that are for 'cartographic purposes only'. I found a useful question and answer here, but it's not quite what I'm looking for.

Answer 1

I propose an approach that only recurs to a geometry generator and a custom function.

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Context

Let's assume to start from this situation (hopefully very close to yours), where the several parking restrictions are stored in the "Condition" field:

I also assume that you know how the QGIS custom functions work, otherwise this post might be helpful: How to create custom functions in QGIS using the function editor.

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Preliminary task - Creating the custom function

After having loaded your layers in QGIS, open the Field Calculator and click on the Function Editor tab.

Then, click on New file and type curb_lines as the name of the new function:

You will see that a new function has been created and it is listed on the left side of the dialog. Now, click on the name of the function and replace the default @qgsfunction with the following code:

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

@qgsfunction(args='auto', group='Custom')
def curb_lines(restrictions_layer_name, roads_layer_name, feature, parent):

    def segments(polyline):
        for x in range(0, len(polyline)-1):
            first_point = polyline[x]
            second_point = polyline[x +1]
            seg = QgsGeometry.fromPolyline([first_point, second_point])
            idsList = index.intersects(seg.boundingBox())
            for id in idsList:
                tmp_geom = all_feats[id].geometry()
                if seg.touches(tmp_geom):
                    num.append(seg)
        return num

    layer  = QgsMapLayerRegistry.instance().mapLayersByName(restrictions_layer_name)[0]
    ro_layer  = QgsMapLayerRegistry.instance().mapLayersByName(roads_layer_name)[0]

    all_feats = {}
    index = QgsSpatialIndex()
    for ft in ro_layer.getFeatures():
        index.insertFeature(ft)
        all_feats[ft.id()] = ft

    input_geometry = QgsGeometry(feature.geometry())
    line_geom = QgsGeometry(input_geometry.geometry().boundary())
    polyline = line_geom.asPolyline()
    num = []
    res = segments(polyline)

    first = True
    for h in num:
        if first:
            geom = QgsGeometry(h)
            first = False
        else:
            geom = geom.combine(h)

    return geom

Once you have done this, click on the Load button and you will be able to see the function from the Custom Menu of the Expression dialog.

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Solution

Firstly, go to Layer Properties | Style and then choose the Rule-based renderer.

Create two different rules (if you have only two categories, otherwise adapt it to your case): I will show you how to get a working solution only for the first category because the procedure is the same for all the others.

Double-click on the first rule and set the filter expression (step 1, see the image below):

"Condition" = 'Residential only'

From the Symbol selector dialog, choose a Geometry generator as symbol layer type (step 2) and Linestring / MultiLinestring as geometry type (step 3).

Now, type this expression (step 4):

curb_lines('restr', 'roads')

where 'restr' and 'roads' are the layer names for the restrictions and roads layers, respectively (adapt them to your case).

If you want to set a different style for each category, go to step 5 and set Color, Pen width, Offset and so on (the answer won't cover this part).

Click on the OK button and repeat the previous steps for the other categories.

Finally, click on the Apply button for applying the changes.

You will see something like this: