Checking that lines lay on polygon boundaries using PyQGIS

Question

I have a polygon layer with one polygon and a line layer with a lot of lines. I need to check lines lie on the polygon boundaries or not and if they are add them into a graph. I couldn't think of anything better that convert polygon boundaries to a line layer and I tried to write such code to build a graph but it doesn't work. In fact I need to add polygon boundaries to a graph.

How can I do this?

import networkx as nx
from qgis.core import *
import processing

from shapely.geometry import Polygon, Point, LineString
from shapely.ops import unary_union

polygon_layer = QgsProject.instance().mapLayersByName("polygon")[0]
polygon_feature = polygon_layer.getFeature(0)
polygon = Polygon(polygon_feature.geometry().asMultiPolygon()[0])
polygons = [p for p in polygon_feature.geometry().asMultiPolygon()]
selected_polygon = Polygon(polygons[0][0])

G = nx.Graph()

for vertex in selected_polygon.exterior.coords:
    G.add_node(vertex)

temp_lines = processing.run("native:polygonstolines", {'INPUT': polygon_layer, 'OUTPUT':'TEMPORARY_OUTPUT'})["OUTPUT"]
QgsProject.instance().addMapLayer(temp_lines)                

for node1 in G.nodes:
    for node2 in G.nodes:
        if node1 != node2:
            line = LineString([node1, node2]) 
            for feature in temp_lines.getFeatures():
                geom = feature.geometry().asMultiPolyline()
                if line.within(geom):
                    G.add_edge(node1, node2, weight=Point(node1).distance(Point(node2)))

I also tried to convert polygon to lines, explode it through PyQGIS and then add it into graph, but I got incorrect geometry error (Too few point).

For example, I have such a polygon (red) and lines (blue). And I need to select only boundaries lines (yellow) or both inside and boundaries lines and add them into a graph. I don't need lines outside or partly outside the polygon.

Answer 1

shapely: intersects(), contains(), crosses(), equals(), touches(),within() ...

from qgis.core import *
import processing

from shapely.geometry import Polygon, Point, LineString
from shapely.wkt import loads

polygonLayer = QgsProject.instance().mapLayersByName("Polygon")[0]
polygonFeature = polygonLayer.getFeatures()
for feature in polygonFeature:
    wkt = feature.geometry().asWkt()
    polygonShapely = loads(wkt)
    
lineLayer = QgsProject.instance().mapLayersByName("Line")[0]
lineFeature = lineLayer.getFeatures()

selection = []

for feature in lineFeature:
    wkt = feature.geometry().asWkt()
    lineShapley = loads(wkt)
    if lineShapley.touches(polygonShapely) or lineShapley.within(polygonShapely):
        selection.append(feature.id())
    else:
        pass

lineLayer.select(selection)

EDIT 14. Jan. 2024:

I've tried to rebuild your polygon. As far as I understand your comment the line 2 and 3 in my new drawing touch the polygon in the vertices. So it will be selected with touches(). So you could change touches() with covered_by().

If you go through the documentation shapley binary predicates you will find a combination that fits for you. You could also try to use not in some cases (just as example):

if not lineShapley.covered_by(polygonShapely)

or filter a selection again in a second loop.

for feature in lineFeature:
    wkt = feature.geometry().asWkt()
    lineShapley = loads(wkt)
    if lineShapley.covered_by(polygonShapely) or lineShapley.within(polygonShapely):
        selection.append(feature.id())
    else:
        pass

If you run into an issue with covered_by() you could think about reducing precession of your geometry, or using a small buffer.