[QGIS-Developer] Reusing network graph

[flatoram]

Hi Prem,

I experienced those performance issues too.
When i process a lot of routing operations between points I will define an
routing area first on the source network layer (e.g. osm_lines).
I create a tempory layer sometimes in this way with pyqgis:

# define bounding box and make it a big larger (just an example)
rectangle = QgsRectangle(point_0, point_1)
rectangle = rectangle.scaled(2)   # depends on your use case

# build request
request = QgsFeatureRequest().setFilterRect(rectangle)
# request = request.setNoAttributes()  # when you do not need any attributes
# request = request.setSubsetOfAttributes(["speed"], layer.fields())  # use
only some fields
# add conditions to request to use only features you need, e.g. ignore
specific street types
new_layer = layer.materialize(request)  # create a memory copy of source
layer

In my use cases the new_layer is definitive smaller and more perfomant than
the main layer for those operations. Use the new layer for routing. But
maybe you have not the full available path in your new layer, when your
bbox was to small.

An other possibility for path finding is QgsTracer class, but it is
different to classic dijsktra/shortestpath when dealing with vertices or
line intersections

# use qgis tracer, but it is a bit different from classic dijkstra
tracer = QgsTracer()
tracer.setLayers([network_layer])
# maybe you have to define the destinationCrs
poly_line = tracer.findShortestPath(start_point, end_point)[0]


Depends on the use cases I saved a lot of calculation time.

- Felix

Am Do., 30. Dez. 2021 um 21:01 Uhr schrieb <
qgis-developer-request at lists.osgeo.org>:

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>    1. Reusing network graph (Prem Kumar)
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> Message: 1
> Date: Thu, 30 Dec 2021 19:16:41 +0530
> From: Prem Kumar <prem.netgis at gmail.com>
> To: qgis-developer <qgis-developer at lists.osgeo.org>
> Subject: [QGIS-Developer] Reusing network graph
> Message-ID:
>         <CAP2_p600CpiF=
> eJfRsCKby7yB1siX4vW7faWbCKSrROtB9cLUg at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi Team,
>
> I have been trying to get the shortest path for a lot of pairs of start and
> end points for a given road layer. Although I'm able to achieve this by
> creating the *QgsVectorLayerDirector *and Graph object for every pair of
> start and end points.
>
> However, I could not reuse the graph object or director object for all the
> pairs of start and end points, eventually the code is as slow as executing
> the* 'native:shortestpathpointtopoint'* processing tool
> programmatically for each pair of start and end points.
>
> Essentially, I'm trying to improve the performance by reusing the graph
> objects. Can someone help me whether there is a possibility to reuse the
> graph object for large sets of pairs? Here is the code snippet I have used.
>
> vectorLayer =
> QgsVectorLayer('testdata/network.gpkg|layername=network_lines',
> 'lines')director = QgsVectorLayerDirector(vectorLayer, -1, '', '', '',
> QgsVectorLayerDirector.DirectionBoth)strategy =
> QgsNetworkDistanceStrategy()director.addStrategy(strategy)builder =
> QgsGraphBuilder(vectorLayer.sourceCrs())startPoint =
> QgsPointXY(1179661.925139,5419188.074362)endPoint =
> QgsPointXY(1180942.970617,5420040.097560)tiedPoints =
> director.makeGraph(builder, [startPoint, endPoint])tStart, tStop =
> tiedPointsgraph = builder.graph()idxStart =
> graph.findVertex(tStart)idxEnd = graph.findVertex(tStop)(tree, costs)
> = QgsGraphAnalyzer.dijkstra(graph, idxStart, 0)
>
> -Prem
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