"Points Along Geometry" not creating points if geometry is multipart

[earthcli]

I found the result of qgis:pointsalonglines with v3.10 and 3.2.2 are different, exactly the new version gives a wrong result. see the screenshot attatched.

[earthcli]

[earthcli]

[gioman]

@earthcli can you attach sample data?

[earthcli]

@gioman Thanks for your quick response, the sample data is located here:
https://github.com/pcoddo/CambodiaFloods/tree/master/1_Depth_Estimation/Layers

the input is the 'flood_lines.shp'

[gioman]

the input is the 'flood_lines.shp'

the lines where the point are not created are multi-part geometries, if you first use "multipart to singleparts" the result is the expected one, but apparently is still a regression anyway.

[earthcli]

I see, many thanks.

[agiudiceandrea]

@earthcli, with QGIS 3.10 are you using gdal:pointsalonglines or native:pointsalonglines?

qgis:pointsalonglines is no longer in QGIS 3.10.

If you are using native:pointsalonglines while in QGIS 3.2 you used qgis:pointsalonglines, the problem could be related to the algorithm port to c++ #30175.

[earthcli]

@agiudiceandrea I use qgis:pointsalonglines in the plugin python console, it does works,
I have also tried run it with the processing box GUI (module: points along geometry) , I get the identical result as in python console.

[agiudiceandrea]

In QGIS 3.10.1 (on my Windows 7 system) hovering the pointer on the algorithm name "Points along geometry" in the processing toolbox I see "Algorithm ID: 'native:pointsalonglines'".

In the python console, executing QgsApplication.processingRegistry().algorithmById("qgis:pointsalonglines").id() I see "'native:pointsalonglines'".
So, in QGIS 3.10, qgis:pointsalonglines (the name of the old python algorithm) is only an alias for native:pointsalonglines (the new c++ algorithm).

[agiudiceandrea]

Both QGIS 3.4.5 (which uses the python version of the algorithm) and QGIS 3.10.1 (which uses the c++ version) seem to behave the same way when a multi part geometry feature is processed by "Points along geometry" alg:

1. points are generated only on the basis of the first part of a multi-part (line/polygn) geometry

This seems more likely related to a change in QgsGeometry::interpolate (c6a91da #7587) on which both the algorithm versions rely on.
Since 3.4, QgsGeometry::interpolate works only on the first part of a multi-part geometry by design.

QGIS/src/core/geometry/qgsgeometry.cpp

Line 2027 in c6a91da

// if multi part, just use first part

Before 3.4 (i.e. 3.2.2), it relies on GEOS in order to interpolate.

2. additional spurious points with null coordinates are generated

This seems related to the algorithm itself (both python and c++ versions) that doesn't take into account the aforementioned change and doesn't test for multi part geometry, so it tries to generate the points along the total length/perimeter of a multi part line/polygon geometry instead of along the length/perimeter of the first part only.

QGIS/src/analysis/processing/qgsalgorithmpointsalonggeometry.cpp

Lines 188 to 189 in b8f1d25

	const double totalLength = geometry.type() == QgsWkbTypes::PolygonGeometry ? geometry.constGet()->perimeter()
	: geometry.length() - endOffset;

QGIS/python/plugins/processing/algs/qgis/PointsAlongGeometry.py

Lines 124 to 127 in e83d02e

	if input_geometry.type == QgsWkbTypes.PolygonGeometry:
	length = input_geometry.constGet().perimeter()
	else:
	length = input_geometry.length() - end_offset