QGIS offset line shows perpendicular tags issue

[MrLando1]

What is the bug or the crash?

I'm adding data to a road network using LRS. I am offsetting the data to show different lanes of the road. In the example below I only have data for 1 lane.

However, some of the data shows an additional perpendicular "tag" towards the original network line. At some scales the "tag" disappears if I change the dimension to "meters at scale". If there is data for multiple lanes, then the tag is showing for the data for each lane. I have checked the network line (using Vector > Geometry Tools > Extract Vertices) and there are no additional vertices which could be responsible for this.

I tried turning off the layer's rendering properties simplify geometry setting

I'm using 3.28.2 but it also occurs on 3.30.1. HOWEVER, I have just discovered that if I load the project into 3.18.3, then issue does not occur (see 2nd image).

Steps to reproduce the issue

1. Use LRS "events" to plot data onto a network line.
2. Set styling so that the data is offset to the left (negative offset) of the network line. (note. if a positive offset is used, the issue still occurs but at other locations along the line)

Versions

Issue occurs on 3.28.2 and 3.30.1.
HOWEVER, I have just discovered that if I load the project into 3.18.3, the issue does not occur

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[agiudiceandrea]

@MrLando1, with "LRS" do you mean the extarnal Python "LRS" plugin https://plugins.qgis.org/plugins/lrs/? If yes, does the issue occur without the need to use such plugin? Could you please provide sample layer and project with which the issue does occur?

[MrLando1]

Hi @agiudiceandrea Yes, It was the external python LRS plugin, but I think that isn't what is causing the issue as the issue doesn't occur in all versions of QGIS..

I can confirm that the issue occurs in v3.28.2 and v3.30.1
I can confirm that the issue does NOT occur in v3.18.3, v3.22.5 or v3.26.3.

I have just created a Test project. I created four network lines, one relatively straight, one concave curve, cone convex curve and a spline (S).

I created dummy data in a csv and added it to the project.

I used v.to.points to create start and end nodes and then changed the Length field to show 0m for the start.
I then used LRS to calibrate a route, and used LRS to add the data using the Events tab.

When looking at the data as is, there isn't an issue, but as soon as I do an offset, the tags appear. To show this clearly, I had done an offset of -40 meters at scale

see attached project.
Tag Test 2.zip

[strk]

I do see those "tags" both using positive and negative offset. The image shows a SINGLE layer with 3 symbology layers being the original line, a left offset and a right offset (both offsets being 40 units). This is happening for me with QGIS 3.31.0 as of
5d6c016eb07 and GEOS 3.12.0dev-CAPI-1.18.0 with the recent fixes in single-sided buffer..

The problem is probably with the line NOT being a single line but a set of joined lines, with the offset curve algorithm not considering the joins, as better shown in this picture:

[strk]

I've just tested the same project with QGIS Release 3.22 (using the same GEOS version) and I see the same behaviour

[strk]

MrLando1 what version of GEOS is your working QGIS reporting ?

[agiudiceandrea]

@strk may it be related to #51583?

[strk]

GEOS version seem to have a role here as I'm getting bigger tags when running geos 3.11.2:

in one case on offsetcurve is almost a full ring (fid=43):

The same map when running against geos 3.10 (a3e863da45608b8a3ae62224fcec99b27f57cf48):

There are still intersections but no pependicular lines:

[strk]

FID=43 has this WKT: MultiLineString ((553772.06458926980849355 177770.05079236582969315, 553780.92358692409470677 177768.99614978794124909, 553781.83254859340377152 177768.41771963477367535))

[strk]

Smaller project with ONLY the single geometry is here: SmallCase.zip

[strk]

This is the operation which changed behaviour between GEOS-3.10 and GEOS-3.11. In the log we have the return from GEOS-3.11, which is the almost-full-ring one:

GEOSOffsetCurve(LINESTRING (417.9785426266025752 432.5653800140134990, 447.6161954912822694 436.0936720217578113, 450.6571140550076962 438.0288020166335627), 133.819, 0, 2, 2) returned LINESTRING (409.6582865640766045 502.4555309383532631, 402.1593597874252737 565.4465158614327720, 431.7970126521049679 568.9748078691770843, 439.7066040716936186 502.5342399454674478, 415.1760359914131868 493.7847818372570714)

[strk]

Interesting note: using the deprecated GEOSSingleSidedBuffer_r C-API signature works better for the above case

[dr-jts]

A work-around for this problem is to always use a Quadrant Segments value of at least 8.

(Note that for Offset Curves specified with a Mitre Join the Quadrant Segments value is essentially irrelevant for the output. It should be forced to 8 internally in the GEOS OffsetCurve code - this will be fixed at some point.)

[strk]

Thanks @dr-jts - I've filed #53234 with the workaround and will keep an eye on locationtech/jts#980 to port that fix to GEOS as soon as ready