simplification makes geometry-based expression values (such as $area) vary for symbology and labels

[qgib]

Author Name: Mathieu Pellerin - nIRV (Mathieu Pellerin - nIRV)
Original Redmine Issue: 14301
Affected QGIS version: master
Redmine category:simplification

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When simplification is activated for a given vector layer, geometry-based expression values - such as $area - will vary within a symbology or label context when the zoom level in changed. I haven't looked at the code, but what I'm assuming happens here is that the expression is run against the simplified geometry, leading to varying values as the geometry is morphed.

That's a significant shortcoming that would be worth addressing. I discovered this when a number of polygons were going missing in a project using a density-based expression to make a graduated symbology.

I'm attaching a small test project to this issue that shows a polygon disappearing when zooming out (due to simplified polygon having an erroneous $area value).

Steps to reproduce

1. Open the attached project
2. Notice the rule-based expression being used: ($area / 10000) > 1309 AND ($area / 10000) < 1310
3. Zoom out onto the canvas a couple of times, you'll see the polygon go missing

If you change the symbology to a single symbol, you can zoom in and out and notice the label (which shows the result of the following expression: $area / 10000) value varies as the simplified polygon returns different $area values.

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• expression_area_simplification.zip (Mathieu Pellerin - nIRV)

[qgib]

Author Name: Alvaro Huarte (@ahuarte47)

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Hi, I would like to propose this pull [https://github.com//pull/2900] to fix the issue.

Best regards
Alvaro

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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Fixed in changeset "d4a482b982b6748a1812efa0ef075b65bf668ba4".

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• status_id was changed from Open to Closed