Point Pattern Fill issue in canvas rendering and exporting images

[qgib]

Author Name: Geo Dev (Geo Dev)
Original Redmine Issue: 18135
Affected QGIS version: 3.5(master)
Redmine category:symbology

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The horizontal and vertical distances in Point Pattern Fill are not always regular when set in map units. At certain map scales, the markers are misaligned and the pattern distances progressively increase from the upper-left area to the lower-right area.

Open the attached vector layer with the associated style to replicate the issue.

The layer contains a square polygon filled with a style composed of:

• a Line Pattern Fill with spacing=100 map units and rotation=0° (horizontal lines);
• a Line Pattern Fill with spacing=100 map units and rotation=90° (vertical lines);
• a Point Pattern Fill of circular markers with the same spacing of the line patterns (horizontal and vertical distance=100 map units).

The two line patterns draw a regular grid and the markers should be exactly placed at the intersections of the horizontal and vertical lines. Instead, at certain zoom levels (e.g. change the map scale starting from 1:500 with step=50: 1:550, 1:600, 1:650 ...), they are quite aligned to the grid in the upper-left area but they become increasingly misaligned going to the lower-right area.
The misalignment of the point pattern with respect to the other two line patterns inhibits the making of a regular composite pattern.

The same issue also occurs in pdf/images exported with the Print Layout. Exporting the map e.g. with resolution=100dpi, the markers are not aligned to the grid even when they are rendered as expected in the canvas (the issue would seem not to occur exporting with resolution=300dpi).

The issue is OS independent and it's occurs both in QGIS 2.99.0 and QGIS 2.18.16.

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• test.geojson (Geo Dev)
• test.qml (Geo Dev)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from master to 2.99(master)

[qgib]

Author Name: Geo Dev (Geo Dev)

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The issue still occurs in QGIS 3.0 and QGIS 3.1(master).

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from 2.99(master) to 3.1(master)

[qgib]

Author Name: Geo Dev (Geo Dev)

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The issue still occurs in QGIS 3.4.1 and QGIS 3.5(master).

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• version was changed from 3.1(master) to 3.5(master)

[pjshelton]

see #28910 for a linear version of this bug.

Both are likely related to #26749

[Pedro-Murteira]

Still an issue on QGIS 3.22.3. (Windows 10). The grid only appears to be correct above 1:640 map scale.