I've to rotate a marker line in a polygon ring by 180 degrees without rotating the same marker line at the polygon border (outline):
How can I do this in QGIS 2.18/2.99?
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Have you read either of these two posts? How to rotate symbols in QGIS & Rotate marker symbols individually in QGIS – whyzar Feb 04 '17 at 17:07
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I don't want to rotate markers by field values. All triangles (marker line) should be placed inside the polygon. It seems this is impossible for polygons with a ring. – eclipsed_by_the_moon Feb 04 '17 at 23:23
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I believe so as well, it's a nice feature to have. However, it would be a property associated with multi-polygon features that identifies the outer and inner rings separately and allowing individual manipulation – whyzar Feb 04 '17 at 23:24
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I could reproduce this in 2.18.3 with a scratch layer of type POLYGON, where i digitised both the outer ring (exterior) and hole (using the add ring tool) in a clockwise direction, and applied 180 degrees to the marker rotation. (Image on left)
I was able to get the result you were after by digitising the hole in an anti-clockwise direction... (Image on right)
It doesn't happen if you create a new SHAPEFILE layer; I assume this automatically corrects the hole so that its vertices go anti-clockwise.
Just to add to the confusion, it looks like you need to use a rotation value of 0 for a MULTIPOLYGON layer..
EDIT tried all 4 combinations of winding for the exterior and hole. It looks like the assumption is the exterior is assumed to be clockwise and holes anti-clockwise. This may not be the case for all formats. But provided holes use a different winding to the exterior, you should be able to get it to work.
Steven Kay
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2The features are saved in a PostGIS polygon layer. I suppose that ST_ForceRHR() fixes the marker line symbology. I'll do some tests in the next days. – eclipsed_by_the_moon Feb 06 '17 at 11:55