Unexpected behavior of collect() in geometry generator

Asked Jan 10 '19 at 19:21

Active Jan 10 '19 at 19:21

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I'm having unexpected results using geometry generator with the following expression:

 buffer(collect($geometry),0)

…which was an answer given in this topic. I have used this previously with the expected result, but now something is strange. It appears that the geometry is being collected (and buffered) not once, but over and over—see the attached screencast. It's not infinite; it will do this about 20-25 times and then stop.

I can't tell which part of the function is the problem. If I just call collect($geometry), I'll get the same result (but rendered faster as it's not buffering). If I just call buffer($geometry, 0), then it renders some features only once, and others a varying multiple of times (see screenshot).

I'm sure it's data related because it doesn't happen on all layers. What can I look for that would cause this multiple rendering?

asked Jan 10 '19 at 19:21

Nathan

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What happens if you use a very small, non-zero buffer? – csk Jan 15 '19 at 19:49
Same result, but with a small, non-zero buffer applied. – Nathan Jan 17 '19 at 18:42
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Did you check for invalid geometries? – csk Jan 17 '19 at 19:50
Yes, valid geometries are required under digitizing settings. – Nathan Jan 18 '19 at 20:50
I found if you have 3 features in the shapefile, QGIS will apply the geometry generator 3 times. What we need is an expression to merge all features, not collect. – James B Sep 10 '19 at 15:11

Unexpected behavior of collect() in geometry generator

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