Fixing polygon with ring self-intersection in QGIS

Question

When I polygonize a raster, I very often have 'pixels' holes/interior ring that touches exterior ring.

An example:

POLYGON((5 0, 10 0, 10 10, 0 10, 0 0, 5 0, 3 3, 5 6, 7 3, 5 0))

It's defined as an invalid geometry in the OGC geometry specifications.

In QGIS, how to fix this?

Answer 1

I've found the solution here but for PostGIS: https://gis.stackexchange.com/a/15301/93097 and I've tested it in Spatialite/Virtual layers, and it works.

Bellow the code I used with a shapefile 'my_layer' with one field "DN" and wrote in the DB Manager:

SELECT "DN", ST_Buffer(geometry, 0.0) as geometry
FROM my_layer;

In the SQL query SELECT [...] FROM [...]; :

• "DN" as mentionned above is the field of my layer. It's a common field that contains raster informations after vectorization, stands for Digital Number. I put it in the query in order to get my attributes with the new virtual layer. If you have more/different fields, just write "my_field1", "my_field2", "my_field3" as in a SQL SELECT query instead of "DN".
• geometry is the default field name for geometry in a QGIS shapefile or memory layer.
• ST_Buffer(geometry, 0.0) : creates a buffer of 0, so a buffer with no distance, and it rewrites the polygon definition (vertices order) and fix self-intersections.