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I can't reproduce it with current trunk on Windows.

Alister Hood wrote:

I can't reproduce it with current trunk on Windows.

Downloaded current osgeo trunk on windows.

The bug is still present. Try to zoom in:

Coordinates  662789.4,5106991.6
 Scale   1:574

you can see the polygons with empty labels, i.e. label .1912 is 3 or 4 polygons higher than its correct position.

Confirmed with my own data. The horizontal(slow) option places the labels in the correct position, but is too slow.
The "over centroid" and "around centroid" options have a "non sense" behaviour

I'd experiencede the same behaviour, It seems to be related with small polygons, the label is moved away from it's correct position.

Confirmed. The PAL library calculates centroids by itself (not using GEOS) and apparently that code is suspectible to numerical errors. GEOS calculates the centroid fine, therefore GEOS should be used instead.

See also #5256

Should be fixed for 'over centroid' (now called 'Offset from centroid') with commit 61a50225

'Around centroid' was not adjusted; the calculation is still done in PAL. The noted pull request might fix that, however.

Please test.

My test says that the bug is fixed, thanks a lot!

But I have another comment...

If you try the attached test layer from trapanator, you can see that some labels are still completely outside their polygons. For example, take a look on the feature with attribute "num" = 2295.

The simple problem here is that the centroid is not ever inside the polygon (if the polygon is not a "simple square"), so the label placement is (imho) not correct.

In postgis there is the function ST_PointOnSurface that "Returns a POINT guaranteed to lie on the surface."
See the doc in http://postgis.refractions.net/documentation/manual-2.0/ST_PointOnSurface.html

I think that the best solution could be to put the label over centroid, but if the centroid is not inside the polygon, move the label inside the surface.

An automatic placement of a label outside its polygon for me has no sense and is confusing

But I have another comment...

If you try the attached test layer from trapanator, you can see that some labels are still completely outside their polygons. For example, take a look on the feature with attribute "num" = 2295.

The simple problem here is that the centroid is not ever inside the polygon (if the polygon is not a "simple square"), so the label placement is (imho) not correct.

In postgis there is the function ST_PointOnSurface that "Returns a POINT guaranteed to lie on the surface."
See the doc in http://postgis.refractions.net/documentation/manual-2.0/ST_PointOnSurface.html

I think that the best solution could be to put the label over centroid, but if the centroid is not inside the polygon, move the label inside the surface.

An automatic placement of a label outside its polygon for me has no sense and is confusing

I guess anyway that this should be closed and this request added in a new ticket(?)

michele zanolli wrote:

...
If you try the attached test layer from trapanator, you can see that some labels are still completely outside their polygons. For example, take a look on the feature with attribute "num" = 2295.

The simple problem here is that the centroid is not ever inside the polygon (if the polygon is not a "simple square"), so the label placement is (imho) not correct.

That is the calculated centroid location (as per the labeling option), so the label location is correct.

In postgis there is the function ST_PointOnSurface that "Returns a POINT guaranteed to lie on the surface."
See the doc in http://postgis.refractions.net/documentation/manual-2.0/ST_PointOnSurface.html

I think that the best solution could be to put the label over centroid, but if the centroid is not inside the polygon, move the label inside the surface.

An automatic placement of a label outside its polygon for me has no sense and is confusing

I agree there should be an option for moving any outlying centroid point to inside its polygon. There are several solutions. GEOS contains a PointOnSurface, though I could not find a public API call for it, it appears to be available via OGR. But the question is where would the on surface point end up?

One way to know where a surface point is going to be located is to translate due left or right from the calculated outlying centroid point and place a label in the center of the first found line or polygon that fully intersects the centroid's polygon. While not be a perfect solution, it may look reasonable in most situations. There are other solutions like finding the closest vertex or segment of the polygon (or other ray-casting techniques), then placing the point inside the polygon from there. See:

Algorithm for finding irrregular polygon centroid (label point)
http://gis.stackexchange.com/questions/2128

There also needs to be a more robust fixing of invalid GEOS geometries (bow ties, etc.) prior to finding the centroid. GEOS can fix some geometries, but there is some new research and GPL-compatible libraries available for this that may give better results, like pprepair:

https://github.com/tudelft-gist/prepair#readme

That is really a feature request and should be a new ticket, as Giovanni suggested.

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I may be able to re-work commit 61a50225 solution to also include support for centroids to be created via GOES for 'around centroid' labeling options. Until someone does something like that, or applies/tests the pull request, I think this issue should remain open.