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There is something called Distance Transform that computes distance from each point to the border. We could probably use it to get nice labeling of complex polygons: the idea is to do this transform for each (rasterized) polygon - that would trigger a kind of heatmap (high score for central areas, low score for areas close to the border) which would be used as a source for costs of label candidates.

http://en.wikipedia.org/wiki/Distance_transform

Doesn't QGIS come with GEOS? It could be worth having a try if label placement would look better by using GEOS PointOnSurface method which is the same as JTS getInteriorPoint
http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/geom/Geometry.html#getInteriorPoint%28%29
The speed is not necessarily much slower.
PostGIS implements this as http://www.postgis.org/docs/ST_PointOnSurface.html.

Yeah, PointOnSurface could be an alternative to centroid - it would be good to try it out and have a comparison!

Using the 'Horizontal' or 'Free' placements in label configuration the text is drawed inside of polygon.
The PAL label library, that QGIS uses, only forces the label inside of polygon with these placements.

See:
https://github.com/qgis/QGIS/blob/master/src/core/qgspallabeling.cpp#L3243
https://github.com/qgis/QGIS/blob/master/src/core/pal/feature.cpp#L1342
https://github.com/qgis/QGIS/blob/master/src/core/pal/feature.cpp#L1034

Best Regards
Alvaro

Hi Alvaro,

Alvaro Huarte wrote:

Using the 'Horizontal' or 'Free' placements in label configuration the text is drawed inside of polygon.
The PAL label library, that QGIS uses, only forces the label inside of polygon with these placements.

Neither of those placements are based upon the polygon's centroid, but instead upon available area. Tim's request is to base it off the centroid and ensure it is inside the polygon.

Hi Larry, I propose a new pull request (https://github.com/qgis/QGIS/pull/1238) using the 'GEOSPointOnSurface' method.

The code has three commits:

+ New 'pointOnSurface' method for QgsGeometry using 'GEOSPointOnSurface'.
+ New option for CentroidFillSymbolLayerV2 to force the location of centroid inside of polygons.
+ Force label position inside of polygons in PAL labeling library.

Best Regards
Alvaro

Yes, that seems like it might be faster, especially if the intersections can be done incrementally (if that matters). For example in this case, after the first two intersections are located, that would be all that is needed.

Though your algorithm skips doing a call to pointOnSurface() first, which should help with performance, don't we still need to do that call, then check if the resultant point is on, or really close, to an edge before doing such an algorithm? In my testing I found that ~75% of the time pointOnSurface() is already good enough. Just need to do something like what you are suggesting for the other ~25%, whenever it happens to fall on an edge. Or, is that what you are already suggesting?

PS: I think the discussion should stay here now, instead of github.com and here. :-)

Larry Shaffer wrote:

Yes, that seems like it might be faster, especially if the intersections can be done incrementally (if that matters). For example in this case, after the first two intersections are located, that would be all that is needed.

Though your algorithm skips doing a call to pointOnSurface() first, which should help with performance, don't we still need to do that call, then check if the resultant point is on, or really close, to an edge before doing such an algorithm? In my testing I found that ~75% of the time pointOnSurface() is already good enough. Just need to do something like what you are suggesting for the other ~25%, whenever it happens to fall on an edge. Or, is that what you are already suggesting?

PS: I think the discussion should stay here now, instead of github.com and here. :-)

When the centroid is not inside of polygon, I think that first it must get all intersection points and after sort these points using the distance to the centroid. Then the algorithm test each middle of pair of points if inside of polygon.