Polylabel algorithm operation

[pavel-avilov]

Hi, could you please explain how does the polylabel algorithm work in the LayerAsCentroid/Centroid methods.

As I understand, the polylabel algorithm should find the point that is the most distant from any edge. I've noticed that some of the house numbers are located a little strangely. I've used the LayerAsCentroid method (just like in your process-openmaptiles.lua) to save the house number info.

Examples of strange housenumber placement with default styles:

For example, on the last screenshot the house number for way is located at the following coordinates:
lat: 51.536720348065
lon: -0.1600334265625

In OSM it looks like this:

I decided to double-check the work of polylabel on this house using Python-Polylabel

I ran the following:

polylabel([[51.5372152, -0.1604443], [51.5371332, -0.1606558], [51.5365747, -0.1600228], [51.5365893, -0.1599317], [51.5366517, -0.1597992], [51.5372152, -0.1604443]])

The code as I checked:

from OSMPythonTools.overpass import Overpass
from polylabel import polylabel

overpass = Overpass()
way_id = '501460502'
query = f'way({way_id}); out geom;'
result = overpass.query(query)
all_coord = []

# Get way coords
for way in result.ways():
    print(f'Way ID: {way.id()}')
    print('Coordinates:')
    for coord in way.nodes():
        print(f'Latitude: {coord.lat()}, Longitude: {coord.lon()}')
        all_coord.append([coord.lat(), coord.lon()])

print(f"polylabel({all_coord}) >")
print(polylabel([all_coord]))

and then I got
lat: 51.53688964928815
lon: -0.16022586209752077
In OSM it looks like this:

Actually, tell me pls, is this behaviour normal?
Why does LayerAsCentroid place the housenumber for https://www.openstreetmap.org/way/501460502 at 51.536720348065, -0.1600334265625, not at 51.53688964928815, -0.16022586209752077?

[cldellow]

tilemaker's polylabel is mainly a port of MapBox's.

One difference might be the default value of the precision parameter. MapBox's precision is 1 (this appears to be the same for python-polylabel), but tilemaker's is 0.00001.

tilemaker chose the smaller precision because tilemaker operates in terms of longitude and latitude, and from MapBox's readme:

Be careful to pick precision appropriate for the input units. E.g. in case of geographic coordinates (longitude and latitude), 0.000001 is appropriate, while the default (1.0) would be too imprecise.

Do you get the same results as tilemaker if you choose a precision of 0.00001?