Out-of-Memory errors on rather small map areas

[hholzgra]

There are cases where relatively small map areas lead to out-of-memory errors when rendering, e.g. when rendering parts of Nuuk/Greenland:

https://print.get-map.org/maps/226476

In the specific case above the culprit is the rock formation inside the bounding box, but there also have been other related cases.

This is not reproducible on all installations, so needs further root cause analysis ...

[hholzgra]

Might be related to:

pygobject/pycairo#239

[hholzgra]

A bit more on this: only the PDF and SVG backends seem to be affected,not the PNG backend.

Which explains why this never really affected tile server use cases ...

[hholzgra]

Turns out the problem is this rendering rule for glaciers, in combination with the Greenland Ice Sheet relation ( https://www.openstreetmap.org/relation/1279614 ):

  <Style filter-mode="first" name="water-areas-natural">
    <Rule>
      <MaxScaleDenominator>750000</MaxScaleDenominator>
      <Filter><![CDATA[([natural] = 'glacier')]]></Filter>
      <LineSymbolizer stroke="#99ccff" stroke-dasharray="4, 2" stroke-width="1.5"/> 

The problem is the stroke-dasharray attribute, without that rendering works fine.

Looks as if the dashing is generated for the whole polygon outline, not just the part visible in the image? With the PNG backend that does not seem to be a problem as pixels outside of the visible area are ignored, whereas the PDF and SVG backends generate a vector object for the full outline, which with an outline was long and as fractal as the Greenland Inland Ice one simply becoming too big ...?

[hholzgra]

New Mapnik bug report: mapnik/mapnik#4476