Rendering artifact on leisure=marina

[lest69]

I just noticed there's a small artifact on the rendering of leisure=marina. It's on every marina area I checked in my region, and appears to always be on the westernmost node of the area.

There are a number of marinas around here if you want to inspect them further.

[gravitystorm]

Yes, I noticed that too and I've no idea what's causing it. I suspect it might be a bug in mapnik, so if anyone wants to create a test case for mapnik then we could report it upstream.

[matthijsmelissen]

This is a consequence of counterintuitive behaviour of negative line-offset.

Example:

line-offset: -5;
line-color: red;
b/line-offset: 0;
b/line-color: blue;

I'm not sure if this is a bug, because I'm not sure what the desired behaviour in general would be, for example if the lines in the upper left continued.

Mapnik input for reference:

  <Rule>
    <MaxScaleDenominator>50000</MaxScaleDenominator>
    <LineSymbolizer offset="-5" stroke="#ff0000" />
    <LineSymbolizer offset="0" stroke="#0000ff" />
  </Rule>

[matthijsmelissen]

It is basically impossible to solve this issue while keeping the current rendering. Using positive line-offset doesn't look good because it causes the blue line to overlap with the land. Personally I like the current rendering apart from the rendering artifact.

Opinions?

[gravitystorm]

That's a great example, and I believe it to be a bug in mapnik - or at least, definitely not desired behaviour! I would advise raising a ticket for mapnik, making clear that this is appearing on polygons (and not closed loop linestrings).

[matthijsmelissen]

Created a bug at Mapnik: mapnik/mapnik#2462

[matkoniecz]

mapnik/mapnik#2851 is a new mapnik ticket about "Predictable offsetting on polygons"

2834 introduces a problematic back compatibility problem for cartographers using line-offset on polygons because it will potentially flip the side of the polygon that the offset occurs on.

This may be a serious blocker for people upgrading to Mapnik 3.0.0.

Let's brainstorm solutions...

[matthijsmelissen]

This should be fixed upstream: mapnik/mapnik#2904

Thanks @springmeyer!

[matkoniecz]

Also, thanks @flippmoke!

[matkoniecz]

Is it also covering rendering border only on one element of multipolygon like at http://www.openstreetmap.org/?mlat=51.5386&mlon=-0.1576#map=12/51.5386/-0.1576 ?

multipolygon: http://www.openstreetmap.org/relation/231792#map=17/51.53532/-0.15541

[matthijsmelissen]

Never noticed that - @springmeyer do you know whether this has been solved or is it worth reporting?

[pnorman]

Confirming fixed

[matthijsmelissen]

This should have been fixed now on openstreetmap.org as Mapnik 3 has now been rolled out. Note that some tiles might not have been updated yet.

[SomeoneElseOSM]

#1237 , which was the same problem as this, is certainly fixed in the standard style now it's on Mapnik 3.