[OSM-talk] Mapnik rendering artifacts

[Dave Stubbs]

On Jan 30, 2008 12:41 PM, Karl Newman <siliconfiend at gmail.com> wrote:

> I'm seeing faint gray lines on the main site slippy map Mapnik base layer,
> and I'm at a loss to explain their source. At first I thought they were
> state borders, but I'm seeing the lines running through the middle of
> states, too (here
> http://www.openstreetmap.org/?lat=42.001&lon=-120.048&zoom=9&layers=B0FTit runs vertically through the middle of Oregon). Then I thought they were
> tile boundaries, but that's not consistent, either. Even weirder is on the
> California-Nevada border, there are two vertical lines right next to each
> other, not always parallel. (See here:
> http://www.openstreetmap.org/?lat=41.62&lon=-120.053&zoom=9&layers=B0FT).
> I downloaded that area in JOSM but all I saw was a single state border way.
> It doesn't seem to have a source in the OSM data, so maybe it's something
> caused by the conversion to pgsql or by the renderer itself (maybe a
> projection issue)? I don't think it's related to the recent mod_tile
> changes, because as far as I know that only affects how the tiles are stored
> and served, not how they're rendered. (If I've said something stupid, please
> forgive me. I *think* I understand the process...)
>
> Any thoughts, anyone?
>
>
These have always been there.

It's an artefact of the way the coastlines are rendered.
Country/state/region polygon shapes are split into a large grid to improve
performance. Each shape is then rendered as land. Unfortunately the edges
meet exactly, but don't overlap, and the boundary is never pixel perfect, so
what I think is an antialiasing effect causes the blue background to seep
through. So you'll see these lines at all the borders known in the shape
file, which include US states, European countries and others, as well as the
grid.
Your double line example is because the shape for california and the tiling
grid almost coincide, but not quite.

You can see a similar effect on the coastline checker, but that has more
tiles in it's grid: http://tile.openstreetmap.nl/coastlines.html
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