[Tagging] Bridges and layers

Dave F. davefox at madasafish.com
Mon Jul 26 14:56:10 BST 2010

  On 26/07/2010 14:07, Richard Mann wrote:
> Dave F (et al),
> Renderers draw roads (typically) by drawing a wide grey line on each
> segment, a grey circle at each node, then a narrower (say) white line
> on each segment, and a white circle at each node. All you see of the
> grey is a thin line on each side of the white line: this is the
> casing. The circles on each node are called caps (end-caps if they are
> at the end of a way, join-caps if they are at an intermediate node).
> If you don't draw the circles on the nodes then you get gaps at
> corners. If you draw all the grey first, regardless of layer, you get
> gaps in the casings when one road goes over another, so it looks like
> they join when they don't (which happens on cyclemap).

>   If you draw the
> grey in the correct layer, then you get little semi-circular arcs of
> grey at the end of bridges (if they are layer=1).

I've never noticed this in Mapnik,or an other. Do you have examples of 
these please.

> So renderers have to do something. Different renderers have come up
> with different solutions, but all produce artefacts because there's a
> piece of data missing

What data is this?

> (which they could pre-process, sure, but there
> are better uses of time). It would be more effective to give them the
> data, and have renderers do it reasonably well consistently.

> What I've suggested isn't the only solution, but it's the most
> economical, I think.
So your saying to save the renders time, the data collectors have to 
waste time adding new tags?

It sounds like a rendering software problem to me & nothing to do with 

I still don't see how layer_change solves anything. AFAICS it just adds 
even more separated sections which cause other render problems such as 
one-way arrows overlapping & cutoff name & ref labels.

incidentally, is there a reason you've joined the cycle path to the 
overhead power line?

Dave F.

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