[Mapcss] "Layer" in MapCSS

Stephan Bösch-Plepelits skunk at xover.htu.tuwien.ac.at
Thu Sep 19 04:29:04 UTC 2013

On Wed, Sep 18, 2013 at 05:01:26PM +0300, Komяpa wrote:
> In that case, "layer" is value of layer= tag, with default of 0. That
> gives automagical perfect rendering of multi-level crossings.
> Probably it should be changed somehow to kick this behaviour in from
> stylesheet itself, but I haven't found a better solution.

On Wed, Sep 18, 2013 at 03:10:58PM +0100, Richard Fairhurst wrote:
> MapCSS has traditionally taken the attitude that stylesheet authors
> shouldn't have to worry about this and has therefore automagically
> abstracted them away.

Yes, I guess for multi-level crossings this would work rather fine, but I
can think of several scenarios where this could go so wrong:

* Landuse-Polygons: Landuse-Polygons usually don't have a layer tag.
  With a default value of 0 they will be rendered after roads with a
  negative layer tag and therefore obstruct them.
* Same with polygons with a layer-tag - I can set a layer=10 on
  landuse=residential and obstruct the whole city :-)
* On the OpenStreetBrowser basemap (which is not MapCSS - yet) I render
  road casings twice, first directly after the landuse polygons with
  linecap=round and the then on their respective layer with linecap=none.
* I would render icons and labels always on the highest layer so as they
  don't get obstructed by other objects.

On Wed, Sep 18, 2013 at 05:01:26PM +0300, Komяpa wrote:
> kothic/komap also have special value to redefine it to "top" or
> "bottom" for the rule.
> https://github.com/kothic/kothic/blob/master/src/komap.py#L391

Yes, I think something like this is needed. Maybe just "layer=-5"?
And default: layer=eval(tag('layer'));

Seid unbequem, seid Sand, nicht Öl im Getriebe der Welt! - Günther Eich
| Stephan Bösch-Plepelits,                                            |
| Technische Universität Wien   -    Studien Informatik & Raumplanung |
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