[Tagging] RENDER

Wed Aug 20 16:16:14 UTC 2014

Also, from one of the talks on SOTM-EU 2014 [1], I learned that Mapnik,
does not allow one to use a value from the database as color in the
rendering.

[1] https://www.youtube.com/watch?v=850JJHQKp_s

On Wed, Aug 20, 2014 at 6:07 PM, Peter Wendorff <wendorff at uni-paderborn.de>
wrote:

> Okay, let me get a bit more verbose.
>
> I want to get Walmarts shown on the map in a different color.
> Before there's a polygon with the following tags:
>
> shop=supermarket
> building=yes
> operator=WalMart
> addr:street=whatever
> addr:housenumber:42
> addr:city=YouLike
> render=blue
>
> Another polygon is tagged
>
> shop=supermarket
> building=yes
> building:levels=2
> operator=WalMart
> addr:street=another street
> addr:city=some city
>
> Let's consider the usual rendering of the osm-carto (default mapnik)
> stylesheet.
>
> without considering the render tag both buildings are drawn as buildings
> (dark outlined polygon filled in slightly lighter gray) and a shopping
> cart icon on top.
>
> Now let's consider the render-tag.
>
> Variant 1: just use it. You get an entirely inconsistent look, as I
> myself wasn't interested in all WalMarts, but probably only those in one
> particular town, so I only added the render tag to some objects.
> Let two others add arbitrary, but different render ideas to other
> objects and the map get's unusable as there's nothing like a map key any
> more: all visuals on the map get more or less meaningless.
>
> Variant 2: Try to use the render tag, but in a consistent way.
> In that case my stylesheet/renderer would have to figure out what is
> meant by the render-tag. It may refer to supermarkets, to WalMarts, to
> stuff in the whatever-street or in the city YouLike. Figuring out a rule
> from that is incredibly hard and very error prone.
>
> Your last sentence might be the misunderstanding:
> "As soon as rendering is defined for an element, it is used instead and
> RENDER is normally ignored".
> But even that is a tricky strategy. Let's stay at the WalMart-Example.
> I want to have a special (!) rendering for WalMarts, so there is no
> rendering defined for it before (as any rendering defined is a fallback
> to a more generic case: supermarkets or even buildings).
> It would therefore lead to cluttering map objects where it is not
> necessary, or doesn't solve anything at all. Although it would break
> down stylesheet innovation even more as you can render your very own
> tags - as long as it isn't rendered on the map itself.
>
> Put the effort to add rendering for missing objects. This is harder to
> achieve, yes; but it is the straightforward way, not a hack around, with
> major drawbacks and side effects.
>
> regards
> Peter
>
> Am 20.08.2014 um 14:46 schrieb André Pirard:
> > On 2014-08-15 16:31, Peter Wendorff wrote :
> >> not a good idea IMHO.
> >> 1) what is the feature this tag should refer to? Consider a polygon that
> >> is tagged as a building (building=yes) and a shop (shop=supermarked) and
> >> a Walmart (operator=WalMart), and the mapper added RENDER=blue. What is
> >> it that should be rendered blue? This object? Any supermarket? any
> >> Walmart?
> > I don't understand what you say very well. "added RENDER" to what?
> > As I say RENDER would typically apply to "an area", to one object, not
> > to "any".
> > That is, you have building=yes + render=blue and that building gets blue.
> >
> >> Any building? How should any rendering decide if the default
> >> rendering should be used or the one defined by the tag you propose?
> > Did you read my sentence:
> >>> As soon as rendering is defined for an element, it is used instead
> >>> and RENDER is normally ignored.
> > ?
> > a.s.o. ...
> >
> > André.
> >
> >
> >> 2) I want to get Walmarts shown on the map in a different color, thus
> >> all Walmarts I want to see in the map get
> >>
> RENDER={mycolor-which-is-not-used-yet-in-the-zoomlevel-I'm-interested-in}.
> >> Now the stylesheet maintainer uses that color for another object -
> >> conflict, damn, fail.
> >> 3) I want to get Walmarts rendered pink on osm-carto, green on HOT,
> >> orange on the cyclemap - what should go to the render-tag (even if the
> >> styles would follow your proposal?
> >>
> >> The only benefit I see in this proposal is just what you said: people
> >> would stop tagging stuff just to get their map to display it the way
> >> they want; but how do you ensure they don't tag stuff to be rendered
> >> with the same style? How do you ensure the map stays usable?
> >>
> >> regards
> >> Peter
> >>
> >> Am 15.08.2014 um 16:12 schrieb André Pirard:
> >>> Hi,
> >>>
> >>> It's a well known fact that many people complain to tag in vain because
> >>> what they tag doesn't show on the map (e.g. mini-golf vs tennis pitch),
> >>> because they're told to open a rendering ticket which replies that only
> >>> official tags are supported, and because they open a vote for an
> >>> official tag and nobody signs.
> >>> As a result they are accused of "tagging for the renderer" instead of
> >>> 'being forced to tag for the renderer".
> >>>
> >>> The solution is simple however.  A RENDER tag that, typically, would
> >>> assign a color to an area.
> >>> I'll let the rendering specialists define what else it can do.
> >>> ⚠ ⚠ ⚠ RENDER only requests *by default* rendering.
> >>> As soon as rendering is defined for an element, it is used instead and
> >>> RENDER is normally ignored.
> >>>
> >>> For a better map,
> >>>
> >>> André.
> >>>
> >
> >
> >
> >
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>
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