[Tagging] RENDER

[André Pirard]

On 2014-08-20 18:07, Peter Wendorff 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".

Yes, this sentence is misunderstood, and by many repliers apparently.
It means that once Mapnik uses a (defined) rendering you cannot change
it (RENDER is ignored).
The main idea behind RENDER is not coloring objects, and I agree it
shouldn't, but showing them.
And the renderer can do that with any single color they like.

André.



> 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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