mapnik::geometry_type

[springmeyer]

Mapnik 2.1 supports a new magic name to allow sorting on geometry type (mapnik/mapnik#546). A a keyword of mapnik::geometry_type is available in the expression engine as if that were an attribute on the data.

Testing now I realize that the :: conflicts with carto attachments so parsing fails.

But it can be made to work by quoting:

#countries['mapnik::geometry_type'=3] {
  polygon-fill: #fff;
}

So, this issue stands to resolve the questions: should we stick with the current name and live with needing quoting? Should it be renamed in mapnik? Should carto start being smart about not interpreting :: inside selectors as attachments? Other ideas?

/cc @artemp and @tmcw.

[kkaefer]

Theoretically, we could support the :: syntax without the quotes as well, since it's unambiguous (you're one level of [ deep and :: can only be used on the top level for attachments. However, having this syntax in addition to the attachments might create a lot of confusion, so maybe we can find a different syntax in Carto for that, e.g.

#countries[mapnik/geometry_type=3] {
  polygon-fill: #fff;
}

[tmcw]

Y, like @kkaefer this is possible to do. I'm not sure if this syntax is the best as far as css-like-ness, could it be something like

#countries[mapnik-geometry-type=3] {
  polygon-fill: #fff;
}

This would conflict with very uncommon fields called the same, but at the same time doesn't introduce a new syntax for this.

[artemp]

mapnik-geometry-type

sounds good.

[tmcw]

Cool, ping here when it's implemented mapnik-side then we'll start thinking about making this backwards/forwards compatible

[springmeyer]

Okay. Makes me realize that Mapnik-reference should likely be responsible as well for declaring these constants.

On Oct 9, 2012, at 5:47 AM, Tom MacWright notifications@github.com wrote:

Cool, ping here when it's implemented mapnik-side then we'll start thinking about making this backwards/forwards compatible

—
Reply to this email directly or view it on GitHub.

[strk]

Can I help in any way here ? I understand the mapnik side is ready ?

[strk]

@artemp, @tmcw, @springmeyer, what about the value type ? Should we aim to use symbolic names rather than (or even better in addition to) the numeric ones ? What about:

 1 == point == puntal
 2 == line == lineal
 3 == polygon == polygonal

[strk]

Forget the above, seems already supported by mapnik

[springmeyer]

yeah, keywords point, linestring, and polygon are already suppored in mapnik.

I'm still ambivalent about if this really needs to be changed as quoting currently works fine:

#countries['mapnik::geometry_type'=3] {
  polygon-fill: #fff;
}

[rcoup]

for Future Rob's reference, @springmeyer says the following works as-is:

#countries['mapnik::geometry_type'=polygon] {
  polygon-fill: #fff;
}
#countries['mapnik::geometry_type'=collection] {
  /* for features with mixed types */
}

[rcoup]

Personally I'd prefer the #countries polygon { ... } model (ie. geometry type as a conceptual equivalent to a CSS element selector), which we talked about in 2011, not sure if @tmcw has had any further thoughts since then?

(just because I like the concept doesn't mean it's straightforward to implement though 😉)

[tmcw]

#countries polygon { ... }

Not sure. What about

#countries polygon point { ... }

Is this valid or no? The concept doesn't map cleanly for me - geometry types are attributes of features and should be filtered on. CSS element selectors are for atoms which are contained in other things - a div inside of body and so on.

polygon { ... }

should be

['mapnik::geometry_type'=polygon] { ... }

Just like any other anonymous filter selector.

[rcoup]

Yup, the filter approach definitely works for geometry type. Memory dredge:

#mylayer linestring {
  line-color: yellow;
  line-width: 1;

  vertex:first { 
    /* make first vertex of line red */
    marker-fill: red;
    marker-width: 1;
  }
}

#countries polygon ring vertex:first {
  /* highlight start vertex of each ring */
  marker-color: red; 
}

Need to dig up Shawn/Josh's notes.

[strk]

I think this ticket can be closed, as ["mapnik::geometry_type"=something] is working fine.
Or am I missing anything ?

[springmeyer]

yes quoting, or ["mapnik::geometry_type"=something] is working fine and changing this is not worth it now. closing