hyperspeed CSS-like map styling
JavaScript Vim script
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Is a stylesheet renderer for Mapnik. It's an evolution of the Cascadenik idea and language, with an emphasis on speed and flexibility.

Reference Documentation



  • MML files are assumed to be JSON, not XML. The files are near-identical to the XML files accepted by Cascadenik, just translated into JSON.
  • Carto will not embed files or download URLs for you. Stylesheets should be embedded directly into your MML JSON and any datasources should be paths (relative or absolute) that would be acceptable in Mapnik XML. The millstone project aims to fill this need.

carto.js MML:

    "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
    "Stylesheet": [{"id":"style.mss","data":"Map {\n  background-color: #fff;\n}\n\n#world {\n  line-color: #ccc;\n  line-width: 0.5;\n  polygon-fill: #eee;\n}"}],
    "Layer": [{
        "id": "world",
        "name": "world",
        "srs": "+proj=latlong +ellps=WGS84 +datum=WGS84 +no_defs",
        "Datasource": {
            "file": "world_borders",
            "type": "shape"

Cascadenik MML

        map-bgcolor: #69f;

        line-width: 1;
        line-color: #696;
        polygon-fill: #6f9;
<Layer srs="+proj=latlong +ellps=WGS84 +datum=WGS84 +no_defs">
        <Parameter name="type">shape</Parameter>
        <Parameter name="file">world_borders</Parameter>

Attachments and Instances


In CSS, a certain object can only have one instance of a property. A <div> has a specific border width and color, rules that match better than others (#id instead of .class) override previous definitions. carto.js acts the same way normally for the sake of familiarity and organization, but Mapnik itself is more powerful.

Layers in Mapnik can have multiple borders and multiple copies of other attributes. This ability is useful in drawing line outlines, like in the case of road borders or 'glow' effects around coasts. carto.js makes this accessible by allowing attachments to styles:

#world {
  line-color: #fff;
  line-width: 3;

  #world::outline {
    line-color: #000;
    line-width: 6;

Attachments are optional: if you don't define them, carto.js does overriding of styles just like Cascadenik.

This brings us to another incompatibility: line-inline and line-outline have been removed from the language, because attachments are capable of the same trick.

While attachments allow creating implicit "layers" with the same data, using instances allows you to create multiple symbolizers in the same style/layer:

#roads {
  casing/line-width: 6;
  casing/line-color: #333;
  line-width: 4;
  line-color: #666;

This makes Mapnik first draw the line of color #333 with a width of 6, and then immediately afterwards, it draws the same line again with width 4 and color #666. Contrast that to attachments: Mapnik would first draw all casings before proceeding to the actual lines.



Instead of the name attribute of the TextSymbolizer and ShieldSymbolizer being a part of the selector, it is a property of a rule. Thus the evaluation is less complex and one can use expressions in names.

cascadenik carto.js
#world NAME {
  text-face-name: "Arial";
#world {
  text-name: "NAME";
  text-face-name: "Arial";



carto.js is only compatible with Mapnik2. Compatibility with Mapnik 0.7.x is not planned.

Rasters and Buildings


Rasters are supported in carto.js - it knows how to download .vrt, .tiff, and soon other raster formats, and the properties of the RasterSymbolizer are exposed in the language.

The BuildingSymbolizer is also supported in carto.js. The code stores symbolizer types and properties in a JSON file (in tree/reference.json), so new Mapnik features can be quickly implemented here.

Variables & Expressions


carto.js inherits from its basis in less.js some new features in CSS. One can define variables in stylesheets, and use expressions to modify them.

@mybackground: #2B4D2D;

Map {
  background-color: @mybackground

#world {
  polygon-fill: @mybackground + #222;
  line-color: darken(@mybackground, 10%);

Nested Styles


carto.js also inherits nesting of rules from less.js.

/* Applies to all layers with .land class */
.land {
  line-color: #ccc;
  line-width: 0.5;
  polygon-fill: #eee;
  /* Applies to #lakes.land */
  #lakes {
    polygon-fill: #000;

This can be a convenient way to group style changes by zoom level:

[zoom > 1] {
  /* Applies to all layers at zoom > 1 */
  polygon-gamma: 0.3;
  #world {
    polygon-fill: #323;
  #lakes {
    polygon-fill: #144;



By defining multiple fonts in a text-face-name definition, you create FontSets in carto.js. These are useful for supporting multiple character sets and fallback fonts for distributed styles.

<pre>#world {

text-name: "[NAME]"; text-size: 11; text-face-name: "Georgia Regular", "Arial Italic"; }

<FontSet name="fontset-0">
  <Font face-name="Georgia Regular"/>
  <Font face-name="Arial Italic"/>
<Style name="world-text">
    <TextSymbolizer fontset-name="fontset-0"


Carto supports a variety of filter styles:

Numeric comparisons:

#world[population > 100]
#world[population < 100]
#world[population >= 100]
#world[population <= 100]

General comparisons:

#world[population = 100]
#world[population != 100]

String comparisons:

/* a regular expression over name */
#world[name =~ "A.*"]



If you're using TileMill, you're already using Carto and don't need to do a thing.

If you're a developer-type and want to use the carto binary with node.js (and you have npm installed),

npm install carto

From the binary

Install millstone to enable support for localizing external resources (URLs and local files) referenced in your mml file.

npm install millstone
carto map_file.json

From code

Currently carto.js is designed to be invoked from node.js. The Renderer interface is the main API for developers, and it takes an MML file as a string as input.

// defined variables:
// - input (the name or identifier of the file being parsed)
// - data (a string containing the MML or an object of MML)
var carto = require('carto');

new carto.Renderer({
        filename: input,
        local_data_dir: path.dirname(input),
    }).render(data, function(err, output) {
        if (err) {
            if (Array.isArray(err)) {
                err.forEach(function(e) {
                    carto.writeError(e, options);
            } else { throw err; }
        } else {


To install, download or clone this repository, then add the vim-carto directory located at build/vim-carto to your ~/.vim file.


carto.js is based on less.js, a CSS compiler written by Alexis Sellier.

It depends on:

Only for running tests:


  • Tom MacWright (tmcw)
  • Konstantin Käfer (kkaefer)
  • AJ Ashton (ajashton)
  • Dane Springmeyer (springmeyer)