maplibre-legend

A Rust crate to dynamically generate SVG layer legends from a MapLibre GL style JSON.

Given a MapLibre style (e.g. style.json), this library parses all layers and produces standalone SVG symbols or a combined legend, complete with optional labels and raster entries.

This project is a work in progress. It doesn't yet support all layer types, but it covers the most common cases. By using each layer's metadata attribute, you can flexibly customize how the legend is generated — especially the labels.

Features

• Parse MapLibre GL style (v8) JSON into a structured Style model.
• Render individual layer legends as SVG: fill, line, circle, symbol, fill-extrusion, background, heatmap, raster.
• Sprite support: sprite field accepts both a single URL string and an array of URLs.
• Stack all layers into one combined SVG with separators.
• Optionally include raster layers.
• Customizable dimensions and label rendering via [LegendConfig].
• Per-layer label overrides through the metadata.legend object.

Installation

[dependencies]
maplibre-legend = "0.5"

Feature flags

Feature	Default	Description
async	✓	Async sprite fetching via reqwest
sync		Blocking sprite fetching — disable default features first

Async (default):

maplibre-legend = "0.5"

Sync:

maplibre-legend = { version = "0.5", default-features = false, features = ["sync"] }

The two features are mutually exclusive.

Usage

Asynchronous

use maplibre_legend::{LegendConfig, MapLibreLegend};
use tokio::fs;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let style_json = fs::read_to_string("style.json").await?;

    let legend = MapLibreLegend::new(
        &style_json,
        LegendConfig {
            default_width: 250,
            ..Default::default()
        },
    )
    .await?;

    // Render all layers into one combined SVG (reversed order)
    let svg = legend.render_all(true)?;
    fs::write("legend.svg", svg).await?;

    // Or render a single layer by ID
    let svg = legend.render_layer("my-layer-id", None)?;
    fs::write("layer.svg", svg).await?;

    Ok(())
}

Synchronous

use maplibre_legend::{LegendConfig, MapLibreLegend};
use std::fs;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let style_json = fs::read_to_string("style.json")?;

    let legend = MapLibreLegend::new(
        &style_json,
        LegendConfig {
            default_width: 250,
            ..Default::default()
        },
    )?;

    let svg = legend.render_all(true)?;
    fs::write("legend.svg", svg)?;

    Ok(())
}

LegendConfig options

LegendConfig {
    default_width: 200,    // SVG width in pixels
    default_height: 40,    // SVG height for single-entry layers
    has_label: true,       // render a title label above each layer
    include_raster: false, // include raster layers in render_all()
}

All fields are public. Use ..Default::default() to keep the rest at their defaults.

Examples

Given a MapLibre style.json with various fill, line, and circle layers:

let svg = legend.render_all(true)?;

style.json	style2.json	style3.json

Customizing legends with metadata

Each layer can override legend behavior through a "legend" object in its metadata:

"metadata": {
    "legend": {
        "label": "Land value 2016 [USD/m²]",
        "default": "Other",
        "custom-labels": [
            "Up to $100",
            "$100 – $250",
            "$250 – $750",
            "Over $750"
        ]
    }
}

Key	Type	Description
label	string	Title for the legend entry. Falls back to the layer id.
default	string	Label for the expression's fallback/default color.
custom-labels	array of strings	Labels for each stop or case in the expression, in order.

Supported expressions

The following MapLibre paint expression types are parsed into legend entries:

Expression	Behavior
match	One entry per value + default
case	One entry per condition + default
interpolate	One entry per stop
step	One entry per threshold + base
coalesce	Delegates to the first inner match/case/interpolate/step
literal	Single entry

Modules

Module	Layer type(s)
fill	fill
line	line
circle	circle
symbol	symbol
fill_extrusion	fill-extrusion
background	background
heatmap	heatmap
raster	raster
default	unknown types (gray fallback)
common	shared types, expression parser, sprite utilities
error	LegendError

Contributing

1. Fork the repo and create a feature branch.
2. Run cargo fmt and cargo clippy before committing.
3. Add tests for new behavior.
4. Submit a pull request — feedback is welcome!

License

BSD-3-Clause. See the LICENSE file for details.