#contour #polygon #geojson #isobands #marching-squares

contour-isobands

Compute isobands and contour polygons (using marching squares algorithm)

8 releases

0.4.3 Mar 11, 2024
0.4.2 Mar 11, 2024
0.4.1 Jan 15, 2024
0.3.1 Oct 4, 2023
0.1.0 Mar 14, 2023

#20 in Geospatial

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AGPL-3.0-or-later

320KB
3K SLoC

Contour-isobands-rs

Build status GitHub Actions Docs.rs version

Compute isobands (i.e. contour polygons which enclose all the points of a grid included between two given values) by applying marching squares to an array of values.

Usage

Basics

Add the following to your Cargo.toml:

[dependencies]
contour-isobands = "0.4.3"

Then, you can use the ContourBuilder to compute isobands:

use contour_isobands::{ContourBuilder, Band};

let values = vec![
    1., 1., 1., 1., 1., 1., 1.,
    1., 5., 5., 5., 5., 5., 1.,
    1., 5., 15., 15., 15., 5., 1.,
    1., 5., 10., 10., 10., 5., 1.,
    1., 5., 5., 5., 5., 5., 1.,
    1., 1., 1., 1., 1., 1., 1.,
];

// These intervals will compute 3 bands:
// - the first one will contain all points between 1 (included) and 5 (excluded)
// - the second one will contain all points between 5 (included) and 7 (excluded)
// - the third one will contain all points between 7 (included) and 15 (included)
let intervals = vec![1., 5., 7., 15.];

let result: Vec<Band> = ContourBuilder::new(7, 6)
    .use_quad_tree(true)
    .contours(&values, &intervals)?;

assert_eq!(result.len(), 3);

The result is a vector of Band structs, each one containing a geometry (MultiPolygon<f64>) and the minimum and maximum values of the band.

Note that you can specify the coordinates of the grid and the distance between points (on x- and y-axis) using the x_origin, y_origin, x_step and y_step parameters of the ContourBuilder constructor :

let result: Vec<Band> = ContourBuilder::new(7, 6)
    .x_origin(-6.144721)
    .y_origin(51.781713)
    .x_step(0.118759)
    .y_step(-0.089932)
    .use_quad_tree(true)
    .contours(&values, &intervals)?;

geojson feature

Each Band struct contains a geometry (MultiPolygon<f64>) and the minimum and maximum values of the band. It can be serialized to geojson using the geojson feature:

[dependencies]
contour-isobands = { version = "0.4.3", features = ["geojson"] }
use contour_isobands::{ContourBuilder, Band};
use geojson::{Feature, FeatureCollection};

let values = vec![
    1., 1., 1., 1., 1., 1., 1.,
    1., 5., 5., 5., 5., 5., 1.,
    1., 5., 15., 15., 15., 5., 1.,
    1., 5., 10., 10., 10., 5., 1.,
    1., 5., 5., 5., 5., 5., 1.,
    1., 1., 1., 1., 1., 1., 1.,
];

let intervals = vec![1., 5., 7., 15.];

let result = ContourBuilder::new(7, 6)
    .use_quad_tree(true)
    .contours(&values, &intervals)?;
    
let features = result.iter()
    .map(|band| band.to_geojson())
    .collect::<Vec<geojson::Feature>>();

let geojson_string = GeoJson::from(
    FeatureCollection {
        bbox: None,
        features,
        foreign_members: None,
    }).to_string();

Note that the polygons exterior rings are oriented in the counter-clockwise direction, while the interior rings are oriented in the clockwise direction (in accordance with the GeoJSON RFC 7946 specification).

parallel feature

[dependencies]
contour-isobands = { version = "0.4.3", features = ["parallel"] }

The parallel feature enables the use of the rayon crate to parallelize the computation of the isobands. By enabling this feature, the ContourBuilder struct exposes a par_contours method :

let result: Vec<Band> = ContourBuilder::new(7, 6)
    .x_origin(-6.144721)
    .y_origin(51.781713)
    .x_step(0.118759)
    .y_step(-0.089932)
    .use_quad_tree(true)
    .par_contours(&values, &intervals)?;

Note that you can still use the contours method if you don't want to use parallelism (indeed, on small grids, the overhead of parallelism can be higher than the gain).

WASM demo

A demo of this crate, compiled to WebAssembly, is available on https://mthh.github.io/contour-isobands-wasm/.

Difference with the contour crate (from mthh/contour-rs repository)

The contour crate computes isolines (cf. wikipedia:Marching_squares) and use them to compute their corresponding contour polygons (i.e. polygons that contain all points above the threshold defined for a given isoline) and isobands (i.e. contour polygons that contain all points between a minimum and a maximum bound). This contour-isobands-rs is dedicated to isobands, also uses marching squares (cf. wikipedia:Marching_squares#Isobands) but uses a slightly different implementation for the disambiguation of saddle points. It also offers parallel computation of isobands using the rayon crate, which can be beneficial when computing isobands on large grids and with many thresholds.

Licence

Since this is mostly a port of https://github.com/RaumZeit/MarchingSquares.js which is licenced under the Affero General Public License v3.0, this project is also licenced under the Affero General Public License v3.0. See the LICENSE file for details.

Dependencies

~0.7–1.4MB
~28K SLoC