Computes isolines, contour polygons and isobands by applying marching squares to a rectangular array of numeric values.
Outputs ring coordinates or geo-types geometries.
The results can also easily be serialised to GeoJSON.
Note : The core of the algorithm is ported from d3-contour.
Add this to your Cargo.toml:
toml
[dependencies]
contour = "0.10.0"
and this to your crate root:
rust
extern crate contour;
The API exposes:
- a ContourBuilder struct, which computes isorings coordinates for a Vec of threshold values and transform them either :
- in Contours (a type containing the threshold value and the geometry as a MultiPolygon), or,
- in Lines (a type containing the threshold value and the geometry as a MultiLineString).
- in Bands (a type containing a minimum value, a maximum value and the geometry as a MultiPolygon).
contour_rings function, which computes isorings coordinates for a single threshold value (returns a Vec of rings coordinates - this is what is used internally by the ContourBuilder).ContourBuilder is the recommended way to use this crate, as it is more flexible and easier to use (it enables to specify the origin and the step of the grid, and to smooth the contours, while contour_rings only speak in grid coordinates and doesn't smooth the resulting rings).
Line, Contour and Band can be serialised to GeoJSON using the geojson feature.
Without defining origin and step:
rust
let c = ContourBuilder::new(10, 10, false); // x dim., y dim., smoothing
let res = c.contours(&vec![
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 1., 1., 1., 0., 0., 0., 0.,
0., 0., 0., 1., 1., 1., 0., 0., 0., 0.,
0., 0., 0., 1., 1., 1., 0., 0., 0., 0.,
0., 0., 0., 1., 1., 1., 0., 0., 0., 0.,
0., 0., 0., 1., 1., 1., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.,
0., 0., 0., 0., 0., 0., 0., 0., 0., 0.
], &[0.5])?; // values, thresholds
With origin and step
```rust let c = ContourBuilder::new(10, 10, true) // x dim., y dim., smoothing .xstep(2) // The horizontal coordinate for the origin of the grid. .ystep(2) // The vertical coordinate for the origin of the grid. .xorigin(100) // The horizontal step for the grid .yorigin(200); // The vertical step for the grid
let res = c.contours(&[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0. ], &[0.5]).unwrap(); // values, thresholds
```
Using the geojson feature
The geojson feature is not enabled by default, so you need to specify it in your Cargo.toml:
toml
[dependencies]
contour = { version = "0.10.0", features = ["geojson"] }
```rust let c = ContourBuilder::new(10, 10, true) // x dim., y dim., smoothing .xstep(2) // The horizontal coordinate for the origin of the grid. .ystep(2) // The vertical coordinate for the origin of the grid. .xorigin(100) // The horizontal step for the grid .yorigin(200); // The vertical step for the grid
let res = c.contours(&[ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 1., 1., 0., 1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0. ], &[0.5]).unwrap(); // values, thresholds println!("{:?}", res[0].to_geojson()); // prints the GeoJSON representation of the first contour ```
Output:
rust
Feature {
bbox: None,
geometry: Some(Geometry {
bbox: None,
value: MultiPolygon([
[[
[110.0, 215.0], [110.0, 213.0], [110.0, 211.0], [110.0, 209.0],
[110.0, 207.0], [109.0, 206.0], [107.0, 206.0], [106.0, 207.0],
[106.0, 209.0], [106.0, 211.0], [106.0, 213.0], [106.0, 215.0],
[107.0, 216.0], [109.0, 216.0], [110.0, 215.0]
]],
[[
[114.0, 215.0], [114.0, 213.0], [114.0, 211.0], [114.0, 209.0],
[114.0, 207.0], [113.0, 206.0], [112.0, 207.0], [112.0, 209.0],
[112.0, 211.0], [112.0, 213.0], [112.0, 215.0], [113.0, 216.0],
[114.0, 215.0]
]]
]),
foreign_members: None
}),
id: None,
properties: Some({"threshold": Number(0.5)}),
foreign_members: None
}
Demo of this crate compiled to WebAssembly and used from JavaScript : wasmdemocontour.
While this crate computes isolines (cf. wikipedia:Marchingsquares) from which it derives contour polygons *(i.e. polygons that contain all points above the threshold defined for a given isoline)* and isobands *(i.e. polygons that contain all points between a minimum and a maximum bound)*, contour-isobands-rs is only dedicated to compute isobands and use a slightly different implementation of the marching squares algorithm for the disambiguation of saddle points (cf. wikipedia:Marchingsquares).
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