simplify

Simplifies the points of a polygon using both the Ramer-Douglas-Peucker algorithm and basic distance-based simplification. Adapted to an ES6 module from the excellent Simplify.js.

Basic example

Removes points that do not change the shape by more than tolerance (in coordinate units) using the Ramer–Douglas–Peucker algorithm: the dashed line is the original, the solid one keeps only the points that matter at a tolerance of 4. Geomap simplifies polygons this way before drawing.

import {simplify} from "@d3plus/math";

const r = n => Math.round(n * 100) / 100;
const poly = Array.from({length: 21}, (_, i) => [
  20 + i * 11,
  r(100 + Math.sin(i / 2) * 40 + ((i % 3) - 1) * 3),
]);

simplify(poly, 4);

Highest quality

By default a quick radial-distance pass drops near-duplicate points before the main algorithm runs. highestQuality: true skips that pass, which can keep a few more points at the same tolerance and runs noticeably slower on large polygons.

import {simplify} from "@d3plus/math";

const r = n => Math.round(n * 100) / 100;
const poly = Array.from({length: 21}, (_, i) => [
  20 + i * 11,
  r(100 + Math.sin(i / 2) * 40 + ((i % 3) - 1) * 3),
]);

simplify(poly, 4, true);

Tolerance

A small tolerance keeps almost every point; a large one keeps only the turns, trading detail for far fewer vertices.

import {simplify} from "@d3plus/math";

const r = n => Math.round(n * 100) / 100;
const poly = Array.from({length: 21}, (_, i) => [
  20 + i * 11,
  r(100 + Math.sin(i / 2) * 40 + ((i % 3) - 1) * 3),
]);

simplify(poly, 1);
simplify(poly, 12);

Parameters

import {simplify} from "@d3plus/math";

polyrequiredArray.<Point>

An Array of points that represent a polygon.

tolerancenumberDefault 1

Affects the amount of simplification (in the same metric as the point coordinates).

highestQualitybooleanDefault false

Excludes distance-based preprocessing step which leads to highest quality simplification but runs ~10-20 times slower.