poly-simplify

Dependancy free JS library to simplify polyline or polygon vertices.

This library is obviously heavily inspired by Volodymyr Agafonkin's brilliant »simplify.js« library which has become kind of a standard.
While most polygon simplification libraries are focused on geodata/map applications, »poly-simplify« aims at a more versatile concept e.g by providing a variety of inputs and including multiple methods for SVG graphic applications.

• Features
• Simplification methods
• 1. Usage
  • 1.1. IIFE
  • 1.2. ESM
  • 1.3 Node.js
• 2. Options
• 3. Quality control
  • 3.1 Relative Quality thresholds
  • 3.2 Absolute thresholds
• 4. Demos: Simplifier Webapp
• Changelog, Updates and rollback
  • Rollback
  • Changelog
• Bug reporting
• Credits and further reading

Features

• input normalisation: you can pass pretty much any reasonable polygon notation such as:

  • point-object arrays – so [{x:0, y:10}, {x:1, y:20}]
  • stringified point data (as provided by SVG <polygon> point attributes – 0 10 1 20
  • nested arrays like [[0 10], [1 20]]
  • JSON stringified presentations of point arrays like [{"x":0,"y":10},{"x":1,"y":20}]
  • SVG pathdata: M0 0 100 200z

• Relative or absolute quality threshold: simplification is controlled via a relative quality parameter similar to raster image compression logics – super tiny geoData polygons won't get distorted

• SVG: node property data (e.g SVG polygons) – constructors like SVGPoint are normalized to get a "decoupled" point array

• Supports compound shapes/paths intersecting polygons interpreted as cut-out shapes

• Dynamic decimal rounding by default decimals are adjusted to match tiny polygon's dimensions

• scaling and alignment: tiny polygons can be scaled to a reasonable size for demonstration purposes. You can also realign coordinates to zero origin

Simplification methods

• "RC" - remove colinear: removes only colinear or zero-length segments. This method can be concidered "lossless" and is ideal for polygons with a lot of excessive point added by a graphic editor (e.g due too shape merging)
• "RD" - simplify by radial distance: removes short segments. Ideal for more aggressive simplifications
• "RDP" – Ramer-Douglas-Peucker algorithm
• "VW" – Visvalingam-Whyatt algorithm
• max vertices simplification: simply omits vertices to get the desired point count – very fast but sloppy

1. Usage

1.1. IIFE

<script src="https://cdn.jsdelivr.net/npm/poly-simplify@latest/dist/poly-simplify.min.js"></script>

<script>
  let pts = [{x:0, y:10}, {x:1, y:20}, {x:2, y:30}];
  let polyPtsSimplified = polySimplify(pts);
</script>

1.2. ESM

<script type="module">
  import {polySimplify} from 'https://cdn.jsdelivr.net/npm/poly-simplify@latest/+esm';

  let pts = [{x:0, y:10}, {x:1, y:20}, {x:2, y:30}];
  let polyPtsSimplified = polySimplify(pts)
</script>

1.3 Node.js

Install

npm install poly-simplify

Usage

import polySimplify from 'poly-simplify';
const simplified = polySimplify('M0 0 h10 h10 v10 v10');
console.log(simplified);

2. Options

| mercator | false | Boolean | Applies Mercator projection – for geodata polygons (or funny distortion effects) |

| unite | false | Boolean | Unites self intersecting polygons |
| scale | 1 | Number | scale polygon |
| scaleToWidth, scaleToHeight | 0 | Number | fit polygon into specified max dimensions |
| alignToZero | false | Boolean | align weird starting coordinates to x/y=0 |
| translateX, translateY | 0 | Number | set manual x/y offsets to realign polygon |
| decimals | -1 | Number | round coordinates to decimals. -1 won't apply rounding. Too small floating point accuracy is auto-adjusted to avoid distortions | | toRelative | false | Boolean | SVG converts commands to relative – more compact |
| toShorthands | false | Boolean | SVG applies shorthand commands where possible h, v – more compact |
| minifyString | false | Boolean | SVG applies string based minification e.g omitting leading zeroes or concatenating subsequent floats .123.456 – more compact |
| meta | false | Boolean | return additional data like: vertices counts (original and simplified) – this option is mostly for debugging e.g for integrety checks |

3. Quality control

3.1 Relative Quality thresholds

By default poly-simplify expects a value between 0–1.
This concept tries to achieve a relative quality similar to raster image compression settings such as Jpg quality levels:

• 1 - we only apply "lossless" simplification by removing only colinear points. More aggressive simplification algorithms like Radial-distance, Ramer-Douglas-Peucker or Visvalingam-Whyatt are disabled
• 0.5 – 1 – tries to find a ballance between visual fidelity and point reduction
• <0.5 – adjusts thresholds for more aggressive simplification This approach will auto-detect reasonable thresholds by approximating the total dimensions of a polygon. So geoData polys that may be smaller than 1px/unit can be simplified without the need to know a suitable tolerance7threshold that might be 0.00001px

3.2 Absolute thresholds

You can also enter an absolute threshold via string input:
1px – sets the simplification threshold to absolute 1 units – bear in mind this may lead to heavy distortions if your polygon is way smaller than the threshold value 150v – the v(vertices) unit will add a maximum point limit to the simplification. In this example: reduce to a maximum of 150 points/vertices.

4. Demos: Simplifier Webapp

You can easily test poly-simplify using the webapp

You can test all settings and generate sample SVG files for demo purposes:

• output object (point array, nested array, SVG or JSON)
• SVG snippet
• JS code snippet

Changelog, Updates and rollback

Rollback

If you encounter any issues with the recent versions you can rollback to a previous version.
See all versions on

• npm
• jsdelivr

Changelog

• 0.1.4 fixed problems with linear polylines

Bug reporting

If you found a bug - feel free to file an issue.
For debugging you may also test your example in the webapp. You can also post in the discussions if you have ideas for missing features.

Credits and further reading

• Volodymyr Agafonkin for »simplify.js«
• Mike Bostock (d3.js) on Line Simplification