Polygon offsetting algorithm, aimed for use with leaflet
npm install polygon-offsetSmall lib for polygon offsetting(margin/padding). See the example of how it can be used with Leaflet. It handles quite well oddly shaped and concave polygons.
!screenshot 2016-07-27 19 05 53
The reason I wrote this is that the only working solution to this problem known to me is Angus Johnson's Clipper library. Library is huge and offsetting in it is subroutine.
This library depends on Martinez polygon clipping algorithm, and combined with it weighs ~14kb.
Node
``bash`
$ npm install polygon-offset
Browserify
`js`
var Offset = require('polygon-offset');
Browser
`html`
`js
// if the first point repeats(for instance GeoJSON) -
// the output will have it as well
var points = [
[0,0], [0,100],
[100,100], [100,0], [0,0]
];
var x = -10;
var offset = new Offset();
var margined = offset.data(points).margin(10);
var padding = offset.data(points).padding(10);
// decides from the sign of x: negative for padding
var unknown = offset.data(points).arcSegments(3).offset(x);
// if you want to work with the polyline - margin only
var polyline = offset.data(points).offsetLine(5);
`
Result will be array of linear rings.
Martinez clipping algorithm
`bash`
$ npm install
$ npm start
$ open http://localhost:3003
$ make
$ npm test
Build
To compile the lib with Google Closure Compiler with ADVANCED_OPTIMIZATIONS
`bash``
$ make
* Hans Muller, Growing and Shrinking Polygons: Round One
* POLYGON OFFSETTING BY COMPUTING WINDING NUMBERS, Xiaorui Chen Sara McMains, Berkeley, 2005
The MIT License (MIT)
Copyright (c) 2016 Alexander Milevski
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