Small 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.

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

$ npm install polygon-offset

Browserify

var Offset = require('polygon-offset');

Browser

<script src="path/to/offset.min.js"></script>

// 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);

Martinez clipping algorithm

$ npm install
$ npm start
$ open http://localhost:3003
$ make
$ npm test

Build

To compile the lib with Google Closure Compiler with ADVANCED_OPTIMIZATIONS

$ 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

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.