flatten-offset

Offset polygon

This package implements algorithm of equidistant offset of polygon. It relies on the FlattenJS library and its polygon model, which is multi polygon comprised from a number of islands and holes, see this interactive notebook for more details.

Algorithm is based on the idea of morphological offset, when each edge of the polygon is mapped to its offset, and then boolean operation performed between original contour and offset edges. When offset value is positive, offset edges are united with original contour, when negative - they are subtracted from original contour.

Installation

You have to install flatten-offset package together with FlattenJS library, in order to use its polygon creation methods.

npm install --save flatten-js flatten-offset 

Usage

The package expose single method offset, which is added to Flatten.Polygon prototype. So we can use it in the way:

    let offsetPolygon = polygon.offset(offset_value)

When using umd minified package:

    let {offset} = Offset;
    let offsetPolygon = offset(polygon, offset_value)

Example:

    let Flatten = require('flatten-js');
    require('flatten-offset');
    let {segment, point, Polygon} = Flatten;
    // Create polygon
    let shapes = [
            segment(point(200,100), point(200,300)),
            segment(point(200,300), point(440,300)),
            segment(point(440,300), point(300,200)),
            segment(point(300,200), point(440,150)),
            segment(point(440,150), point(500,150)),
            segment(point(500,150), point(640,200)),
            segment(point(640,200), point(500,300)),
            segment(point(500,300), point(740,300)),
            segment(point(740,300), point(740,100)),
            segment(point(740,100), point(200,100))
        ];
    
    let polygon = new Polygon();
    polygon.addFace(shapes);
    
    // Apply polygon offset
    let offsetPolygon = polygon.offset(20);

See Offset Polygon interactive notebook to explore this algorithm in work.