graph-draw

A JavaScript library for tessellating undirected planar graphs for Node and browsers.

The algorithm is designed to avoid local overdraw. A typical non local overdraw (expected) situation:

The library can be used for example to draw boundaries or polylines on a Leaflet map using leaflet-pixi-overlay (see the demos).

Demo

A very basic demo.

A polyline on a map (173 edges tessellated in 335 triangles).

French cities boundaries (186722 edges tessellated in 769041 triangles).

A rail network.

Installation

graph-draw is available as a npm package:

npm install graph-draw

In Node:

const graphDraw = require('graph-draw');

In browsers, include one file from the dist directory. (Files with name that contains “bundle” include libtess.)

Usage

const vertices = [[0, 0], [100, 0], [100, 100], [0, 100]];
// coordinates of the vertices
const edges = [[0, 1], [1, 2], [2, 3], [3, 0], [1, 3]];
// each edge is specified with the indices of the linked vertices
// each edge must appear exactly once in the list
// ([0, 1] and [1, 0] are the same edge)
const graph = {vertices, edges};
const strokeWidth = 10;
const polygons = [];
const polygonCallBack = convexPolygon => polygons.push(convexPolygon);
graphDraw(graph, strokeWidth, polygonCallBack);

The polygonCallBack is executed on each polygon of the tessellation. Now, polygons contains a list of convex polygons (which can be easily converted into triangle strips or triangle fans):

[
  [[x1, y1], [x2, y2], [x3, y3], [x4, y4]],
  [[a1, b1], [a2, b2], [a3, b3]],
  ...
]

Those convex polygons can have between 3 and 8 edges.

Limiting miters

When the angle between two consecutive edges is close to 2π, long miter situations occur. For example:

const vertices = [
  [0, -200],
  [100 , -100],
  [30, -200]
];
const edges = [
  [0, 1],
  [1, 2]
];
const graph = {vertices, edges};
const strokeWidth = 20;
graphDraw(graph, strokeWidth, polygonCallBack);

produces:

To avoid this, graphDraw function accepts a fourth (optional) maxAngle parameter which is an angle between π and 2π. If the angle between two consecutive edges is above maxAngle, the miter will be replaced by two triangles approximating a round join. For example:

graphDraw(graph, strokeWidth, polygonCallBack, Math.PI);

will produce:

Sponsors

Thanks to Stadia Maps for providing the Stamen Toner Lite map tiles in our project demos.