linetrim-pslg

This module trims the edges in a planar straight line graph (pslg) based on another planar straight line graph.

This code is a modified version of Mikola Lysenko's overlay-pslg code. It is correct to the best of my knowledge, but I suspect there are simpler correct algorithms in the literature.

Example

Here is a simple example showing how to use this module to compute the intersection of two PSLGs:

//Load the module
var linetrim = require('linetrim-pslg')
 
//Red PSLG - Define a triangle
var linePoints = [
  [0.5, 0.25],
  [0.25, 0.5],
  [0.75, 0.75]
]
var lineEdges = [ [0,1], [1,2], [2,0] ]
 
//Blue PSLG - Define a square
var polyPoints = [
  [0.25, 0.25],
  [0.25,  0.6],
  [0.6, 0.6],
  [0.6, 0.25]
]
var polyEdges = [ [0,1], [1,2], [2,3], [3,0] ]
 
//Construct intersection
console.log(linetrim(linePoints, lineEdges, polyPoints, polyEdges, false))

Output

The result of this module is the following JSON:

{ points: 
   [ [ 0.75, 0.75 ],
     [ 0.44999999999999996, 0.6 ],
     [ 0.6, 0.44999999999999996 ] ],
  edges: [ [ 0, 1 ], [ 0, 2 ] ] }

We can visualize this result as follows:

Install

To install this module, you can use npm. The command is as follows:

npm i linetrim-pslg

It works in any reasonable CommonJS environment like node.js. If you want to use it in a browser, you should use browserify.

API

require('linetrim-pslg')(linePoints, lineEdges, bluePoints, blueEdges[, intersectMode])

Computes a Boolean operation between two planar straight line graphs.

• linePoints, lineEdges are the points and edges of the lines that will be cut up
• bluePoints, blueEdges are the points and edges of the polygons
• intersectMode specifies if we're in intersect mode (true) or subtract mode (false)

Returns An object encoding a planar straight line graph with the remaining line segments

• points are the points of the result
• edges are the edges we have kept, indexing into the points array.

Note The interiors of the polygon are computed using cdt2d. It counts the parity of the path with the fewest number of boundary crossings for each point. Even parity points are in the exterior, odd parity in the interior.

License

(c) 2017 Joseph Gentle, Mikola Lysenko. MIT License