2d-polygon-self-intersections-lua's Introduction

2d-polygon-self-intersections-lua

This is a Lua port of JS library 2d-polygon-self-intersections to find self-intersections in a 2d polygon.

This library may not be fast, but it is robust. Robust in the fact that it will find all of the self-intersections in a polygon - minus of course shared endpoints.

You can expect a time complexity of O(n^2)

How to use it

local intersections = require("intersections")

local poly = { 0,0, 10,0, 0,10, 10,10 }

local isects = intersections(poly)

for i=1,#isects-1,2 do
    print("x: "..isects[i], "y: "..isects[i+1])
end

-- outputs: x: 5    y: 5

Description

intersections(polygon, filterFunc)

polygon - A table with a sequence of vertices (i.e. a triangle { 0,0, 10,0, 10,10 })
filterFunc - A filter function called whenever an intersection is found: filterFunc(ix,iy, i, x1,y1, x2,y2, j, x3,y3, x4,y4, unique)
ix,iy - current intersection (e.g. x: 5, y: 5) - mutations in this array get collected
i - index of the segment (e.g 1)
x1,y1 - start of the first segment (e.g 0,5)
x2,y2 - start of the first segment (e.g 10,5)
j - index of the segment (e.g 3)
x3,y3 - start of the first segment (e.g 5,0)
x4,y4 - start of the first segment (e.g 5,10)
unique - boolean representing whether or not this intersection point has been seen before
return true to collect and false to discard

returns nil if no intersection is found or a table listing all intersections in a row.

NOTE: this library assumes the polygon is closed, so manually adding the start point as the end point has no effect.

Also note that there are 2 intersections per crossing, this library by default will only report one - all intersections will be unique. This behavior can be changed with the filterFunc.