polyline-normals/index.js

gets miter normals for a 2D polyline

var util = require('polyline-miter-util')

var lineA = [0, 0]
var lineB = [0, 0]
var tangent = [0, 0]
var miter = [0, 0]

module.exports = function(points, closed) {
    var curNormal = null
    var out = []
    if (closed) {
        points = points.slice()
        points.push(points[0])
    }

    var total = points.length
    for (var i=1; i<total; i++) {
        var last = points[i-1]
        var cur = points[i]
        var next = i<points.length-1 ? points[i+1] : null

        util.direction(lineA, cur, last)
        if (!curNormal)  {
            curNormal = [0, 0]
            util.normal(curNormal, lineA)
        }

        if (i === 1) //add initial normals
            addNext(out, curNormal, 1)

        if (!next) { //no miter, simple segment
            util.normal(curNormal, lineA) //reset normal
            addNext(out, curNormal, 1)
        } else { //miter with last
            //get unit dir of next line
            util.direction(lineB, next, cur)

            //stores tangent & miter
            var miterLen = util.computeMiter(tangent, miter, lineA, lineB, 1)
            addNext(out, miter, miterLen)
        }
    }

    //if the polyline is a closed loop, clean up the last normal
    if (points.length > 2 && closed) {
        var last2 = points[total-2]
        var cur2 = points[0]
        var next2 = points[1]

        util.direction(lineA, cur2, last2)
        util.direction(lineB, next2, cur2)
        util.normal(curNormal, lineA)
        
        var miterLen2 = util.computeMiter(tangent, miter, lineA, lineB, 1)
        out[0][0] = miter.slice()
        out[total-1][0] = miter.slice()
        out[0][1] = miterLen2
        out[total-1][1] = miterLen2
        out.pop()
    }

    return out
}

function addNext(out, normal, length) {
    out.push([[normal[0], normal[1]], length])
}