Round-polygon NPM

round-polygon

Small, typed, dependency-free tool to round corners of any 2d-polygon provided by an array of { x, y } points.

The algorithm prevents rounding overlaps, so if you pass an oversized radius, it won't break the shape, but instead calculate the maximum radius of each point, just like you expect.

preview

Demo page

Installation

by npm:

npm i round-polygon

and then

import roundPolygon from "round-polygon"

or if you don't use npm you can import module from unpkg:

import roundPolygon from "https://unpkg.com/round-polygon@latest/dist/round-polygon.es.js"

or by script tag (with a link to IIFE module, which pollutes global scope with roundPolygon and getSegments functions) in your html page:

<script src="https://unpkg.com/round-polygon@latest/dist/round-polygon.iife.js"></script>

Usage

Types

if you use TypeScript you can also import input type "InitPoint" and output type "RoundedPoint"

import roundPolygon, { InitPoint, RoundedPoint } from "round-polygon"

let polygonToRound: InitPoint[],
    roundedPolygon: RoundedPoint[],
    radius: number

Input

roundPolygon function takes two arguments: an array of initial points and a radius

polygonToRound = [
  { x: 100, y:   0 },
  { x:   0, y: 150 },
  { x: 200, y: 150 },
  { x: 200, y:   0 },
  { x: 150, y: 200 },
]

radius = 20

roundedPolygon = roundPolygon(polygonToRound, radius)

To set a certain radius for a certain point, just add "r" property to the initial point. The radius passed as the argument to the function won't affect these points. Keep in mind that the algorithm rounds these points with higher priority.

{ x: 100, y: 0, r: 50 }

Output

a rounded point is an object with provided properties:

{
  id: number, // index of the current point
  x: number, // x-coordinate of the initial point
  y: number, // y-coordinate of the initial point
  offset: number, // length between the initial point and the start or the end of a rounding arc
  angle: { // in radians
    main: number, // the angle between the previous point, the current one, and the next one
    prev: number, // the angle between prev-to-curr-line to x-Axis
    next: number, // the angle between next-to-curr-line to x-Axis
    bis: number, // the bisector angle to x-Axis
    dir: 1 | -1 // whether clockwise (1) or counter-clockwise (-1) is the main angle direction (from the "prev" to the "next" angle)
  },
  arc: {
    radius: number, // the rounding radius of the current point (might be less then provided as an argument (caused by rounding overlapping))
    x: number, // x-coordinate of rounding center
    y: number // y-coordinate of rounding center
  },
  in: {
    length: number, // the length of prev-to-curr-line
    x: number, // x-coordinate where the arc begins laying on prev-to-curr-line
    y: number // y-coordinate where the arc begins laying on prev-to-curr-line
  },
  out: {
    length: number, // the length of next-to-curr-line
    x: number, // x-coordinate where arc ends laying on next-to-curr-line
    y: number // y-coordinate where arc ends laying on next-to-curr-line
  },
  prev: {...}, // a getter, returns prev-indexed rounded point
  next: {...} // a getter, returns next-indexed rounded point
}

Segments

You might need the rounded polygon provided with only { x, y } points, so every arc should be divided by segments of stright lines. Given getSegments function does that, it calculates coordinates of segmented arcs points. It takes array of rounded points, type of arc division (by LENGTH of segments or by AMOUNT of points) and a value to divide by corresponded to this type. It returns an array of { x, y } points.

import roundPolygon, { getSegments, Point } from "round-polygon"

let segments: Point[]

roundedPolygon = roundPolygon(polygonToRound, radius)
segments = getSegments(roundedPolygon, "LENGTH", 20)

Summary

so the whole approach to draw a rounded shape using, for example, Canvas API looks like this:

// init
const
  canvas = document.querySelector("canvas"),
  ctx = canvas.getContext("2d"),
  polygonToRound = [
    { x: 100, y:   0, r: 60 },
    { x:   0, y: 150 },
    { x: 200, y: 150, r: 60 },
    { x: 200, y:   0 },
    { x: 150, y: 200 },
  ],
  radius = 1000,
  roundedPolygon = roundPolygon(polygonToRound, radius),
  segments = getSegments(roundedPolygon, "LENGTH", 10)

// draw rounded shape
ctx.beginPath()
roundedPolygon.forEach((p, i) => {
  !i && ctx.moveTo(p.in.x, p.in.y)
  ctx.arcTo(p.x, p.y, p.out.x, p.out.y, p.arc.radius)
  ctx.lineTo(p.next.in.x, p.next.in.y)
})
ctx.stroke()

// draw segments of rounded shape
segments.forEach((p) => {
  ctx.beginPath()
  ctx.arc(p.x, p.y, 2, 0, Math.PI*2)
  ctx.fill()
})

example