Draw perfect pressure-sensitive freehand lines.

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Table of Contents

• Installation
• Usage
• Support
• Discussion
• Author

Installation

npm install perfect-freehand

or

yarn add perfect-freehand

Usage

This package exports a function named getStroke that:

• accepts an array of points and an (optional) options object
• returns a stroke outline as an array of points formatted as [x, y]

import { getStroke } from 'perfect-freehand'

You may format your input points as array or an object. In both cases, the value for pressure is optional (it will default to .5).

getStroke([
  [0, 0, 0],
  [10, 5, 0.5],
  [20, 8, 0.3],
])

getStroke([
  { x: 0, y: 0, pressure: 0 },
  { x: 10, y: 5, pressure: 0.5 },
  { x: 20, y: 8, pressure: 0.3 },
])

Options

The options object is optional, as are each of its properties.

Property	Type	Default	Description
size	number	8	The base size (diameter) of the stroke.
thinning	number	.5	The effect of pressure on the stroke's size.
smoothing	number	.5	How much to soften the stroke's edges.
streamline	number	.5	How much to streamline the stroke.
simulatePressure	boolean	true	Whether to simulate pressure based on velocity.
easing	function	t => t	An easing function to apply to each point's pressure.
start	{ }		Tapering options for the start of the line.
end	{ }		Tapering options for the end of the line.
last	boolean	true	Whether the stroke is complete.

Note: When the last property is true, the line's end will be drawn at the last input point, rather than slightly behind it.

The start and end options accept an object:

Property	Type	Default	Description
cap	boolean	true	Whether to draw a cap.
taper	number	0	The distance to taper.
easing	function	t => t	An easing function for the tapering effect.

Note: The cap property has no effect when taper is more than zero.

getStroke(myPoints, {
  size: 8,
  thinning: 0.5,
  smoothing: 0.5,
  streamline: 0.5,
  easing: (t) => t,
  simulatePressure: true,
  last: true,
  start: {
    cap: true,
    taper: 0,
    easing: (t) => t,
  },
  end: {
    cap: true,
    taper: 0,
    easing: (t) => t,
  },
})

Tip: To create a stroke with a steady line, set the thinning option to 0.

Tip: To create a stroke that gets thinner with pressure instead of thicker, use a negative number for the thinning option.

Rendering

While getStroke returns an array of points representing the outline of a stroke, it's up to you to decide how you will render these points.

The function below will turn the points returned by getStroke into SVG path data.

function getSvgPathFromStroke(stroke) {
  if (!stroke.length) return ''

  const d = stroke.reduce(
    (acc, [x0, y0], i, arr) => {
      const [x1, y1] = arr[(i + 1) % arr.length]
      acc.push(x0, y0, (x0 + x1) / 2, (y0 + y1) / 2)
      return acc
    },
    ['M', ...stroke[0], 'Q']
  )

  d.push('Z')
  return d.join(' ')
}

To use this function, first use perfect-freehand to turn your input points into a stroke outline, then pass the result to getSvgPathFromStroke.

import { getStroke } from 'perfect-freehand'

const myStroke = getStroke(myInputPoints)

const pathData = getSvgPathFromStroke(myStroke)

You could then pass this string either to an SVG path element:

<path d={pathData} />

Or, if you are rendering with HTML Canvas, you can pass the result to a Path2D constructor).

const myPath = new Path2D(pathData)

ctx.fill(myPath)

Flattening

To render a stroke as a "flattened" polygon, add the polygon-clipping package and use the following function together with the getSvgPathFromStroke.

import polygonClipping from 'polygon-clipping'

function getFlatSvgPathFromStroke(stroke) {
  const poly = polygonClipping.union([stroke])

  const d = []

  for (let face of poly) {
    for (let points of face) {
      d.push(getSvgPathFromStroke(points))
    }
  }

  return d.join(' ')
}

Tip: For implementations in Typescript, see the example project included in this repository.

Example

import * as React from 'react'
import { getStroke } from 'perfect-freehand'
import { getSvgPathFromStroke } from './utils'

export default function Example() {
  const [points, setPoints] = React.useState([])

  function handlePointerDown(e) {
    e.target.setPointerCapture(e.pointerId)
    setPoints([[e.pageX, e.pageY, e.pressure]])
  }

  function handlePointerMove(e) {
    if (e.buttons !== 1) return
    setPoints([...points, [e.pageX, e.pageY, e.pressure]])
  }

  const stroke = getStroke(points, {
    size: 16,
    thinning: 0.5,
    smoothing: 0.5,
    streamline: 0.5,
  })

  const pathData = getSvgPathFromStroke(stroke)

  return (
    <svg
      onPointerDown={handlePointerDown}
      onPointerMove={handlePointerMove}
      style={{ touchAction: 'none' }}
    >
      {points && <path d={pathData} />}
    </svg>
  )
}

Advanced Usage

For advanced usage, the library also exports smaller functions that getStroke uses to generate its outline points.

getStrokePoints

A function that accepts an array of points (formatted either as [x, y, pressure] or { x: number, y: number, pressure: number}) and a streamline value. Returns a set of adjusted points as { point, pressure, vector, distance, runningLength }. The path's total length will be the runningLength of the last point in the array.

import { getStrokePoints } from 'perfect-freehand'
import samplePoints from "./samplePoints.json'

const strokePoints = getStrokePoints(samplePoints)

getOutlinePoints

A function that accepts an array of points (formatted as { point, pressure, vector, distance, runningLength }, i.e. the output of getStrokePoints) and returns an array of points ([x, y]) defining the outline of a pressure-sensitive stroke.

import { getStrokePoints, getOutlinePoints } from 'perfect-freehand'
import samplePoints from "./samplePoints.json'

const strokePoints = getStrokePoints(samplePoints)
const outlinePoints = getOutlinePoints(strokePoints)

StrokeOptions

A TypeScript type for the options object. Useful if you're defining your options outside of the getStroke function.

import { StrokeOptions, getStroke } from 'perfect-freehand'

const options: StrokeOptions = {
  size: 16,
}

const stroke = getStroke(options)

Support

Please open an issue for support.

Discussion

Have an idea or casual question? Visit the discussion page.

Author

• @steveruizok