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Encode GIFs with Node

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# gif-encoder-2

Encode GIFs with Node.js

## Contents

- [Installation](#installation)
- [Overview](#overview)
- [Usage](#usage)
  - [Constructor](#constructor)
  - [Methods](#methods)
- [Examples](#examples)
  - [Canvas Animation](#canvas-animation)
  - [Sequencial Images](#sequencial-images)
- [Algorithms](#algorithms)
- [Optimizer](#optimizer)
- [Progess Event](#progress-event)

## Installation

```
npm install gif-encoder-2
```

## Overview

This library builds on top of previous _JavaScript_ _GIF_ encoders including [jsgif](https://github.com/antimatter15/jsgif) and [gifencoder](https://github.com/eugeneware/gifencoder).

This library adds the [Octree](https://en.wikipedia.org/wiki/Octree) quantization algorithm as an alternative to the original _NeuQuant_ algorithm.

This library adds a simple optimizer to speed up overall processing time of both algorithms.

This library adds a progress event.

This library is designed to be used in a _Node_ environment, including the [Electron](https://electronjs.org/) renderer process. [Node Canvas](https://github.com/Automattic/node-canvas) can be a useful peer library but isn't required. The [HTML Canvas API] can be used in _Electron_.

## Usage

### Constructor

`GIFEncoder(width, height, algorithm, useOptimizer, totalFrames)`

```javascript
const encoder = new GIFEncoder(500, 500)
const encoder = new GIFEncoder(1200, 800, 'octree', false)
const encoder = new GIFEncoder(720, 480, 'neuquant', true, 20)
```

### Methods

## Examples

### Canvas Animation

Draw a square that changes color as it moves.

```javascript
const GIFEncoder = require('gif-encoder-2')
const { createCanvas } = require('canvas')
const { writeFile } = require('fs')
const path = require('path')

const size = 200
const half = size / 2

const canvas = createCanvas(size, size)
const ctx = canvas.getContext('2d')

function drawBackground() {
  ctx.fillStyle = '#ffffff'
  ctx.fillRect(0, 0, size, size)
}

const encoder = new GIFEncoder(size, size)
encoder.setDelay(500)
encoder.start()

drawBackground()
ctx.fillStyle = '#ff0000'
ctx.fillRect(0, 0, half, half)
encoder.addFrame(ctx)

drawBackground()
ctx.fillStyle = '#00ff00'
ctx.fillRect(half, 0, half, half)
encoder.addFrame(ctx)

drawBackground()
ctx.fillStyle = '#0000ff'
ctx.fillRect(half, half, half, half)
encoder.addFrame(ctx)

drawBackground()
ctx.fillStyle = '#ffff00'
ctx.fillRect(0, half, half, half)
encoder.addFrame(ctx)

encoder.finish()

const buffer = encoder.out.getData()

writeFile(path.join(__dirname, 'output', 'beginner.gif'), buffer, error => {
  // gif drawn or error
})
```

### Sequencial Images

Create a function that reads a directory of images and turns them into a _GIF_.

```javascript
const GIFEncoder = require('gif-encoder-2')
const { createCanvas, Image } = require('canvas')
const { createWriteStream, readdir } = require('fs')
const { promisify } = require('util')
const path = require('path')

const readdirAsync = promisify(readdir)
const imagesFolder = path.join(__dirname, 'input')

async function createGif(algorithm) {
  return new Promise(async resolve1 => {
    // read image directory
    const files = await readdirAsync(imagesFolder)

// find the width and height of the image
    const [width, height] = await new Promise(resolve2 => {
      const image = new Image()
      image.onload = () => resolve2([image.width, image.height])
      image.src = path.join(imagesFolder, files[0])
    })

// base GIF filepath on which algorithm is being used
    const dstPath = path.join(__dirname, 'output', `intermediate-${algorithm}.gif`)
    // create a write stream for GIF data
    const writeStream = createWriteStream(dstPath)
    // when stream closes GIF is created so resolve promise
    writeStream.on('close', () => {
      resolve1()
    })

const encoder = new GIFEncoder(width, height, algorithm)
    // pipe encoder's read stream to our write stream
    encoder.createReadStream().pipe(writeStream)
    encoder.start()
    encoder.setDelay(200)

const canvas = createCanvas(width, height)
    const ctx = canvas.getContext('2d')

// draw an image for each file and add frame to encoder
    for (const file of files) {
      await new Promise(resolve3 => {
        const image = new Image()
        image.onload = () => {
          ctx.drawImage(image, 0, 0)
          encoder.addFrame(ctx)
          resolve3()
        }
        image.src = path.join(imagesFolder, file)
      })
    }
  })
}

createGif('neuquant')
createGif('octree')
```

**NeuQuant Algorithm**

**Octree Algorithm**

## Algorithms

- _NeuQuant_ tends to perform faster than _Octree_
- _Octree_ tends to output a smaller file than _NeuQuant_
- _Octree_ produces a slight banding effect

The example above encodes 20 images measuring 300px x 240px. The output file from _NeuQuant_ is 1172KB and the _Octree_ is less than half of that at 515KB.

## Optimizer

The optimizer works by reusing the color palette from the previous image on the current image. This can reduce the overall processing time signifigantly but its best suited for a sequence of similarly colored images. Use the `setThreshold` method to set a percentage determining how similar the two images must be to trigger the optimizer. The default is `90%`. The optimizer is only used if `true` is passed as the 4th argument to the constructor.

## Progress Event

Works if `totalFrames` is expressed in constructor, otherwise this value will be 0.

```javascript
encoder.on('progress', percent => {
  // do something with percent value
})
```

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