minimal animated GIF encoder in 50 lines

encode_anim_gif.js

// encode_anim_gif.js
// minimal code to make animated GIFs from arrays of pixels
// - no compression
// - supports 127 colors (127 shades of gray by default)

function encode_anim_gif(frames,w,h,delay=5){
  let bytes = [];
  bytes.push(0x47,0x49,0x46,0x38,0x39,0x61);
  bytes.push(w&0xff);
  bytes.push((w>>8)&0xff);
  bytes.push(h&0xff);
  bytes.push((h>>8)&0xff);
  bytes.push(0xf6);
  bytes.push(0,0);
  for (let i = 0; i < 127; i++){
    bytes.push(i*2,i*2,i*2); //change this for alternative color table
  }
  bytes.push(0xff,0xff,0xff);
  bytes.push(0x21,0xff,0x0b);
  bytes.push(0x4E,0x45,0x54,0x53,0x43,0x41,0x50,0x45,0x32,0x2E,0x30);
  bytes.push(0x03,0x01,0xff,0xff,0x00);

  for (let k = 0; k < frames.length; k++){
    let data = frames[k];
    bytes.push(0x21,0xf9,0x04,0b00000100);
    bytes.push(delay&0xff); // change this for different delay each frame
    bytes.push((delay>>8)&0xff);
    bytes.push(0xff,0x00);
    bytes.push(0x2c,0,0,0,0);
    bytes.push(w&0xff);
    bytes.push((w>>8)&0xff);
    bytes.push(h&0xff);
    bytes.push((h>>8)&0xff);
    bytes.push(0,7);

    let n = ~~(w*h/126);
    let inc = n*126;
    let exc = w*h-inc;
    for (let i = 0; i < n; i++){
      bytes.push(0x7f,0x80);
      for (let j = 0; j < 126; j++){
        bytes.push(~~(data[i*126+j]/2));
      }
    }
    if (exc){
      bytes.push(exc+1);
      bytes.push(0x80);
      for (let i = 0; i < exc; i++){
        bytes.push(~~(data[inc+i]/2));
      }
    }
    bytes.push(0x01,0x81,0x00);
  }
  bytes.push(0x3B);
  return bytes;
}

// ------ end library -------


// ----- begin example ------

let frames = [];
let W = 255;
let H = 255;
for (let k = 0; k < 100; k++){
  let data = [];
  for (let i = 0; i < H; i++){
    for (let j = 0; j < W; j++){
      data.push((j+k*5)%255);
    }
  }
  frames.push(data);
}
const fs = require('fs');
fs.writeFileSync("test.gif",Buffer.from(encode_anim_gif(frames,W,H,10)))