encodeToPNG.js

var dumpImage = (function() {
  var crcTable = [];
  for (var i = 0; i < 256; i++) {
    var c = i;
    for (var h = 0; h < 8; h++) {
      if (c & 1)
        c = 0xedB88320 ^ ((c >> 1) & 0x7fffffff);
      else
        c = (c >> 1) & 0x7fffffff;
    }
    crcTable[i] = c;
  }

  function crc32(data, start, end){
    var crc = -1;
    for (var i = start; i < end; i++) {
      var a = (crc ^ data[i]) & 0xff;
      var b = crcTable[a];
      crc = (crc >>> 8) ^ b;
    }
    return crc ^ -1;
  }

  function createPngChunk(type, data) {
    var chunk = new Uint8Array(12 + data.length);
    var p = 0;

    var len = data.length;
    chunk[p] = len >> 24 & 0xff;
    chunk[p + 1] = len >> 16 & 0xff;
    chunk[p + 2] = len >> 8 & 0xff;
    chunk[p + 3] = len & 0xff;

    chunk[p + 4] = type.charCodeAt(0) & 0xff;
    chunk[p + 5] = type.charCodeAt(1) & 0xff;
    chunk[p + 6] = type.charCodeAt(2) & 0xff;
    chunk[p + 7] = type.charCodeAt(3) & 0xff;

    if (data instanceof Uint8Array)
      chunk.set(data, 8);

    p = 8 + len;

    var crc = crc32(chunk, 4, p);
    chunk[p] = crc >> 24 & 0xff;
    chunk[p + 1] = crc >> 16 & 0xff;
    chunk[p + 2] = crc >> 8 & 0xff;
    chunk[p + 3] = crc & 0xff;

    return chunk;
  }

  function adler32(data, start, end) {
    var a = 1;
    var b = 0;
    for (var i = start; i < end; ++i) {
      a = (a + (data[i] & 0xff)) % 65521;
      b = (b + a) % 65521;
    }
    return (b << 16) | a;
  }

  function encode(imgData, kind) {
    var width = imgData.width;
    var height = imgData.height;
    var bitDepth;
    var colorType;

    var bytes = imgData.data;
    var lineSize;
    switch (kind) {
      case ImageKind.GRAYSCALE_1BPP:
        colorType = 0;
        bitDepth = 1;
        lineSize = (width + 7) >> 3;
        break;
      case ImageKind.RGB_24BPP:
        colorType = 2;
        bitDepth = 8;
        lineSize = width * 3;
        break;
      case ImageKind.RGBA_32BPP:
        colorType = 6;
        bitDepth = 8;
        lineSize = width * 4;
        break;
      default:
        throw new Error('invalid format');
    }

    var literals = new Uint8Array((1 + lineSize) * height);
    var offsetLiterals = 0, offsetBytes = 0;
    for (var y = 0; y < height; ++y) {
      literals[offsetLiterals++] = 0;
      literals.set(bytes.subarray(offsetBytes, offsetBytes + lineSize), offsetLiterals);
      offsetBytes += lineSize;
      offsetLiterals += lineSize;
    }

    var ihdr = new Uint8Array([
        width >> 24 & 0xff,
        width >> 16 & 0xff,
        width >> 8 & 0xff,
        width & 0xff,
        height >> 24 & 0xff,
        height >> 16 & 0xff,
        height >> 8 & 0xff,
        height & 0xff,
      bitDepth, // bit depth
      colorType, // color type
      0x00, // compression method
      0x00, // filter method
      0x00 // interlace method
    ]);

    var len = literals.length;
    var maxBlockLength = 0xFFFF;

    var idat = new Uint8Array(2 + len + Math.ceil(len / maxBlockLength) * 5 + 4);
    var pi = 0;
    idat[pi++] = 0x78; // compression method and flags
    idat[pi++] = 0x9c;  // flags

    var pos = 0;
    while (len > maxBlockLength) {
      idat[pi++] = 0x00;
      idat[pi++] = 0xff;
      idat[pi++] = 0xff;
      idat[pi++] = 0x00;
      idat[pi++] = 0x00;
      idat.set(literals.subarray(pos, pos + maxBlockLength), pi);
      pi += maxBlockLength;
      pos += maxBlockLength;
      len -= maxBlockLength;
    }

    idat[pi++] = 0x01;
    idat[pi++] = len & 0xff;
    idat[pi++] = len >> 8 & 0xff;
    idat[pi++] = (~len & 0xffff) & 0xff;
    idat[pi++] = (~len & 0xffff) >> 8 & 0xff;

    idat.set(literals.subarray(pos), pi);
    pi += literals.length - pos;

    var adler = adler32(literals, 0, literals.length); // checksum
    idat[pi++] = adler >> 24 & 0xff;
    idat[pi++] = adler >> 16 & 0xff;
    idat[pi++] = adler >> 8 & 0xff;
    idat[pi++] = adler & 0xff;

    var chunks = [
      new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]),
      createPngChunk('IHDR', ihdr),
      createPngChunk('IDAT', idat),
      createPngChunk('IEND', '')
    ];

    var blob = new Blob(chunks, { type: 'image/png' });
    return URL.createObjectURL(blob);
  }

  var d = null;
  return function dumpImage(imgData) {
    if (d === null) {
      var w = window.open('about:blank', '_blank');
      d = w.document.open('text/html');
    }

    var kind = imgData.kind === undefined ? ImageKind.GRAYSCALE_1BPP : imgData.kind;
    var url = encode(imgData, kind);
    d.writeln(imgData.width + 'x' + imgData.height + ' ' + kind + '<br>');
    d.writeln('<img src=' + url + '><br>');
    d.writeln('<hr>');
  }
})();

https://github.com/mozilla/shumway/blob/95e002869175eb886b3c99038e867d8ab5a9912d/src/swf/bitmap.js