Worker addition for gif-engine-js

worker_addition.js

const getDisposals = frameObj => frameObj.graphicExtension && frameObj.graphicExtension.disposalMethod || 0;
const getDelays = frameObj => frameObj.graphicExtension && frameObj.graphicExtension.delay - 1 || 0;
const copyColorsTransparent = async (source, target, fWidth, fHeight, oLeft, oTop, cWidth, flag) => {
  for (let row = 0, pointer = -1; fHeight > row; ++row)
  for (let column = 0; fWidth > column; ++column) {
    let offset = (column + oLeft + (row + oTop) * cWidth) * 4;
    if (flag && source[pointer + 4] === 0) {
      pointer += 4;
      continue;
    }
    target[  offset] = source[++pointer];
    target[++offset] = source[++pointer];
    target[++offset] = source[++pointer]; ++pointer;
    target[++offset] = flag ? source[pointer] : 255;
  }
};
const copyColors = async (source, target, fWidth, fHeight, oLeft, oTop, cWidth) => {
  for (let row = 0, pointer = -1; fHeight > row; ++row)
  for (let column = 0; fWidth > column; ++column) {
    let offset = (column + oLeft + (row + oTop) * cWidth) * 4;
    target[++pointer] = source[  offset];
    target[++pointer] = source[++offset];
    target[++pointer] = source[++offset];
    target[++pointer] = source[++offset];
  }
};
const messageHandler = async e => {
  const o = await GIF(
    await (
      await fetch(e.data, { headers: { pragma: "no-cache", "cache-control": "no-cache" }})
    ).arrayBuffer()
  );
  const frameCount = o.frames.length;
  const compiledFrames = new Array(frameCount);
  const delays = o.frames.map(getDelays);
  const canvasWidth = o.descriptor.width;
  const canvasHeight = o.descriptor.height;
  const disposals = o.frames.map(getDisposals);
  const canvas = new Uint8ClampedArray(canvasWidth * canvasHeight * 4);
  let index = 0;
  do {
    const frame = o.frames[index];
    const transparentColorFlag = frame.graphicExtension && frame.graphicExtension.transparentColorFlag;
    const [
      {
        data: frameImageData,
        width: frameWidth,
        height: frameHeight
      },
      offsetLeft,
      offsetTop
    ] = await o.toImageData(index);
    switch(disposals[index]) {
     case 2:
      for (let row = 0; frameHeight > row; ++row)
      for (let column = 0; frameWidth > column; ++column) {
        let offset = (column + offsetLeft + (row + offsetTop) * canvasWidth) * 4;
        canvas[  offset] = 0;
        canvas[++offset] = 0;
        canvas[++offset] = 0;
        canvas[++offset] = transparentColorFlag ? 0 : 255;
      } break;
     case 3:
      if (index > 0) {
        const [
          {
            data: frameImageData,
            width: frameWidth,
            height: frameHeight
          },
          offsetLeft,
          offsetTop
        ] = compiledFrames[index - 1];
        await copyColors(frameImageData, canvas, frameWidth, frameHeight, offsetLeft, offsetTop, canvasWidth);
      } break;
    }
    await copyColorsTransparent(frameImageData, canvas, frameWidth, frameHeight, offsetLeft, offsetTop, canvasWidth, transparentColorFlag);
    await copyColors(canvas, frameImageData, frameWidth, frameHeight, offsetLeft, offsetTop, canvasWidth);
    compiledFrames[index] = [new ImageData(frameImageData, frameWidth, frameHeight), offsetLeft, offsetTop];
  } while(++index < frameCount);
  postMessage([compiledFrames, delays, canvasWidth, canvasHeight]);
};
(global => {
  global.onmessage = messageHandler;
  global.onerror = e => {
    postMessage(["log", e]);
  };
})((() => this)());