OffscreenCanvasとWeb Workerで画像処理を実装する

export interface WorkerRequestBody {
  buffer: ArrayBuffer;
  type: string;
  size: {
    height: number;
    width: number;
  };
}

export interface WorkerResponseBody {
  buffer: ArrayBuffer;
  contentType: string;
}

async function imageProcessing(
  data: WorkerRequestBody
): Promise<WorkerResponseBody> {
  const { buffer, type, size } = data;

  // ArrayBufferからBlobに戻す
  const blob = new Blob([buffer], { type });

  // OffscreenCanvasを生成
  const canvas = new OffscreenCanvas(size.width, size.height);

  // BlobからBitmapを作成
  const bitmap = await createImageBitmap(blob);

  // CanvasRenderingContext2Dを取得
  const ctx = canvas.getContext("2d");
  if (!ctx) throw new Error("Context2D is not defined");

  // 画像をCanvasに描画
  ctx.drawImage(bitmap, 0, 0);

  // WebPに変換
  const webp = await canvas.convertToBlob({
    type: "image/webp",
    quality: 0.85,
  });

  // WebPのBlobオブジェクトをArrayBufferに変換
  const buf = await webp.arrayBuffer();

  return {
    buffer: buf,
    contentType: "image/webp",
  };
}

self.addEventListener("message", async (e: MessageEvent<WorkerRequestBody>) => {
  const output = await imageProcessing(e.data);
  // ArrayBufferをUIスレッドに移譲するため、第二引数を指定し、所有権を渡す
  self.postMessage(output, [output.buffer]);
});

import MyWorker from "./worker.ts";

const worker = new MyWorker();

const fileInput = document.querySelector<HTMLInputElement>("#js-file-input");

worker.addEventListener("message", (e: MessageEvent<WorkerResponseBody>) => {
  const { buffer, contentType } = e.data;
  const blob = new Blob([buffer], { type: contentType });

  // do something
});

fileInput.addEventListener("change", async (e) => {
  const [file] = Array.from(fileInput.files);
  if (!file) return;

  const buffer = await file.arrayBuffer();
  const type = file.type;
  const size = { height: 1080, width: 1980 }; // 画像サイズを取得

  // WebWorkerに送信&所有権移譲
  worker.postMessage(
    {
      buffer,
      type,
      size
    },
    [buffer]
  );
});