squoosh/src/codecs/resize/processor.ts

import { WorkerResizeOptions } from './processor-meta';
import { getContainOffsets } from './util';
import { resize as codecResize } from '../../../codecs/resize/pkg';

function crop(data: ImageData, sx: number, sy: number, sw: number, sh: number): ImageData {
  const inputPixels = new Uint32Array(data.data.buffer);

  // Copy within the same buffer for speed and memory efficiency.
  for (let y = 0; y < sh; y += 1) {
    const start = ((y + sy) * data.width) + sx;
    inputPixels.copyWithin(y * sw, start, start + sw);
  }

  return new ImageData(
    new Uint8ClampedArray(inputPixels.buffer.slice(0, sw * sh * 4)),
    sw, sh,
  );
}

/** Resize methods by index */
const resizeMethods: WorkerResizeOptions['method'][] = [
  'triangle', 'catrom', 'mitchell', 'lanczos3',
];

export async function resize(data: ImageData, opts: WorkerResizeOptions): Promise<ImageData> {
  let input = data;

  if (opts.fitMethod === 'contain') {
    const { sx, sy, sw, sh } = getContainOffsets(data.width, data.height, opts.width, opts.height);
    input = crop(input, Math.round(sx), Math.round(sy), Math.round(sw), Math.round(sh));
  }

  const result = codecResize(
    new Uint8Array(input.data.buffer), input.width, input.height, opts.width, opts.height,
    resizeMethods.indexOf(opts.method), opts.premultiply, opts.linearRGB,
  );

  return new ImageData(new Uint8ClampedArray(result.buffer), opts.width, opts.height);
}