Crop image by percentage in sharp

router.use('/', (req, res) => {
  const parameters = {
    left: 0.5, //procent wycięcia od góry
    top: 0.2, //procent wycięcia z lewej
    size: 0.5 //szerokość i wysokość zdjęcia brane z szerokości oryginalnego ()
  }
  const fileStream = this.s3.getObject({
    Bucket: '<my-bucket-name>',
    Key: '<photo-key>'
  }).createReadStream();

  let imageInfo;

  const transformer = sharp().on('info', info => {
    imageInfo = info;
  })

  const cropTransformer = sharp().extract({
    left: parameters.left * imageInfo.width,
    top: parameters.top * imageInfo.height,
    width: parameters.size * imageInfo.width,
    height: parameters.size * imageInfo.width
  });

  return fileStream.pipe(transformer).pipe(cropTransformer).pipe(res);
})