hometown/app/javascript/mastodon/utils/resize_image.js

import EXIF from 'exif-js';

const MAX_IMAGE_PIXELS = 1638400; // 1280x1280px

const getImageUrl = inputFile => new Promise((resolve, reject) => {
  if (window.URL && URL.createObjectURL) {
    try {
      resolve(URL.createObjectURL(inputFile));
    } catch (error) {
      reject(error);
    }
    return;
  }

  const reader = new FileReader();
  reader.onerror = (...args) => reject(...args);
  reader.onload  = ({ target }) => resolve(target.result);

  reader.readAsDataURL(inputFile);
});

const loadImage = inputFile => new Promise((resolve, reject) => {
  getImageUrl(inputFile).then(url => {
    const img = new Image();

    img.onerror = (...args) => reject(...args);
    img.onload  = () => resolve(img);

    img.src = url;
  }).catch(reject);
});

const getOrientation = (img, type = 'image/png') => new Promise(resolve => {
  if (type !== 'image/jpeg') {
    resolve(1);
    return;
  }

  EXIF.getData(img, () => {
    const orientation = EXIF.getTag(img, 'Orientation');
    resolve(orientation);
  });
});

const processImage = (img, { width, height, orientation, type = 'image/png' }) => new Promise(resolve => {
  const canvas  = document.createElement('canvas');

  if (4 < orientation && orientation < 9) {
    canvas.width  = height;
    canvas.height = width;
  } else {
    canvas.width  = width;
    canvas.height = height;
  }

  const context = canvas.getContext('2d');

  switch (orientation) {
  case 2: context.transform(-1, 0, 0, 1, width, 0); break;
  case 3: context.transform(-1, 0, 0, -1, width, height); break;
  case 4: context.transform(1, 0, 0, -1, 0, height); break;
  case 5: context.transform(0, 1, 1, 0, 0, 0); break;
  case 6: context.transform(0, 1, -1, 0, height, 0); break;
  case 7: context.transform(0, -1, -1, 0, height, width); break;
  case 8: context.transform(0, -1, 1, 0, 0, width); break;
  }

  context.drawImage(img, 0, 0, width, height);

  // The Tor Browser and maybe other browsers may prevent reading from canvas
  // and return an all-white image instead. Assume reading failed if the resized
  // image is perfectly white.
  const imageData = context.getImageData(0, 0, width, height);
  if (imageData.data.every(value => value === 255)) {
    throw 'Failed to read from canvas';
  }

  canvas.toBlob(resolve, type);
});

const resizeImage = (img, type = 'image/png') => new Promise((resolve, reject) => {
  const { width, height } = img;

  const newWidth  = Math.round(Math.sqrt(MAX_IMAGE_PIXELS * (width / height)));
  const newHeight = Math.round(Math.sqrt(MAX_IMAGE_PIXELS * (height / width)));

  getOrientation(img, type)
    .then(orientation => processImage(img, {
      width: newWidth,
      height: newHeight,
      orientation,
      type,
    }))
    .then(resolve)
    .catch(reject);
});

export default inputFile => new Promise((resolve, reject) => {
  if (!inputFile.type.match(/image.*/) || inputFile.type === 'image/gif') {
    resolve(inputFile);
    return;
  }

  loadImage(inputFile).then(img => {
    if (img.width * img.height < MAX_IMAGE_PIXELS) {
      resolve(inputFile);
      return;
    }

    resizeImage(img, inputFile.type)
      .then(resolve)
      .catch(() => resolve(inputFile));
  }).catch(reject);
});