https://joshwright.com/tips/how-to-upload-photos-to-instagram-with-the-highest-possible-quality/

instagram-compression-analysis.js

const fs = require('fs');
const _ = require('lodash');
const PNG = require('pngjs').PNG;
const { promisify } = require('util');
const md5File = require('md5-file');
const images = [
  { filename: 'graphic-gif.gif' },
  { filename: 'graphic-jpg-0.jpg' },
  { filename: 'graphic-jpg-76.jpg' },
  { filename: 'graphic-jpg-100.jpg' },
  { filename: 'graphic-png.png' },
];

for (const image of images) {
  image.orig = `${__dirname}/files/${image.filename}`;
  image.insta = image.orig.split('.')[0] + '-after.jpg';
  image.instapng = image.orig.split('.')[0] + '-after-png.png';
}

const getPixels = promisify((filename, callback) => {
  fs.createReadStream(filename)
    .pipe(new PNG({ colorType: 2 }))
    .on("parsed", function () {
      const pixels = [];
      for (var y = 0; y < this.height; y++) {
        for (var x = 0; x < this.width; x++) {
          var idx = (this.width * y + x) << 2;
          pixels.push({
            r: this.data[idx + 0],
            g: this.data[idx + 1],
            b: this.data[idx + 2],
          });
        }
      }
      callback(null, pixels);
    });
});

const comparePixels = (e1, e2) => {
  const rmean = (e1.r + e2.r) / 2;
  const r = e1.r - e2.r;
  const g = e1.g - e2.g;
  const b = e1.b - e2.b;
  const v = Math.sqrt((((512+rmean)*r*r)>>8) + 4*g*g + (((767-rmean)*b*b)>>8));
  const perc = Math.min(1, Math.max(v / 764, 0));
  return perc; // 0 is identical, 1 is opposite
};


(async () => {

  // const origPixels = await getPixels('./files/photoshop-png.png');
  const origPixels = await getPixels('./files/graphic-png.png');

  for (const image of images) {
    const imgPixels = await getPixels(image.instapng);

    let percentDiff = 0;
    for (const [i, aPixel] of origPixels.entries()){
      const bPixel = imgPixels[i];
      const pixelDiff = comparePixels(aPixel, bPixel);
      percentDiff += pixelDiff;
    }
    percentDiff = percentDiff / origPixels.length;
    image.diff = percentDiff;


    let exactDiff = 0;
    for (const [i, aPixel] of origPixels.entries()){
      const bPixel = imgPixels[i];
      const exactSame = JSON.stringify(aPixel) === JSON.stringify(bPixel);
      exactDiff += exactSame ? 0 : 1;
    }
    image.exactdiff = exactDiff / origPixels.length;

    image.origkb = Math.round(fs.statSync(image.orig).size/1024);
    image.instakb = Math.round(fs.statSync(image.insta).size/1024);

    const origHash = await md5File(image.orig);
    const instaHash = await md5File(image.insta);
    image.hashmatch = origHash === instaHash;
  }

  // results
  for (const image of _.sortBy(images, i => i.diff)) {
    console.log([
      image.filename,
      ((1-image.diff)*100).toFixed(4)+'%',
      ((1-image.exactdiff)*100).toFixed(4)+'%',
      image.origkb+'kb',
      image.instakb+'kb',
      image.hashmatch,
    ].join(','));
  }

})();