phash.js

var Jimp = require("Jimp");

/*
 *  Compute Perceptual Hash of an Image
 *  Javascript equivalent of https://github.com/bjlittle/imagehash
 *
 *  These hashes differ by ~1-3 Hamming distance from original python implementation
 *
 */

// Copied from https://gist.github.com/bellbind/6fee86bd8027b57991f9
function dct2d(mat, m, n) {
  // 0.0-1.0 2D matrix
  var isqrt2 = 1 / Math.sqrt(2);
  var px = Math.PI / m;
  var py = Math.PI / n;
  var r = [];
  for (var ry = 0; ry < n; ry++) {
    for (var rx = 0; rx < m; rx++) {
      var c = Math.pow(isqrt2, (rx === 0) + (ry === 0));
      var t = 0;
      for (var y = 0; y < n; y++) {
        for (var x = 0; x < m; x++) {
          var v = mat[y * m + x];
          t += v * Math.cos(px * (x + 0.5) * rx) * Math.cos(py * (y + 0.5) * ry);
        }
      }
      r.push(c * t / 4);
    }
  }
  return r;
};


function phashQ(imgUrl, hashsize = 8, hfreq_fact = 4) {
  const LUMASCALE = [0.2989, 0.5870, 0.1140];                  //ITU-R 601-2 luma RGB -> greyscale transform
  const OCTLEN = 8;
  var imgsize = hashsize * hfreq_fact;
  var midpoint = Math.floor((hashsize * hashsize) / 2);
  var noctets = Math.floor((hashsize * hashsize) / OCTLEN);
  return new Promise((resolve, reject) => {
    Jimp.read(imgUrl).then(img => {
      var pixels = [];
      img.resize(imgsize, imgsize).scan(0, 0, imgsize, imgsize, function(x, y, idx) {
        pixels.push(
          this.bitmap.data[idx] * LUMASCALE[0] + 
          this.bitmap.data[idx + 1] * LUMASCALE[1] + 
          this.bitmap.data[idx + 2] * LUMASCALE[2]);
      });
      var coef = dct2d(pixels, imgsize, imgsize);
      var lofreqcoef = coef.filter((v, idx) => (Math.floor(idx / imgsize) < hashsize) && ((idx % imgsize) < hashsize));
      var median = lofreqcoef.slice().sort((a, b) => a - b)[midpoint];
      var bits = new Uint8Array(noctets);
      for (var i=0; i<noctets; i++) {
        bits[i] = lofreqcoef.slice(i * OCTLEN, (i + 1) * OCTLEN).
        reduce((acc, val, idx) => acc |= ((val > median) ? 01 : 00) << (OCTLEN - idx - 1), 00);
      }
      resolve(bits.reduce((acc, val) => acc += val.toString(16).padStart(2, "0"), ""));
    });
  });
}