The better DCT perceptual hash algorithm

In my previous blog entry I derived a simple but efficient method of constructing a DCT hash which facilitates sorting the hash values, basically producing a perceptual sort to a set of images - at least for well-behaving "normal" images.

My first solution was to transform the existing "traditional" DCT hashes into sort-friendly hash. As I don't think it to be very productive to first construct the DCT hashes in traditional way and then running a re-index. I present in this blog post a class to calculate the sort-friendly hash directly.

The class is as follows:

<?php
  // This class constructs perceptual hash for an image
  // It is based on the basic DCT hash but by reorganizing the hash bits
  // the hash is more or less sortable, so building basically a perceptual index
  //
  // DCT code initial author: Elliot Shepherd
  //
  // Sort-friendly transformation author: The Nekkid PHP Programmer
  // http://nekkidphpprogrammer.blogspot.fi
  // nekkidphpprogrammer<at>gmail.com
  //
  class Phash_Bis {
    private $size=32;
    private $small_size=8;  
    private $c=array();
    private $reorder=array();
       
    private function InitCoefficients() {
      $this->c[0]=1.0/sqrt(2);
      for ($i=1; $i < $this->size ; $i++)
        $this->c[$i]=1.0;                   
//
// here we intialize the matrix for placing most significant frequencies to 

// the beginning of the hash
//
      for ($l=0;$l<$this->small_size;$l++) {
        $stp=$l*$l;
        for ($u=0;$u<=$l;$u++) {
          $this->reorder[$stp]=array($u,$l);
          $stp+=2;
        }
        $stp=$l*$l+1;
        for ($v=0;$v<$l;$v++) {
          $indexer=$l*$this->small_size+$v;
          $this->reorder[$stp]=array($l,$v);
          $stp+=2;
        }
      }
      ksort($this->reorder);
    }
    
    private function blue($img,$x,$y) {
      return imagecolorat($img,$x, $y) & 0xff;
    }
        
    public function __construct($my_size=32,$my_small_size=8) {
      $this->size=$my_size;
      $this->small_size=$my_small_size;
      $this->InitCoefficients();
    }
    
    private function ApplyDCT($f) {
      $n=$this->size;
      $F=array();
      for ($u=0;$u<$n;$u++) {
        for ($v=0;$v<$n;$v++) {
          $sum=0.0;
          for ($i=0;$i<$n;$i++) {
            for ($j=0;$j<$n;$j++) {
              $sum+=cos(((2*$i+1)/(2.0*$n))*$u*M_PI)*cos(((2*$j+1)/(2.0*$n))*$v*M_PI)*($f[$i][$j]);
            }
          }
          $sum*=(($this->c[$u]*$this->c[$v])/4.0);
          $F[$u][$v]=$sum;
        }
      } 
      return $F;
    }
    
    public function hash($image) {
      $timing=microtime(true);
      $hash="missing";
      if (file_exists($image)) {
        $size=$this->size;
        $res=imagecreatefromstring(file_get_contents($image));
        $img = imagecreatetruecolor($size, $size);
        imagecopyresampled($img, $res, 0, 0, 0, 0, $size, $size, imagesx($res), imagesy($res));
        imagecopymergegray($img, $res, 0, 0, 0, 0, $size, $size, 50);
        $vals=array();
        for ($x=0;$x<$size;$x++) {
          for ($y=0;$y<$size;$y++) {
            $vals[$x][$y]=$this->blue($img,$x,$y);
          }
        }
        $dct_vals=$this->ApplyDCT($vals);
        $total=0.0;
        for ($x=0;$x<$this->small_size;$x++) {
          for ($y=0;$y<$this->small_size;$y++) {
            $total += $dct_vals[$x][$y];
          }
        }
        $total-=$dct_vals[0][0];
        $avg=$total/($this->small_size*$this->small_size-1);
        $hash=0;
//
// Transformed hash generation
//        
        foreach ($this->reorder as $ptr) {
          $hash = gmp_mul($hash, 2);
          if ($dct_vals[$ptr[0]][$ptr[1]]>$avg)
            $hash=gmp_add($hash, 1); ;
        }
//
//  Hash is returned by hexadecimal string, my preference
//        
        $hash_len=$this->small_size*$this->small_size/4;
        return substr("0000000000000000".gmp_strval($hash,$hash_len),-$hash_len);
        
        
      }
      return $hash;
    }
  }
?>

I hope this helps you to find those pesky duplicates more effectively :)

If you encounter an error or any other problem, you may comment here and I'll see if I can be of help.