DifferenceHash.php
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<?php
namespace Grafika\Imagick\ImageHash;
use Grafika\Imagick\Editor;
use Grafika\Imagick\Image;
/**
* DifferenceHash
*
* Algorithm:
* Reduce size. The fastest way to remove high frequencies and detail is to shrink the image. In this case, shrink it to 9x8 so that there are 72 total pixels.
* Reduce color. Convert the image to a grayscale picture. This changes the hash from 72 pixels to a total of 72 colors.
* Compute the difference. The algorithm works on the difference between adjacent pixels. This identifies the relative gradient direction. In this case, the 9 pixels per row yields 8 differences between adjacent pixels. Eight rows of eight differences becomes 64 bits.
* Assign bits. Each bit is simply set based on whether the left pixel is brighter than the right pixel.
*
* http://www.hackerfactor.com/blog/index.php?/archives/529-Kind-of-Like-That.html
*
* @package Grafika\Imagick\ImageHash
*/
class DifferenceHash
{
/**
* Generate and get the difference hash of image.
*
* @param Image $image
*
* @param Editor $editor
*
* @return string
*/
public function hash($image, $editor)
{
$width = 9;
$height = 8;
$image = clone $image; // Make sure we are working on the clone if Image is passed
$editor->resizeExact($image, $width, $height); // Resize to exactly 9x8
$imagick = $image->getCore();
// Build hash
$hash = '';
for ($y = 0; $y < $height; $y++) {
// Get the pixel value for the leftmost pixel.
$rgba = $imagick->getImagePixelColor(0, $y)->getColor();
$left = floor(($rgba['r'] + $rgba['g'] + $rgba['b']) / 3);
for ($x = 1; $x < $width; $x++) {
// Get the pixel value for each pixel starting from position 1.
$rgba = $imagick->getImagePixelColor($x, $y)->getColor();
$right = floor(($rgba['r'] + $rgba['g'] + $rgba['b']) / 3);
// Each hash bit is set based on whether the left pixel is brighter than the right pixel.
if ($left > $right) {
$hash .= '1';
} else {
$hash .= '0';
}
// Prepare the next loop.
$left = $right;
}
}
$editor->free( $image );
return $hash;
}
}