hash()
hash(\Grafika\Gd\Image $image, \Grafika\Gd\Editor $editor) : string
Generate and get the difference hash of image.
Parameters
| \Grafika\Gd\Image | $image | |
| \Grafika\Gd\Editor | $editor |
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
hash(\Grafika\Gd\Image $image, \Grafika\Gd\Editor $editor) : string
Generate and get the difference hash of image.
| \Grafika\Gd\Image | $image | |
| \Grafika\Gd\Editor | $editor |
<?php
namespace Grafika\Gd\ImageHash;
use Grafika\Gd\Editor;
use Grafika\Gd\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\Gd\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
$gd = $image->getCore();
// Build hash
$hash = '';
for ($y = 0; $y < $height; $y++) {
// Get the pixel value for the leftmost pixel.
$rgba = imagecolorat($gd, 0, $y);
$r = ($rgba >> 16) & 0xFF;
$g = ($rgba >> 8) & 0xFF;
$b = $rgba & 0xFF;
$left = floor(($r + $g + $b) / 3);
for ($x = 1; $x < $width; $x++) {
// Get the pixel value for each pixel starting from position 1.
$rgba = imagecolorat($gd, $x, $y);
$r = ($rgba >> 16) & 0xFF;
$g = ($rgba >> 8) & 0xFF;
$b = $rgba & 0xFF;
$right = floor(($r + $g + $b) / 3);
// Each hash bit is set based on whether the left pixel is brighter than the right pixel.
if ($left > $right) {
$hash .= '1';
} else {
$hash .= '0';
}
// Prepare the next loop.
$left = $right;
}
}
$editor->free( $image );
return $hash;
}
}