garygreen · May 28, 2020 13:00 · May 28, 2020
diff --git a/GdDifferenceHash.php b/GdDifferenceHash.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace App\Image;
+
+use Jenssegers\ImageHash\Hash;
+use Jenssegers\ImageHash\Implementation;
+
+class GdDifferenceHash implements Implementation
+{
+	/**
+	 * @var int
+	 */
+	protected $size;
+
+	/**
+	 * @param int $size
+	 */
+	public function __construct($size = 8)
+	{
+		$this->size = $size;
+	}
+
+	/**
+	 * @inheritdoc
+	 */
+	public function hash($image)
+	{
+		$source = imagecreatefromjpeg($image);
+
+		// For this implementation we create a 8x9 image.
+		$width = $this->size + 1;
+		$height = $this->size;
+
+		// Resize the image.
+		// $resized = $image->resize($width, $height);
+		$resized = imagecreatetruecolor($width, $height);
+
+		list($origwidth, $origheight) = getimagesize($image);
+		imagecopyresized($resized, $source, 0, 0, 0, 0, $width, $height, $origwidth, $origheight);
+
+		$bits = [];
+		for ($y = 0; $y < $height; $y++) {
+			// Get the pixel value for the leftmost pixel.
+			$rgb = $this->pickColor($resized, 0, $y);
+			$left = (int) floor(($rgb[0] * 0.299) + ($rgb[1] * 0.587) + ($rgb[2] * 0.114));
+
+			for ($x = 1; $x < $width; $x++) {
+				// Get the pixel value for each pixel starting from position 1.
+				$rgb = $this->pickColor($resized, $x, $y);
+				$right = (int) floor(($rgb[0] * 0.299) + ($rgb[1] * 0.587) + ($rgb[2] * 0.114));
+
+				// Each hash bit is 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
+				$bits[] = (int) ($left > $right);
+
+				// Prepare the next loop.
+				$left = $right;
+			}
+		}
+
+		return Hash::fromBits($bits);
+	}
+
+	protected function pickColor($image, $x, $y)
+	{
+		$colorIndex = imagecolorsforindex($image, imagecolorat($image, $x, $y));
+
+		return [$colorIndex['red'], $colorIndex['green'], $colorIndex['blue']];
+	}
+}