Kohana_Image_GD with grayscale conversion

gd.php

<?php defined('SYSPATH') or die('No direct script access.');

class Image_GD extends Kohana_Image_GD {

	public function grayscale()
	{
		// Creating the Canvas 
		$bwimage = $this->_create($this->width, $this->height); 
		
		$this->_load_image();

		//Creates the 256 color palette		
		$palette = array();
		
		for ($c = 0; $c < 256; $c++)
		{
			$palette[$c] = imagecolorallocate($bwimage, $c, $c, $c);
		}
		
		//Reads the original colors pixel by pixel 
		for ($y = 0; $y < $this->height; $y++) 
		{
			for ($x = 0; $x < $this->width; $x++)
			{
				$rgb 	= imagecolorat($this->_image, $x, $y);
				$r 		= ($rgb >> 16) & 0xFF;
				$g 		= ($rgb >> 8) & 0xFF;
				$b 		= $rgb & 0xFF;

				// This is where we actually use yiq to modify our rgb values, and then convert them to our grayscale palette
				$gs = $this->yiq($r, $g, $b);
				
				imagesetpixel($bwimage, $x, $y, $palette[$gs]);
			}
		}
		
		imagecopy($this->_image, $bwimage, 0, 0, 0, 0, $this->width, $this->height);
		
		imagedestroy($this->_image);
		
		$this->_image = $bwimage;
		
		return $this;
	}

	protected function yiq($r, $g, $b) 
	{
		return (($r * 0.299) + ($g * 0.587) + ($b * 0.114));
	}

}