Image Processing PHP

Ada yang bisa menjelaskan maksud dari source ini?

list($width, $height) = getimagesize($file);
	$source = imagecreatefromjpeg($file);
	$bwimg = imagecreate($width, $height);
	for ($c=0; $c < 256; $c++) {
		$palette[$c]=imagecolorallocate($bwimg, $c, $c, $c);
	}
	for ($y=0; $y < $height; $y++) {
		for ($x=0; $x < $width; $x++) {
			$rgb = imagecolorat($source, $x, $y);
			$r = ($rgb>>16)&0xFF;
			$g = ($rgb>>8)&0xFF;
			$b = $rgb&0xFF;

			$gs = yiq1($r,$g,$b);
			imagesetpixel($bwimg, $x, $y, $palette[$gs]);

		}
	}