function crop($x1,$y1,$x2,$y2) {
		$tmp=imagecreatetruecolor($width=$x2-$x1+1,$height=$y2-$y1+1);
		imagesavealpha($tmp,TRUE);
		imagefill($tmp,0,0,IMG_COLOR_TRANSPARENT);
		imagecopyresampled($tmp,$this->data,
			0,0,$x1,$y1,$width,$height,$width,$height);
		imagedestroy($this->data);
		$this->data=$tmp;
		return $this->save();
	}