public function verticalFlip() {
		if (function_exists('imageflip')) {
			imageflip($this->_image, IMG_FLIP_VERTICAL);
		} else {
			$image = imagecreatetruecolor($this->width, $this->height);
			$dst_x = 0;
			$src_x = 0;
			$coordinate = ($this->height - 1);
			foreach (range($this->height, 0) as $range) {
				$src_y = $range;
				$dst_y = $coordinate - $range;
				imagecopy($image, $this->_image, $dst_x, $dst_y, $src_x, $src_y, $this->width, 1);
			}
			imagedestroy($this->_image);
			$this->_image = $image;
		}
		return $this;
	}