:returns: a gray-scale (h, w) uint8 image

def _current_state(self):
        """
        :returns: a gray-scale (h, w) uint8 image
        """
        ret = self._grab_raw_image()
        # max-pooled over the last screen
        ret = np.maximum(ret, self.last_raw_screen)
        if self.viz:
            if isinstance(self.viz, float):
                cv2.imshow(self.windowname, ret)
                cv2.waitKey(int(self.viz * 1000))
        ret = ret.astype('float32')
        # 0.299,0.587.0.114. same as rgb2y in torch/image
        ret = cv2.cvtColor(ret, cv2.COLOR_RGB2GRAY)[:, :]
        return ret.astype('uint8')