Load HHA features from Gupta et al. ECCV14.
        See https://github.com/s-gupta/rcnn-depth/blob/master/rcnn/saveHHA.m

def load_hha(self, idx):
        """
        Load HHA features from Gupta et al. ECCV14.
        See https://github.com/s-gupta/rcnn-depth/blob/master/rcnn/saveHHA.m
        """
        im = Image.open('{}/data/hha/img_{}.png'.format(self.nyud_dir, idx))
        hha = np.array(im, dtype=np.float32)
        hha -= self.mean_hha
        hha = hha.transpose((2,0,1))
        return hha