code: def append_flipped_images(self):
          
          logger.info('%s append flipped images to roidb' % self._name)
          roidb_flipped = []
          for roi_rec in self._roidb:
              boxes = roi_rec['boxes'].copy()
              oldx1 = boxes[:, 0].copy()
              oldx2 = boxes[:, 2].copy()
              boxes[:, 0] = roi_rec['width'] - oldx2 - 1
              boxes[:, 2] = roi_rec['width'] - oldx1 - 1
              assert (boxes[:, 2] >= boxes[:, 0]).all()
              roi_rec_flipped = roi_rec.copy()
              roi_rec_flipped['boxes'] = boxes
              roi_rec_flipped['flipped'] = True
              roidb_flipped.append(roi_rec_flipped)
          self._roidb.extend(roidb_flipped)
docs: Only flip boxes coordinates, images will be flipped when loading into network