def prepare_roidb(imdb):
  """Enrich the imdb's roidb by adding some derived quantities that
  are useful for training. This function precomputes the maximum
  overlap, taken over ground-truth boxes, between each ROI and
  each ground-truth box. The class with maximum overlap is also
  recorded.
  """

  roidb = imdb.roidb
  if not (imdb.name.startswith('coco')):
    cache_file = os.path.join(imdb.cache_path, imdb.name + '_sizes.pkl')
    if os.path.exists(cache_file):
      print('Image sizes loaded from %s' % cache_file)
      with open(cache_file, 'rb') as f:
        sizes = pickle.load(f)
    else:
      print('Extracting image sizes... (It may take long time)')
      sizes = [PIL.Image.open(imdb.image_path_at(i)).size
                for i in range(imdb.num_images)]
      with open(cache_file, 'wb') as f:
        pickle.dump(sizes, f)
      print('Done!!')
         
  for i in range(len(imdb.image_index)):
    roidb[i]['img_id'] = imdb.image_id_at(i)
    roidb[i]['image'] = imdb.image_path_at(i)
    if not (imdb.name.startswith('coco')):
      roidb[i]['width'] = sizes[i][0]
      roidb[i]['height'] = sizes[i][1]
    # need gt_overlaps as a dense array for argmax
    gt_overlaps = roidb[i]['gt_overlaps'].toarray()
    # max overlap with gt over classes (columns)
    max_overlaps = gt_overlaps.max(axis=1)
    # gt class that had the max overlap
    max_classes = gt_overlaps.argmax(axis=1)
    roidb[i]['max_classes'] = max_classes
    roidb[i]['max_overlaps'] = max_overlaps
    # sanity checks
    # max overlap of 0 => class should be zero (background)
    zero_inds = np.where(max_overlaps == 0)[0]
    assert all(max_classes[zero_inds] == 0)
    # max overlap > 0 => class should not be zero (must be a fg class)
    nonzero_inds = np.where(max_overlaps > 0)[0]
    assert all(max_classes[nonzero_inds] != 0)
Enrich the imdb's roidb by adding some derived quantities that
  are useful for training. This function precomputes the maximum
  overlap, taken over ground-truth boxes, between each ROI and
  each ground-truth box. The class with maximum overlap is also
  recorded.
  
lib/roi_data_layer/roidb.py