What is image mask?

[vvjn]

Hi,
Can you tell me what the image mask (from path_to_train_img_mask and path_to_valid_img_mask below) is and what it is used for?
Thanks.

python train_mmod.py \
 -data demo \
 -save_model demo_modelname \
 -path_to_train_img_feats train-resnet50-res4frelu.npy \
 -path_to_valid_img_feats val-resnet50-res4frelu.npy \
 -path_to_train_attr train_obj.npy \
 -path_to_valid_attr val_obj.npy \
 -path_to_train_img_mask train_obj_mask.npy \
 -path_to_valid_img_mask val_obj_mask.npy \
 -encoder_type transformer -decoder_type transformer --multimodal_model_type dcap

[ShaoDonCui]

I ran the preprocess.py but got this error, why?
(base) C:\Users\cui\Desktop\my like code\DCCN-master>python preprocess.py -train_src train.lc.norm.tok.en -train_tgt train.lc.norm.tok.de -valid_src val.lc.norm.tok.en -valid_tgt val.lc.norm.tok.de -save_data demo
Extracting features...

• number of source features: 0.
• number of target features: 0.
  Building Fields object...
  Building & saving training data...
  Traceback (most recent call last):
  File "C:\Users\cui\Desktop\my like code\DCCN-master\onmt\io\TextDataset.py", line 369, in iter
  raise StopIteration
  StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "C:\Users\cui\Desktop\my like code\DCCN-master\preprocess.py", line 209, in
main()
File "C:\Users\cui\Desktop\my like code\DCCN-master\preprocess.py", line 194, in main
train_dataset_files = build_save_dataset('train', fields, opt)
File "C:\Users\cui\Desktop\my like code\DCCN-master\preprocess.py", line 128, in build_save_dataset
return build_save_text_dataset_in_shards(
File "C:\Users\cui\Desktop\my like code\DCCN-master\preprocess.py", line 96, in build_save_text_dataset_in_shards
dataset = onmt.io.TextDataset(
File "C:\Users\cui\Desktop\my like code\DCCN-master\onmt\io\TextDataset.py", line 76, in init
for ex_values in example_values:
File "C:\Users\cui\Desktop\my like code\DCCN-master\onmt\io\TextDataset.py", line 68, in
example_values = ([ex[k] for k in keys] for ex in examples_iter)
File "C:\Users\cui\Desktop\my like code\DCCN-master\onmt\io\TextDataset.py", line 55, in
examples_iter = (self._join_dicts(src, tgt) for src, tgt in zip(src_examples_iter, tgt_examples_iter))
RuntimeError: generator raised StopIteration

[ShaoDonCui]

can you help me? thank you.