The ICDAR 2019 Competition on Post-OCR Text Correction dataset (zenodo record) contains text files in the following format:
[OCR_toInput] This is a cxample...
[OCR_aligned] This is a@ cxample...
[ GS_aligned] This is an example.@@
01234567890123The first line contains the ocr input text. The second line contains the aligned ocr and the third line contains the aligned gold standard. @ is the aligment character and # represents characters in the OCR that do not occur in the gold standard.
For working with this data, the first 14 characters have to be removed. We also remove leading and trailing whitespace.
remove_label_and_nl (line:str)
Task 1 of the competition is about finding tokens with OCR mistakes. In this context a token refers to a string between two whitespaces.
AlignedToken (ocr:str, gs:str, ocr_aligned:str, gs_aligned:str, start:int, len_ocr:int)
Dataclass for storing aligned tokens
tokenize_aligned (ocr_aligned:str, gs_aligned:str)
Get a list of AlignedTokens from the aligned OCR and GS strings
[AlignedToken(ocr='This', gs='This', ocr_aligned='This', gs_aligned='This', start=0, len_ocr=4),
AlignedToken(ocr='is', gs='is', ocr_aligned='is', gs_aligned='is', start=5, len_ocr=2),
AlignedToken(ocr='a', gs='an', ocr_aligned='a@', gs_aligned='an', start=8, len_ocr=1),
AlignedToken(ocr='cxample...', gs='example.', ocr_aligned='cxample...', gs_aligned='example.##', start=10, len_ocr=10)]The OCR text of an AlignedToken may still consist of multiple tokens. This is the case when the OCR text contains one or more spaces. To make sure the (sub)tokenization of a token is the same, no matter if it was not yet tokenized completely, another round of tokenization is added.
InputToken (ocr:str, gs:str, start:int, len_ocr:int, label:int)
Dataclass for the tokenization within AlignedTokens
leading_whitespace_offset (string:str)
Return the leading whitespace offset for an aligned ocr string
If an aligned ocr string contains leading whitespace, the offset needs to be added to the start index of the respective input tokens.
Args: string (str): aligned ocr string to determine the leading whitespace offset for
Returns: int: leading whitespace offset for input tokens
get_input_tokens (aligned_token:__main__.AlignedToken)
Tokenize an AlignedToken into subtokens and assign task 1 labels
t = AlignedToken("Major", "Major", "Major", "Major", 19, 5)
print(t)
for inp_tok in get_input_tokens(t):
print(inp_tok)AlignedToken(ocr='Major', gs='Major', ocr_aligned='Major', gs_aligned='Major', start=19, len_ocr=5)
InputToken(ocr='Major', gs='Major', start=19, len_ocr=5, label=0)t = AlignedToken("INEVR", "I NEVER", "I@NEV@R", "I NEVER", 0, 5)
print(t)
for inp_tok in get_input_tokens(t):
print(inp_tok)AlignedToken(ocr='INEVR', gs='I NEVER', ocr_aligned='I@NEV@R', gs_aligned='I NEVER', start=0, len_ocr=5)
InputToken(ocr='INEVR', gs='I NEVER', start=0, len_ocr=5, label=1)t = AlignedToken("Long ow.", "Longhow.", "Long ow.", "Longhow.", 24, 8)
print(t)
for inp_tok in get_input_tokens(t):
print(inp_tok)AlignedToken(ocr='Long ow.', gs='Longhow.', ocr_aligned='Long ow.', gs_aligned='Longhow.', start=24, len_ocr=8)
InputToken(ocr='Long', gs='Longhow.', start=24, len_ocr=4, label=1)
InputToken(ocr='ow.', gs='', start=29, len_ocr=3, label=2)Next, we need functions for processing a text in the ICDAR data format.
Text (ocr_text:str, tokens:list, input_tokens:list, score:float)
Dataclass for storing a text in the ICDAR data format
clean (string:str)
Remove alignment characters from a text
normalized_ed (ed:int, ocr:str, gs:str)
Returns the normalized editdistance
process_text (in_file:pathlib.Path)
Process a text from the ICDAR dataset
Extract AlignedTokens, InputTokens, and calculate normalized editdistance.
Processing the example text:
Text(ocr_text='This is a cxample...', tokens=[AlignedToken(ocr='This', gs='This', ocr_aligned='This', gs_aligned='This', start=0, len_ocr=4), AlignedToken(ocr='is', gs='is', ocr_aligned='is', gs_aligned='is', start=5, len_ocr=2), AlignedToken(ocr='a', gs='an', ocr_aligned='a@', gs_aligned='an', start=8, len_ocr=1), AlignedToken(ocr='cxample...', gs='example.', ocr_aligned='cxample...', gs_aligned='example.@@', start=10, len_ocr=10)], input_tokens=[InputToken(ocr='This', gs='This', start=0, len_ocr=4, label=0), InputToken(ocr='is', gs='is', start=5, len_ocr=2, label=0), InputToken(ocr='a', gs='an', start=8, len_ocr=1, label=1), InputToken(ocr='cxample...', gs='example.', start=10, len_ocr=10, label=1)], score=0.2)File structure of the ICDAR dataset
.
├── <data_dir>
│ ├── <language>
│ │ ├── <language (set)>1
│ │ ...
│ │ └── <language (set)>n
│ ...
...generate_data (in_dir:pathlib.Path)
Process all texts in the dataset and return a dataframe with metadata
get_intermediate_data (zip_file:str)
Get the data and metadata for the train and test sets from the zip file.
extract_icdar_data (out_dir:str, zip_file:str)
The following functions can be used to generate sequences of a certain length with possible overlap.
window (iterable, size=2)
Given an iterable, return all subsequences of a certain size
generate_sentences (df:pandas.core.frame.DataFrame, data:Dict[str,__main__.Text], size:int=15, step:int=10)
Generate sequences of a certain length and possible overlap
process_input_ocr (text:str)
Generate Text object for OCR input text (without aligned gold standard)
text = process_input_ocr("This is a cxample...")
data = {"ocr_input": text}
md = pd.DataFrame(
{
"language": ["?"],
"file_name": ["ocr_input"],
"score": [text.score],
"num_tokens": [len(text.tokens)],
"num_input_tokens": [len(text.input_tokens)],
}
)
df = generate_sentences(md, data, size=2, step=2)
assert 2 == df.shape[0]
assert [0, 2] == list(df.start_token_id)
df1it [00:00, 2198.27it/s]| key | start_token_id | score | tokens | tags | language | |
|---|---|---|---|---|---|---|
| 0 | ocr_input | 0 | 0.0 | [This, is] | [0, 0] | oc |
| 1 | ocr_input | 2 | 0.0 | [a, cxample...] | [0, 0] | oc |