unicharambigs - Man Page

The unicharambigs file (a component of traineddata, see combine_tessdata(1) ) is used by Tesseract to represent possible ambiguities between characters, or groups of characters.

The file contains a number of lines, laid out as follow:

[num] <TAB> [char(s)] <TAB> [num] <TAB> [char(s)] <TAB> [num]

Characters appearing in fields two and four should appear in unicharset. The numbers in fields one and three refer to the number of unichars (not bytes).

v1
2       ' '     1       "     1
1       m       2       r n   0
3       i i i   1       m     0

The first line is a version identifier. In this example, all instances of the 2 character sequence '' will always be replaced by the 1 character sequence "; a 1 character sequence m may be replaced by the 2 character sequence rn, and the 3 character sequence may be replaced by the 1 character sequence m.

Version 3.03 and on supports a new, simpler format for the unicharambigs file:

v2
'' " 1
m rn 0
iii m 0

In this format, the "error" and "correction" are simple UTF-8 strings separated by a space, and, after another space, the same type specifier as v1 (0 for optional and 1 for mandatory substitution). Note the downside of this simpler format is that Tesseract has to encode the UTF-8 strings into the components of the unicharset. In complex scripts, this encoding may be ambiguous. In this case, the encoding is chosen such as to use the least UTF-8 characters for each component, ie the shortest unicharset components will make up the encoding.

The unicharambigs file first appeared in Tesseract 3.00; prior to that, a similar format, called DangAmbigs (dangerous ambiguities) was used: the format was almost identical, except only mandatory replacements could be specified, and field 5 was absent.

This is a documentation "bug": it’s not currently clear what should be done in the case of ligatures (such as fi) which may also appear as regular letters in the unicharset.

tesseract(1), unicharset(5) https://tesseract-ocr.github.io/tessdoc/Training-Tesseract-3.03%E2%80%933.05.html#the-unicharambigs-file

The Tesseract OCR engine was written by Ray Smith and his research groups at Hewlett Packard (1985-1995) and Google (2006-present).

combine_tessdata(1), tesseract(1).