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https://github.com/tesseract-ocr/tesseract.git
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0f9d507740
The last contribution from Google was in 2018
(see commit ce88adbf32).
Signed-off-by: Stefan Weil <sw@weilnetz.de>
134 lines
5.1 KiB
Plaintext
134 lines
5.1 KiB
Plaintext
UNICHARSET(5)
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=============
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:doctype: manpage
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NAME
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----
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unicharset - character properties file used by tesseract(1)
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DESCRIPTION
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-----------
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Tesseract's unicharset file contains information on each symbol
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(unichar) the Tesseract OCR engine is trained to recognize.
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A unicharset file (i.e. 'eng.unicharset') is distributed as part of a
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Tesseract language pack (i.e. 'eng.traineddata'). For information on
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extracting the unicharset file, see combine_tessdata(1).
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The first line of a unicharset file contains the number of unichars in
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the file. After this line, each subsequent line provides information for
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a single unichar. The first such line contains a placeholder reserved for
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the space character. Each unichar is referred to within Tesseract by its
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Unichar ID, which is the line number (minus 1) within the unicharset file.
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Therefore, space gets unichar 0.
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Each unichar line in the unicharset file (v2+) may have four space-separated fields:
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'character' 'properties' 'script' 'id'
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Starting with Tesseract v3.02, more information may be given for each unichar:
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'character' 'properties' 'glyph_metrics' 'script' 'other_case' 'direction' 'mirror' 'normed_form'
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Entries:
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'character':: The UTF-8 encoded string to be produced for this unichar.
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'properties':: An integer mask of character properties, one per bit.
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From least to most significant bit, these are: isalpha, islower, isupper,
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isdigit, ispunctuation.
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'glyph_metrics':: Ten comma-separated integers representing various standards
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for where this glyph is to be found within a baseline-normalized coordinate
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system where 128 is normalized to x-height.
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* min_bottom, max_bottom: the ranges where the bottom of the character can
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be found.
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* min_top, max_top: the ranges where the top of the character may be found.
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* min_width, max_width: horizontal width of the character.
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* min_bearing, max_bearing: how far from the usual start position does the
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leftmost part of the character begin.
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* min_advance, max_advance: how far from the printer's cell left do we
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advance to begin the next character.
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'script':: Name of the script (Latin, Common, Greek, Cyrillic, Han, null).
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'other_case':: The Unichar ID of the other case version of this character
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(upper or lower).
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'direction':: The Unicode BiDi direction of this character, as defined by
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ICU's enum UCharDirection. (0 = Left to Right, 1 = Right to Left,
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2 = European Number...)
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'mirror':: The Unichar ID of the BiDirectional mirror of this character.
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For example the mirror of open paren is close paren, but Latin Capital C
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has no mirror, so it remains a Latin Capital C.
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'normed_form':: The UTF-8 representation of a "normalized form" of this unichar
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for the purpose of blaming a module for errors given ground truth text.
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For instance, a left or right single quote may normalize to an ASCII quote.
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EXAMPLE (v2)
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------------
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..............
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; 10 Common 46
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b 3 Latin 59
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W 5 Latin 40
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7 8 Common 66
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= 0 Common 93
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..............
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";" is a punctuation character. Its properties are thus represented by the
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binary number 10000 (10 in hexadecimal).
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"b" is an alphabetic character and a lower case character. Its properties are
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thus represented by the binary number 00011 (3 in hexadecimal).
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"W" is an alphabetic character and an upper case character. Its properties are
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thus represented by the binary number 00101 (5 in hexadecimal).
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"7" is just a digit. Its properties are thus represented by the binary number
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01000 (8 in hexadecimal).
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"=" is not punctuation nor a digit nor an alphabetic character. Its properties
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are thus represented by the binary number 00000 (0 in hexadecimal).
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Japanese or Chinese alphabetic character properties are represented by the
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binary number 00001 (1 in hexadecimal): they are alphabetic, but neither
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upper nor lower case.
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EXAMPLE (v3.02)
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---------------
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..................................................................
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110
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NULL 0 NULL 0
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N 5 59,68,216,255,87,236,0,27,104,227 Latin 11 0 1 N
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Y 5 59,68,216,255,91,205,0,47,91,223 Latin 33 0 2 Y
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1 8 59,69,203,255,45,128,0,66,74,173 Common 3 2 3 1
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9 8 18,66,203,255,89,156,0,39,104,173 Common 4 2 4 9
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a 3 58,65,186,198,85,164,0,26,97,185 Latin 56 0 5 a
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. . .
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..................................................................
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CAVEATS
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-------
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Although the unicharset reader maintains the ability to read unicharsets
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of older formats and will assign default values to missing fields,
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the accuracy will be degraded.
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Further, most other data files are indexed by the unicharset file,
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so changing it without re-generating the others is likely to have dire
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consequences.
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HISTORY
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-------
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The unicharset format first appeared with Tesseract 2.00, which was the
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first version to support languages other than English. The unicharset file
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contained only the first two fields, and the "ispunctuation" property was
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absent (punctuation was regarded as "0", as "=" is in the above example.
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SEE ALSO
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--------
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tesseract(1), combine_tessdata(1), unicharset_extractor(1)
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<https://tesseract-ocr.github.io/tessdoc/Training-Tesseract.html>
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AUTHOR
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------
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The Tesseract OCR engine was written by Ray Smith and his research groups
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at Hewlett Packard (1985-1995) and Google (2006-2018).
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