This manual is for liblouis (version 2.5.3, 21 June 2013), a Braille Translation and Back-Translation Library derived from the Linux screen reader BRLTTY.
Copyright © 1999-2006 by the BRLTTY Team.
Copyright © 2004-2007 ViewPlus Technologies, Inc. www.viewplus.com.
Copyright © 2007,2009 Abilitiessoft, Inc. www.abilitiessoft.com.
This file is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser (or library) General Public License (LGPL) as published by the Free Software Foundation; either version 3, or (at your option) any later version.
This file is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser (or Library) General Public License LGPL for more details.
You should have received a copy of the GNU Lesser (or Library) General Public License (LGPL) along with this program; see the file COPYING. If not, write to the Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
Liblouis is an open-source braille translator and back-translator derived from the translation routines in the BRLTTY screen reader for Linux. It has, however, gone far beyond these routines. It is named in honor of Louis Braille. In Linux and Mac OSX it is a shared library, and in Windows it is a DLL. For installation instructions see the README file. Please report bugs and oddities to the maintainer, john.boyer@abilitiessoft.com
This documentation is derived from Chapter 7 of the BRLTTY manual, but it has been extensively rewritten to cover new features.
Please read the following copyright and warranty information. Note that this information also applies to all source code, tables and other files in this distribution of liblouis. It applies similarly to the sister library liblouisxml.
This file is maintained by John J. Boyer john.boyer@abilitiessoft.com.
Persons who wish to program with liblouis but will not be writing translation tables may want to skip ahead to Programming with liblouis.
A number of test programs are provided as part of the liblouis
package. They are intended for testing liblouis and for debugging
tables. None of them is suitable for braille transcription. An
application that can be used for transcription is xml2brl,
which is part of the liblouisxml package (see Introduction in Liblouisxml User’s and Programmer’s Manual). The source
code of the test programs can be studied to learn how to use the
liblouis library and they can be used to perform the following
functions.
All of these programs recognize the --help and --version options.
Print a usage message listing all available options, then exit successfully.
Print the version number, then exit successfully.
The lou_debug tool is intended for debugging liblouis
translation tables. The command line for lou_debug is:
lou_debug [OPTIONS] TABLE[,TABLE,...]
The command line options that are accepted by lou_debug are
described in common options.
The table (or comma-separated list of tables) is compiled. If no errors are found a brief command summary is printed, then the prompt ‘Command:’. You can then input one of the command letters and get output, as described below.
Most of the commands print information in the various arrays of
TranslationTableHeader. Since these arrays are pointers to
chains of hashed items, the commands first print the hash number, then
the first item, then the next item chained to it, and so on. After
each item there is a prompt indicated by ‘=>’. You can then press
enter (RET) to see the next item in the chain or the first
item in the next chain. Or you can press h (for next-(h)ash) to
skip to the next hash chain. You can also press e to exit the
command and go back to the ‘command:’ prompt.
Brings up a screen of somewhat more extensive help.
Display the first forward-translation rule in the first non-empty hash
bucket. The number of the bucket is displayed at the beginning of the
chain. Each rule is identified by the word ‘Rule:’. The fields
are displayed by phrases consisting of the name of the field, an equal
sign, and its value. The before and after fields are displayed only if
they are nonzero. Special opcodes such as the correct opcode (see correct) and
the multipass opcodes are shown with the code that instructs the
virtual machine that interprets them. If you want to see only the
rules for a particular character string you can type p at the
‘command:’ prompt. This will take you to the ‘particular:’
prompt, where you can press f and then type in the string. The
whole hash chain containing the string will be displayed.
Display back-translation rules. This display is very similar to that of forward translation rules except that the dot pattern is displayed before the character string.
Display character definitions, again within their hash chains.
Displays single-cell dot definitions. If a character-definition opcode gives a multi-cell dot pattern, it is displayed among the back-translation rules.
Display the character-to-dots map. This is set up by the
character-definition opcodes and can also be influenced by the
display opcode (see display).
Display the dot to character map, which shows which single-cell dot patterns map to which characters.
Show the multi-cell dot patterns which have been assigned to the characters from 0 to 255 to comply with computer braille codes such as a 6-dot code. Note that the character-definition opcodes should use 8-dot computer braille.
Bring up a secondary (‘particular:’) prompt from which you can examine particular character strings, dot patterns, etc. The commands (given in its own command summary) are very similar to those of the main ‘command:’ prompt, but you can type a character string or dot pattern. They include h, f, b, c, d, C, D, z and x (to exit this prompt), but not p, i and m.
Show braille indicators. This shows the dot patterns for various
opcodes such as the capsign opcode (see capsign) and the numsign opcode (see numsign).
It also shows emphasis dot patterns, such as those for the
italword,
the firstletterbold opcode (see firstletterbold), etc. If a given
opcode has not been used nothing is printed for it.
Display various miscellaneous information about the table, such as the number of passes, whether certain opcodes have been used, and whether there is a hyphenation table.
Exit the program.
When working on translation tables it is sometimes useful to determine
what rules were applied when translating a string. lou_trace
helps with exactly that. It list all the the applied rules for a given
translation table and an input string.
lou_trace [OPTIONS] TABLE[,TABLE,...]
lou_trace accepts all the standard options (see common options). Once started you can type an input string followed by
RET. lou_trace will print the braille
translation followed by list of rules that were applied to produce the
translation. A possible invocation is listed in the following example:
$ lou_trace tables/en-us-g2.ctb the u.s. postal service ! u4s4 po/al s}vice 1. largesign the 2346 2. repeated 0 3. lowercase u 136 4. punctuation . 46 5. context _$l["."]$l @256 6. lowercase s 234 7. postpunc . 256 8. repeated 0 9. begword post 1234-135-34 10. largesign a 1 11. lowercase l 123 12. repeated 0 13. lowercase s 234 14. always er 12456 15. lowercase v 1236 16. lowercase i 24 17. lowercase c 14 18. lowercase e 15 19. pass2 $s1-10 @0 20. pass2 $s1-10 @0 21. pass2 $s1-10 @0
To use this program type the following:
lou_checktable [OPTIONS] TABLE
Aside from the standard options (see common options)
lou_checktable also accepts the following options:
Do not write to standard error if there are no errors.
If the table contains errors, appropriate messages will be displayed. If there are no errors the message ‘no errors found.’ will be shown.
This program tests every capability of the liblouis library. It is completely interactive. Invoke it as follows:
lou_allround [OPTIONS]
The command line options that are accepted by lou_allround
are described in common options.
You will see a few lines telling you how to use the program. Pressing one of the letters in parentheses and then enter will take you to a message asking for more information or for the answer to a yes/no question. Typing the letter ‘r’ and then RET will take you to a screen where you can enter a line to be processed by the library and then view the results.
This program translates whatever is on the standard input unit and
prints it on the standard output unit. It is intended for large-scale
testing of the accuracy of translation and back-translation. The
command line for lou_translate is:
lou_translate [OPTION] TABLE[,TABLE,...]
Aside from the standard options (see common options) this program also accepts the following options:
Do a forward translation.
Do a backward translation.
To use it to translate or back-translate a file use a line like
lou_translate --forward en-us-g2.ctb <liblouis.txt >testtrans
This program checks the accuracy of hyphenation in Braille translation for both translated and untranslated words. It is completely interactive. Invoke it as follows:
lou_checkhyphens [OPTIONS]
The command line options that are accepted by
lou_checkhyphens are described in common options.
You will see a few lines telling you how to use the program.
Many translation (contraction) tables have already been made up. They are included in this distribution in the tables directory and should be studied as part of the documentation. The most helpful (and normative) are listed in the following table:
Character definitions for U.S. tables
Remove excessive whitespace
Uncontracted American English
Contracted or Grade 2 American English
Make liblouis output conform to BRF standard
8-dot computer braille for use in coding examples
6-dot computer braille
Nemeth Code translation for use with liblouisxml
Fixes errors at the boundaries of math and text
The names used for files containing translation tables are completely arbitrary. They are not interpreted in any way by the translator. Contraction tables may be 8-bit ASCII files, UTF-8, 16-bit big-endian Unicode files or 16-bit little-endian Unicode files. Blank lines are ignored. Any leading and trailing whitespace (any number of blanks and/or tabs) is ignored. Lines which begin with a number sign or hatch mark (‘#’) are ignored, i.e. they are comments. If the number sign is not the first non-blank character in the line, it is treated as an ordinary character. If the first non-blank character is less-than (‘<’) the line is also treated as a comment. This makes it possible to mark up tables as xhtml documents. Lines which are not blank or comments define table entries. The general format of a table entry is:
opcode operands comments
Table entries may not be split between lines. The opcode is a mnemonic that specifies what the entry does. The operands may be character sequences, braille dot patterns or occasionally something else. They are described for each opcode, please see Opcode Index. With some exceptions, opcodes expect a certain number of operands. Any text on the line after the last operand is ignored, and may be a comment. A few opcodes accept a variable number of operands. In this case a number sign begins a comment unless it is preceded by a backslash (‘\’).
Here are some examples of table entries.
# This is a comment. always world 456-2456 A word and the dot pattern of its contraction
Most opcodes have both a "characters" operand and a "dots" operand, though some have only one and a few have other types.
The characters operand consists of any combination of characters and escape sequences proceeded and followed by whitespace. Escape sequences are used to represent difficult characters. They begin with a backslash (‘\‘). They are:
backslash
form feed
new line
carriage return
blank (space)
horizontal tab
vertical tab
"escape" character (hex 1b, dec 27)
4-digit hexadecimal value of a character
If liblouis has been compiled for 32-bit Unicode the following are also recognized.
5-digit (20 bit) character
Full 32-bit value.
The dots operand is a braille dot pattern. The real braille dots, 1 through 8, must be specified with their standard numbers. liblouis recognizes "virtual dots," which are used for special purposes, such as distinguishing accent marks. There are seven virtual dots. They are specified by the number 9 and the letters ‘a’ through ‘f’. For a multi-cell dot pattern, the cell specifications must be separated from one another by a dash (‘-’). For example, the contraction for the English word ‘lord’ (the letter ‘l’ preceded by dot 5) would be specified as 5-123. A space may be specified with the special dot number 0.
An opcode which is helpful in writing translation tables is
include. Its format is:
include filename
It reads the file indicated by filename and incorporates or
includes its entries into the table. Included files can include other
files, which can include other files, etc. For an example, see what
files are included by the entry include en-us-g1.ctb in the table
en-us-g2.ctb. If the included file is not in the same directory
as the main table, use a full path name for filename. Tables can also be
specified in a table list, in which the table names are separated by
commas and given as a single table name in calls to the translation
functions.
The order of the various types of opcodes or table entries is
important. Character-definition opcodes should come first. However, if
the optional display opcode (see display) is used it should precede
character-definition opcodes. Braille-indicator opcodes should come
next. Translation opcodes should follow. The context opcode (see context) is a
translation opcode, even though it is considered along with the
multipass opcodes. These latter should follow the translation opcodes.
The correct opcode (see correct) can be used anywhere after the
character-definition opcodes, but it is probably a good idea to group
all correct opcodes together. The include opcode (see include) can be
used anywhere, but the order of entries in the combined table must
conform to the order given above. Within each type of opcode, the
order of entries is generally unimportant. Thus the translation
entries can be grouped alphabetically or in any other order that is
convenient. Hyphenation tables may be specified either with an
include opcode or as part of a table list. They should come after
everything else. Character-definition opcodes are necessary for
hyphenation tables to work.
Hyphenation tables are necessary to make opcodes such as the
nocross opcode (see nocross) function properly. There are no opcodes for
hyphenation table entries because these tables have a special format.
Therefore, they cannot be specified as part of an ordinary table.
Rather, they must be included using the include opcode (see include) or as part
of a table list. The liblouis hyphenation algorithm was adopted from the
one used by OpenOffice. Note that Hyphenation tables must follow
character definitions and should preferably be the last. For an example
of a hyphenation table, see hyph_en_US.dic.
These opcodes are needed to define attributes such as digit,
punctuation, letter, etc. for all characters and their dot patterns.
liblouis has no built-in character definitions, but such definitions
are essential to the operation of the context opcode (see context), the
correct opcode (see correct), the multipass opcodes and the back-translator. If
the dot pattern is a single cell, it is used to define the mapping
between dot patterns and characters, unless a display opcode (see display) for
that character-dot-pattern pair has been used previously. If only a
single-cell dot pattern has been given for a character, that dot
pattern is defined with the character’s own attributes. If more than
one cell is given and some of them have not previously been defined as
single cells, the undefined cells are entered into the dots table with
the space attribute. This is done for backward compatibility with
old tables, but it may cause problems with the above opcodes or
back-translation. For this reason, every single-cell dot pattern
should be defined before it is used in a multi-cell character
representation. The best way to do this is to use the 8-dot computer
braille representation for the particular braille code. If a character
or dot pattern used in any rule, except those with the display
opcode, the repeated opcode (see repeated) or the replace opcode (see replace), is not
defined by one of the character-definition opcodes, liblouis will give
an error message and refuse to continue until the problem is fixed. If
the translator or back-translator encounters an undefined character in
its input it produces a succinct error indication in its output, and
the character is treated as a space.
space character dotsDefines a character as a space and also defines the dot pattern as such. for example:
space \s 0 \s is the escape sequence for blank; 0 means no dots.
punctuation character dotsAssociates a punctuation mark in the particular language with a braille representation and defines the character and dot pattern as punctuation. For example:
punctuation . 46 dot pattern for period in NAB computer braille
digit character dotsAssociates a digit with a dot pattern and defines the character as a digit. For example:
digit 0 356 NAB computer braille
uplow characters dots [,dots]The characters operand must be a pair of letters, of which the first is uppercase and the second lowercase. The first dots suboperand indicates the dot pattern for the upper-case letter. It may have more than one cell. The second dots suboperand must be separated from the first by a comma and is optional, as indicated by the square brackets. If present, it indicates the dot pattern for the lower-case letter. It may also have more than one cell. If the second dots suboperand is not present the first is used for the lower-case letter as well as the upper-case letter. This opcode is needed because not all languages follow a consistent pattern in assigning Unicode codes to upper and lower case letters. It should be used even for languages that do. The distinction is important in the forward translator. for example:
uplow Aa 17,1
grouping name characters dots ,dotsThis opcode is used to indicate pairs of grouping symbols used in
processing mathematical expressions. These symbols are usually
generated by the MathML interpreter in liblouisxml. They are used in
multipass opcodes. The name operand must contain only letters, but
they may be upper- or lower-case. The characters operand must contain
exactly two Unicode characters. The dots operand must contain exactly
two braille cells, separated by a comma. Note that grouping dot
patterns also need to be declared with the exactdots opcode (see exactdots). The
characters may need to be declared with the math opcode (see math).
grouping mrow \x0001\x0002 1e,2e grouping mfrac \x0003\x0004 3e,4e
letter character dotsAssociates a letter in the language with a braille representation and defines the character as a letter. This is intended for letters which are neither uppercase nor lowercase.
lowercase character dotsAssociates a character with a dot pattern and defines the character as a lowercase letter. Both the character and the dot pattern have the attributes lowercase and letter.
uppercase character dotsAssociates a character with a dot pattern and defines the character as
an uppercase letter. Both the character and the dot pattern have the
attributes uppercase and letter. lowercase and uppercase
should be used when a letter has only one case. Otherwise use the
uplow opcode (see uplow).
litdigit digit dotsAssociates a digit with the dot pattern which should be used to represent it in literary texts. For example:
litdigit 0 245 litdigit 1 1
sign character dotsAssociates a character with a dot pattern and defines both as a sign. This opcode should be used for things like at sign (‘@’), percent (‘%’), dollar sign (‘$’), etc. Do not use it to define ordinary punctuation such as period and comma. For example:
sign % 4-25-1234 literary percent sign
math character dotsAssociates a character and a dot pattern and defines them as a mathematical symbol. It should be used for less than (‘<’), greater than(‘>’), equals(‘=’), plus(‘+’), etc. For example:
math + 346 plus
Braille indicators are dot patterns which are inserted into the braille text to indicate such things as capitalization, italic type, computer braille, etc. The opcodes which define them are followed only by a dot pattern, which may be one or more cells.
capsign dotsThe dot pattern which indicates capitalization of a single letter. In English, this is dot 6. For example:
capsign 6
begcaps dotsThe dot pattern which begins a block of capital letters. For example:
begcaps 6-6
endcaps dotsThe dot pattern which ends a block of capital letters within a word. For example:
endcaps 6-3
letsign dotsThis indicator is needed in Grade 2 to show that a single letter is not a contraction. It is also used when an abbreviation happens to be a sequence of letters that is the same as a contraction. For example:
letsign 56
noletsign lettersThe letters in the operand will not be proceeded by a letter sign.
More than one noletsign opcode can be used. This is equivalent
to a single entry containing all the letters. In addition, if a single
letter, such as ‘a’ in English, is defined as a word
(see word) or largesign
(see largesign), it will be
treated as though it had also been specified in a noletsign
entry.
noletsignbefore charactersIf any of the characters proceeds a single letter without a space a
letter sign is not used. By default the characters apostrophe
(‘'’) and period (‘.’) have this property. Use of a
noletsignbefore entry cancels the defaults. If more than one
noletsignbefore entry is used, the characters in all entries
are combined.
noletsignafter charactersIf any of the characters follows a single letter without a space a
letter sign is not used. By default the characters apostrophe
(‘'’) and period (‘.’) have this property. Use of a
noletsignafter entry cancels the defaults. If more than one
noletsignafter entry is used the characters in all entries are
combined.
numsign dotsThe translator inserts this indicator before numbers made up of digits
defined with the litdigit opcode (see litdigit) to show that they are a number
and not letters or some other symbols. For example:
numsign 3456
These also define braille indicators, but they require more explanation. There are four sets, for italic, bold, underline and computer braille. In each of the first three sets there are seven opcodes, for use before the first word of a phrase, for use before the last word, for use after the last word, for use before the first letter (or character) if emphasis starts in the middle of a word, for use after the last letter (or character) if emphasis ends in the middle of a word, before a single letter (or character), and to specify the length of a phrase to which the first-word and last-word-before indicators apply. This rather elaborate set of emphasis opcodes was devised to try to meet all contingencies. It is unlikely that a translation table will contain all of them. The translator checks for their presence. If they are present, it first looks to see if the single-letter indicator should be used. Then it looks at the word (or phrase) indicators and finally at the multi-letter indicators.
The translator will apply up to two emphasis indicators to each phrase
or string of characters, depending on what the typeform
parameter in its calling sequence indicates (see Programming with liblouis).
For computer braille there are only two braille indicators, for the
beginning and end of a sequence of characters to be rendered in
computer braille. Such a sequence may also have other emphasis. The
computer braille indicators are applied not only when computer braille
is indicated in the typeform parameter, but also when a
sequence of characters is determined to be computer braille because it
contains a subsequence defined by the compbrl opcode (see compbrl) or the
literal opcode (see literal).
Here are the various emphasis opcodes.
firstwordital dotsThis is the braille indicator to be placed before the first word of an
italicized phrase that is longer than the value given in the
lenitalphrase opcode (see lenitalphrase). For example:
firstwordital 46-46 English indicator
lastworditalbefore dotsThis is the braille indicator to be placed before the last word of an
italicized phrase. In addition, if firstwordital is not used,
this braille indicator is doubled and placed before the first word. Do
not use lastworditalbefore and lastworditalafter in the
same table. For example:
lastworditalbefore 4-6
lastworditalafter dotsThis is the braille indicator to be placed after the last word of an
italicized phrase. Do not use lastworditalbefore and
lastworditalafter in the same table. See also the
lenitalphrase opcode (see lenitalphrase) for more information.
firstletterital dotsThis is the braille indicator to be placed before the first letter (or character) if italicization begins in the middle of a word.
lastletterital dotsThis is the braille indicator to be placed after the last letter (or character) when italicization ends in the middle of a word.
singleletterital dotsThis braille indicator is used if only a single letter (or character) is italicized.
lenitalphrase numberIf lastworditalbefore is used, an italicized phrase is checked
to see how many words it contains. If this number is less than or
equal to the number given in the lenitalphrase opcode, the
lastworditalbefore sign is placed in front of each word. If it
is greater, the firstwordital indicator is placed before the
first word and the lastworditalbefore indicator is placed after
the last word. Note that if the firstwordital opcode is not
used its indicator is made up by doubling the dot pattern given in the
lastworditalbefore entry. For example:
lenitalphrase 4
firstwordbold dotsThis is the braille indicator to be placed before the first word of a bold phrase. For example:
firstwordbold 456-456
lastwordboldbefore dotsThis is the braille indicator to be placed before the last word of a
bold phrase. In addition, if firstwordbold is not used, this
braille indicator is doubled and placed before the first word. Do not
use lastwordboldbefore and lastwordboldafter in the same
table. For example:
lastwordboldbefore 456
lastwordboldafter dotsThis is the braille indicator to be placed after the last word of a
bold phrase. Do not use lastwordboldbefore and
lastwordboldafter in the same table.
firstletterbold dotsThis is the braille indicator to be placed before the first letter (or character) if bold emphasis begins in the middle of a word.
lastletterbold dotsThis is the braille indicator to be placed after the last letter (or character) when bold emphasis ends in the middle of a word.
singleletterbold dotsThis braille indicator is used if only a single letter (or character) is in boldface.
lenboldphrase numberIf lastwordboldbefore is used, a bold phrase is checked to see
how many words it contains. If this number is less than or equal to
the number given in the lenboldphrase opcode, the
lastwordboldbefore sign is placed in front of each word. If it
is greater, the firstwordbold indicator is placed before the
first word and the lastwordboldbefore indicator is placed after
the last word. Note that if the firstwordbold opcode is not
used its indicator is made up by doubling the dot pattern given in the
lastwordboldbefore entry.
firstwordunder dotsThis is the braille indicator to be placed before the first word of an underlined phrase.
lastwordunderbefore dotsThis is the braille indicator to be placed before the last word of an
underlined phrase. In addition, if firstwordunder is not used,
this braille indicator is doubled and placed before the first word.
lastwordunderafter dotsThis is the braille indicator to be placed after the last word of an underlined phrase.
firstletterunder dotsThis is the braille indicator to be placed before the first letter (or character) if underline emphasis begins in the middle of a word.
lastletterunder dotsThis is the braille indicator to be placed after the last letter (or character) when underline emphasis ends in the middle of a word.
singleletterunder dotsThis braille indicator is used if only a single letter (or character) is underlined.
lenunderphrase numberIf lastwordunderbefore is used, an underlined phrase is checked
to see how many words it contains. If this number is less than or
equal to the number given in the lenunderphrase opcode, the
lastwordunderbefore sign is placed in front of each word. If it
is greater, the firstwordunder indicator is placed before the
first word and the lastwordunderbefore indicator is placed
after the last word. Note that if the firstwordunder opcode is
not used its indicator is made up by doubling the dot pattern given in
the lastwordunderbefore entry.
begcomp dotsThis braille indicator is placed before a sequence of characters
translated in computer braille, whether this sequence is indicated in
the typeform parameter (see Programming with liblouis) or
inferred because it contains a subsequence specified by the
compbrl opcode (see compbrl).
endcomp dotsThis braille indicator is placed after a sequence of characters
translated in computer braille, whether this sequence is indicated in
the typeform parameter (see Programming with liblouis) or
inferred because it contains a subsequence specified by the
compbrl opcode (see compbrl).
These opcodes define certain symbols, such as the decimal point, which require special treatment.
decpoint character dotsThis opcode defines the decimal point. The character operand must have only one character. For example, in en-us-g1.ctb we have:
decpoint . 46
hyphen character dotsThis opcode defines the hyphen, that is, the character used in compound words such as have-nots. The back-translator uses it to determine the end of individual words.
These opcodes cause special processing to be carried out.
capsnocontThis opcode has no operands. If it is specified, words or parts of words in all caps are not contracted. This is needed for languages such as Norwegian.
These opcodes define the braille representations for character sequences. Each of them defines an entry within the contraction table. These entries may be defined in any order except, as noted below, when they define alternate representations for the same character sequence.
Each of these opcodes specifies a condition under which the translation is legal, and each also has a characters operand and a dots operand. The text being translated is processed strictly from left to right, character by character, with the most eligible entry for each position being used. If there is more than one eligible entry for a given position in the text, then the one with the longest character string is used. If there is more than one eligible entry for the same character string, then the one defined first is is tested for legality first. (This is the only case in which the order of the entries makes a difference.)
The characters operand is a sequence or string of characters preceded and followed by whitespace. Each character can be entered in the normal way, or it can be defined as a four-digit hexadecimal number preceded by ‘\x’.
The dots operand defines the braille representation for the characters operand. It may also be specified as an equals sign (‘=’). This means that the the default representation for each character (see Character-Definition Opcodes) within the sequence is to be used.
In what follows the word ‘characters’ means a sequence of one or more consecutive letters between spaces and/or punctuation marks.
noback opcode ...This is an opcode prefix, that is to say, it modifies the operation of the opcode that follows it on the same line. noback specifies that no back-translation is to be done using this line.
noback always ;\s; 0
nofor opcode ...This is an opcode prefix which modifies the operation of the opcode following it on the same line. nofor specifies that forward translation is not to use the information on this line.
compbrl charactersIf the characters are found within a block of text surrounded by
whitespace the entire block is translated according to the default
braille representations defined by the Character-Definition Opcodes, if 8-dot computer braille is enabled or according to the dot
patterns given in the comp6 opcode (see comp6), if 6-dot computer braille is
enabled. For example:
compbrl www translate URLs in computer braille
comp6 character dotsThis opcode specifies the translation of characters in 6-dot computer braille. It is necessary because the translation of a single character may require more than one cell. The first operand must be a character with a decimal representation from 0 to 255 inclusive. The second operand may specify as many cells as necessary. The opcode is somewhat of a misnomer, since any dots, not just dots 1 through 6, can be specified. This even includes virtual dots.
nocont charactersLike compbrl, except that the string is uncontracted.
prepunc opcode (see prepunc) and postpunc opcode (see postpunc) rules are applied,
however. This is useful for specifying that foreign words should not
be contracted in an entire document.
replace characters {characters}Replace the first set of characters, no matter where they appear, with
the second. Note that the second operand is NOT a dot pattern.
It is also optional. If it is omitted the character(s) in the first
operand will be discarded. This is useful for ignoring characters. It
is possible that the "ignored" characters may still affect the
translation indirectly. Therefore, it is preferable to use
correct opcode (see correct).
always characters dotsReplace the characters with the dot pattern no matter where they
appear. Do NOT use an entry such as always a 1. Use the
uplow, letter, etc. character definition opcodes
instead. For example:
always world 456-2456 unconditional translation
repeated characters dotsReplace the characters with the dot pattern no matter where they appear. Ignore any consecutive repetitions of the same character sequence. This is useful for shortening long strings of spaces or hyphens or periods. For example:
repeated --- 36-36-36 shorten separator lines made with hyphens
repword characters dotsWhen characters are encountered check to see if the word before this string matches the word after it. If so, replace characters with dots and eliminate the second word and any word following another occurrence of characters that is the same. This opcode is used in Malaysian braille. In this case the rule is:
repword - 123456
largesign characters dotsReplace the characters with the dot pattern no matter where they
appear. In addition, if two words defined as large signs follow each
other, remove the space between them. For example, in
en-us-g2.ctb the words ‘and’ and ‘the’ are both
defined as large signs. Thus, in the phrase ‘the cat and the dog’
the space would be deleted between ‘and’ and ‘the’, with the
result ‘the cat andthe dog’. Of course, ‘and’ and ‘the’
would be properly contracted. The term largesign is a bit of
braille jargon that pleases braille experts.
word characters dotsReplace the characters with the dot pattern if they are a word, that is, are surrounded by whitespace and/or punctuation.
syllable characters dotsAs its name indicates, this opcode defines a "syllable" which must be represented by exactly the dot patterns given. Contractions may not cross the boundaries of this "syllable" either from left or right. The character string defined by this opcode need not be a lexical syllable, though it usually will be. The equal sign in the following example means that the the default representation for each character within the sequence is to be used (see Translation Opcodes):
syllable horse = sawhorse, horseradish
nocross characters dotsReplace the characters with the dot pattern if the characters are all
in one syllable (do not cross a syllable boundary). For this opcode to
work, a hyphenation table must be included. If this is not done,
nocross behaves like the always opcode (see always). For example, if
the English Grade 2 table is being used and the appropriate
hyphenation table has been included nocross sh 146 will cause
the ‘sh’ in ‘monkshood’ not to be contracted.
joinword characters dotsReplace the characters with the dot pattern if they are a word which
is followed by whitespace and a letter. In addition remove the
whitespace. For example, en-us-g2.ctb has joinword to
235. This means that if the word ‘to’ is followed by another
word the contraction is to be used and the space is to be omitted. If
these conditions are not met, the word is translated according to any
other opcodes that may apply to it.
lowword characters dotsReplace the characters with the dot pattern if they are a word
preceded and followed by whitespace. No punctuation either before or
after the word is allowed. The term lowword derives from the
fact that in English these contractions are written in the lower part
of the cell. For example:
lowword were 2356
contraction charactersIf you look at en-us-g2.ctb you will see that some words are
actually contracted into some of their own letters. A famous example
among braille transcribers is ‘also’, which is contracted as
‘al’. But this is also the name of a person. To take another
example, ‘altogether’ is contracted as ‘alt’, but this is
the abbreviation for the alternate key on a computer keyboard.
Similarly ‘could’ is contracted into ‘cd’, but this is the
abbreviation for compact disk. To prevent confusion in such cases, the
letter sign (see letsign opcode (see letsign)) is placed before such letter
combinations when they actually are abbreviations, not contractions.
The contraction opcode tells the translator to do this.
sufword characters dotsReplace the characters with the dot pattern if they are either a word or at the beginning of a word.
prfword characters dotsReplace the characters with the dot pattern if they are either a word or at the end of a word.
begword characters dotsReplace the characters with the dot pattern if they are at the beginning of a word.
begmidword characters dotsReplace the characters with the dot pattern if they are either at the beginning or in the middle of a word.
midword characters dotsReplace the characters with the dot pattern if they are in the middle of a word.
midendword characters dotsReplace the characters with the dot pattern if they are either in the middle or at the end of a word.
endword characters dotsReplace the characters with the dot pattern if they are at the end of a word.
partword characters dotsReplace the characters with the dot pattern if the characters are anywhere in a word, that is, if they are proceeded or followed by a letter.
exactdots @dotsNote that the operand must begin with an at sign (‘@’). The dot pattern following it is evaluated for validity. If it is valid, whenever an at sign followed by this dot pattern appears in the source document it is replaced by the characters corresponding to the dot pattern in the output. This opcode is intended for use in liblouisxml semantic-action files to specify exact dot patterns, as in mathematical codes. For example:
exactdots @4-46-12356
will produce the characters with these dot patterns in the output.
prepunc characters dotsReplace the characters with the dot pattern if they are part of punctuation at the beginning of a word.
postpunc characters dotsReplace the characters with the dot pattern if they are part of punctuation at the end of a word.
begnum characters dotsReplace the characters with the dot pattern if they are at the
beginning of a number, that is, before all its digits. For example, in
en-us-g1.ctb we have begnum # 4.
midnum characters dotsReplace the characters with the dot pattern if they are in the middle
of a number. For example, en-us-g1.ctb has midnum . 46.
This is because the decimal point has a different dot pattern than the
period.
endnum characters dotsReplace the characters with the dot pattern if they are at the end of
a number. For example en-us-g1.ctb has endnum th 1456.
This handles things like ‘4th’. A letter sign is NOT
inserted.
joinnum characters dotsReplace the characters with the dot pattern. In addition, if whitespace and a number follows omit the whitespace. This opcode can be used to join currency symbols to numbers for example:
joinnum \x20AC 15 (EURO SIGN) joinnum \x0024 145 (DOLLAR SIGN) joinnum \x00A3 1234 (POUND SIGN) joinnum \x00A5 13456 (YEN SIGN)
These opcodes define and use character classes. A character class associates a set of characters with a name. The name then refers to any character within the class. A character may belong to more than one class.
The basic character classes correspond to the character definition
opcodes, with the exception of the uplow opcode (see uplow), which defines
characters belonging to the two classes uppercase and
lowercase. These classes are:
spaceWhitespace characters such as blank and tab
digitNumeric characters
letterBoth uppercase and lowercase alphabetic characters
lowercaseLowercase alphabetic characters
uppercaseUppercase alphabetic characters
punctuationPunctuation marks
signSigns such as percent (‘%’)
mathMathematical symbols
litdigitLiterary digit
undefinedNot properly defined
The opcodes which define and use character classes are shown below. For examples see fr-abrege.ctb.
class name charactersDefine a new character class. The characters operand must be specified as a string. A character class may not be used until it has been defined.
after class opcode ...The specified opcode is further constrained in that the matched character sequence must be immediately preceded by a character belonging to the specified class. If this opcode is used more than once on the same line then the union of the characters in all the classes is used.
before class opcode ...The specified opcode is further constrained in that the matched character sequence must be immediately followed by a character belonging to the specified class. If this opcode is used more than once on the same line then the union of the characters in all the classes is used.
The swap opcodes are needed to tell the context opcode (see context), the
correct opcode (see correct) and multipass opcodes which dot patterns to swap
for which characters. There are three, swapcd, swapdd
and swapcc. The first swaps dot patterns for characters. The
second swaps dot patterns for dot patterns and the third swaps
characters for characters. The first is used in the context
opcode and the second is used in the multipass opcodes. Dot patterns
are separated by commas and may contain more than one cell.
swapcd name characters dots, dots, dots, ...See above paragraph for explanation. For example:
swapcd dropped 0123456789 356,2,23,...
swapdd name dots, dots, dots ... dotpattern1, dotpattern2, dotpattern3, ...The swapdd opcode defines substitutions for the multipass
opcodes. In the second operand the dot patterns must be single cells,
but in the third operand multi-cell dot patterns are allowed. This is
because multi-cell patterns in the second operand would lead to
ambiguities.
swapcc name characters charactersThe swapcc opcode swaps characters in its second operand for
characters in the corresponding places in its third operand. It is
intended for use with correct opcodes and can solve problems
such as formatting phone numbers.
The context and multipass opcodes (pass2, pass3
and pass4) provide translation capabilities beyond those of the
basic translation opcodes (see Translation Opcodes) discussed
previously. The multipass opcodes cause additional passes to be made
over the string to be translated. The number after the word
pass indicates in which pass the entry is to be applied. If no
multipass opcodes are given, only the first translation pass is made.
The context opcode is basically a multipass opcode for the
first pass. It differs slightly from the multipass opcodes per se. The
format of all these opcodes is opcode test action. The specific
opcodes are invoked as follows:
context test actionpass2 test actionpass3 test actionpass4 test actionThe test and action operands have suboperands. Each
suboperand begins with a non-alphanumeric character and ends when
another non-alphanumeric character is encountered. The suboperands and
their initial characters are as follows.
a string of characters. This string must be terminated by another
double quote. It may contain any characters. If a double quote is
needed within the string, it must be preceded by a backslash
(‘\’). If a space is needed, it must be represented by the escape
sequence \s. This suboperand is valid only in the test part of the
context opcode.
a sequence of dot patterns. Cells are separated by hyphens as usual. This suboperand is not valid in the test part of the context and correct opcodes.
If this is the beginning of the string being translated this suboperand is true. It is valid only in the test part and must be the first thing in this operand.
If this is the end of the string being translated this suboperand is true. It is valid only in the test part and must be the last thing in this operand.
a string of attributes, such as ‘d’ for digit, ‘l’ for letter, etc. More than one attribute can be given. If you wish to check characters with any attribute, use the letter ‘a’. Input characters are checked to see if they have at least one of the attributes. The attribute string can be followed by numbers specifying how many characters are to be checked. If no numbers are given, 1 is assumed. If two numbers separated by a hyphen are given, the input is checked to make sure that at least the first number of characters with the attributes are present, but no more than the second number. If only one number is present, then exactly that many characters must have the attributes. A period instead of the numbers indicates an indefinite number of characters (for technical reasons the number of characters that are actually matched is limited to 65535).
This suboperand is valid in all test parts but not in action parts. For the characters which can be used in attribute strings, see the following table.
reverses the logical meaning of the suboperand which follows. For example, !$d is true only if the character is NOT a digit. This suboperand is valid in test parts only.
the name of a class defined by the class opcode (see class) or the name of a
swap set defined by the swap opcodes (see Swap Opcodes). Names may
contain only letters. The letters may be upper or
lower-case. The case matters. Class names may be used in test parts
only. Swap names are valid everywhere.
Name: the name of a grouping pair. The left brace indicates that the first (or left) member of the pair is to be used in matching. If this is between replacement brackets it must be the only item. This is also valid in the action part.
Name: the name of a grouping pair. The right brace indicates that the second (or right) member is to be used in matching. See the remarks on the left brace immediately above.
Search the input for the expression following the slash and return true if found. This can be used to set a variable.
Move backward. If a number follows, move backward that number of characters. The program never moves backward beyond the beginning of the input string. This suboperand is valid only in test parts.
start replacement here. This suboperand must always be paired with a right bracket and is valid only in test parts. Multiple pairs of square brackets in a single expression are not allowed.
end replacement here. This suboperand must always be paired with a left bracket and is valid only in test parts.
test or set a variable. Variables are referred to by numbers 1 to 50,
for example, #1, #2, #25. Variables may be set by
one context or multipass opcode and tested by another. Thus, an
operation that occurs at one place in a translation can tell an
operation that occurs later about itself. This feature will be used in
math translation, and it may also help to alleviate the need for new
opcodes. This suboperand is valid everywhere.
Variables are set in the action part. To set a variable use an
expression like #1=1, #2=5, etc. Variables are also
incremented and decremented in the action part with expressions like
#1+, #3-, etc. These operators increment or decrement
the variable by 1.
Variables are tested in the test part with expressions like
#1=2, #3<4, #5>6, etc.
Copy the characters or dot patterns in the input within the replacement brackets into the output and discard anything else that may match. This feature is used, for example, for handling numeric subscripts in Nemeth. This suboperand is valid only in action parts.
Valid only in the action part. The characters to be replaced are simply ignored. That is, they are replaced with nothing. If either member of a grouping pair is in the replace brackets the other member at the same level is also removed.
The characters which can be used in attribute strings are as follows:
any attribute
digit
literary digit
letter
math
punctuation
sign
space
uppercase
lowercase
first user-defined class
second user-defined class
third user-defined class
fourth user-defined class
The following illustrates the algorithm how text is evaluated with multipass expressions:
Loop over context, pass2, pass3 and pass4 and do the following for each pass:
correct test actionBecause some input (such as that from an OCR program) may contain
systematic errors, it is sometimes advantageous to use a
pre-translation pass to remove them. The errors and their corrections
are specified by the correct opcode. If there are no
correct opcodes in a table, the pre-translation pass is not
used. The format of the correct opcode is very similar to that
of the context opcode (see context). The only difference is that in the action
part strings may be used and dot patterns may not be used. Some
examples of correct opcode entries are:
correct "\\" ? Eliminate backslashes correct "cornf" "comf" fix a common "scano" correct "cornm" "comm" correct "cornp" "comp" correct "*" ? Get rid of stray asterisks correct "|" ? ditto for vertical bars correct "\s?" "?" drop space before question mark
include filenameRead the file indicated by filename and incorporate or include
its entries into the table. Included files can include other files,
which can include other files, etc. For an example, see what files are
included by the entry include en-us-g1.ctb in the table
en-us-g2.ctb. If the included file is not in the same directory
as the main table, use a full path name for filename.
locale charactersNot implemented, but recognized and ignored for backward compatibility.
undefined dotsIf this opcode is used in a table any characters which have not been
defined in the table but are encountered in the text will be replaced by
the dot pattern. If this opcode is not used, any undefined characters
are replaced by '\xhhhh', where the h’s are hexadecimal digits.
display character dotsAssociates dot patterns with the characters which will be sent to a braille embosser, display or screen font. The character must be in the range 0-255 and the dots must specify a single cell. Here are some examples:
# When the character a is sent to the embosser or display, # it will produce a dot 1. display a 1
# When the character L is sent to the display or embosser # it will produce dots 1-2-3. display L 123
The display opcode is optional. It is used when the embosser or
display has a different mapping of characters to dot patterns than
that given in Character-Definition Opcodes. If used, display
entries must proceed character-definition entries.
A possible use case would be to define display opcodes so that the result is Unicode braille for use on a display and a second set of display opcodes (in a different file) to produce plain ASCII braille for use with an embosser.
multind dots opcode opcode ...The multind opcode tells the back-translator that a sequence of
braille cells represents more than one braille indicator. For example,
in en-us-g1.ctb we have multind 56-6 letsign capsign.
The back-translator can generally handle single braille indicators,
but it cannot apply them when they immediately follow each other. It
recognizes the letter sign if it is followed by a letter and takes
appropriate action. It also recognizes the capital sign if it is
followed by a letter. But when there is a letter sign followed by a
capital sign it fails to recognize the letter sign unless the sequence
has been defined with multind. A multind entry may not
contain a comment because liblouis would attempt to interpret it as an
opcode.
The following opcodes are an early attempt to handle emphasis. They have been deprecated by more specific opcodes, but are kept for backward compatibility.
italsign dotsThis opcode is deprecated. Use the lastworditalbefore opcode (see lastworditalbefore) instead.
begital dotsThis opcode is deprecated. Use the firstletterital opcode (see firstletterital) instead.
endital dotsThis opcode is deprecated. Use the lastletterital opcode (see lastletterital) instead.
boldsign dotsThis opcode is deprecated. Use the lastwordboldbefore opcode (see lastwordboldbefore) instead.
begbold dotsThis opcode is deprecated. Use the firstletterbold opcode (see firstletterbold) instead.
endbold dotsThis opcode is deprecated. Use the lastletterbold opcode (see lastletterbold) instead.
undersign dotsThis opcode is deprecated. Use the lastwordunderbefore opcode (see lastwordunderbefore) instead.
begunder dotsThis opcode is deprecated. Use the firstletterunder opcode (see firstletterunder) instead.
endunder dotsThis opcode is deprecated. Use the lastletterunder opcode (see lastletterunder) instead.
literal charactersThis opcode is deprecated. Use the compbrl opcode (see compbrl) instead.
There are a number of automated tests for liblouis and they are proving to be of tremendous value. When changing the code the developers can run the tests to see if anything broke.
For testing the translation tables there are basically two approaches: there are the harness tests and the doctests. They were created at roughly the same time using different technologies, have influenced each other and have gone through improvements and technology changes. For now they are both based on Python so you need to have that installed. The philosophies of the two are slightly different:
The harness tests are data driven, i.e. you give the test data, i.e. a string to translate and the expected output. The data is in a standard format, i.e. json. They work with both Python2 and Python3, however since the format is json it is perceivable that somebody would write some C code which takes the data in the harness file and runs it through liblouis so they could also run without Python and without ucs4.
The doctests on the other hand are based on a technology used in Python where you define your tests as if you were sitting at a terminal session with a Python interpreter. So the tests look like you typed a command and got some output, e.g.
>>> translate(['table.ctb'], "Hello", mode=compbrlLeftCursor)
("HELLO", [0,1,2,3], [0,1,2,3], 0)
There is a convenience wrapper which hides away much of the complexity of above example so you can write stuff like
>>> t.braille('the cat sat on the mat')
u'! cat sat on ! mat'
But essentially you are writing code, so the doctests allow you to do more flexible tests that are much closer to the raw iron. For technical reasons the doctests will probably only ever work in either Python2 or Python3 but not both and they will never run from C.
To sum it up, the recommendation is that for normal table testing you should use the test harness. It has a lot of momentum and the format is a standard. If you want to be closer to the raw Python API of liblouis, if you want to test some more intricate scenarios (involving inpos, modes, etc) then the doctests are for you.
Each harness file is a simple utf8 encoded json file, which has two entries.
tablesA list containing table names, which the tests should be run against. This is usually just one table, but for some situations more than one table is required.
testsA list of sections of tests, which should be processed independantly. Each test section is a dictionary of two items.
flagsThe flags that apply for all the test cases in this section. For example, they could all be forward translation tests, or they should all be run as computer braille tests.
dataA list of test cases, each one containing the specific test data needed to perform a test.
These are the valid fields for the flags section:
commentA field describing the reason for the tests, the transformation rule or any useful info that might be needed in case the test breaks (optional).
cursorPosThe position of the cursor within the given text (optional). Useful when simulating screenreader interaction, to debug contraction and cursor behaviour.
modeThe liblouis translation mode that should be used for this test (optional). If not defined defaults to 0.
outputUniBrlFor a forward translation test, the output should be in unicode braille. For a backward translation test, the input is in unicode braille.
testmodeThe optional testmode field can have three values: "translate" (default if undeclaired), "backtranslate" or "hyphenate". Declares what tests should be performed on the test data.
Each test case has the following entries:
inputThe unicode text to be tested (required).
outputThe expected braille output (required). The dots should be encoded in the liblouis ascii-braille like encoding.
brlCursorPosThe expected position of the braille cursor in the braille output (optional). Useful when simulating screenreader interaction, to debug contraction and cursor behaviour.
Variables defined in the flags section can be overwridden by individual test cases, but if several tests need the same options, they should idealy be split into their own section, complete with their own flags and data.
For examples please see *_harness.txt in the harness directory in the source distribution.
For examples on how to create doctests please see *_test.txt in the doctest directory in the source distribution.
Back-translation is carried out by the function
lou_backTranslateString. Its calling sequence is described in
Programming with liblouis. Tables containing no
context opcode (see context), correct opcode (see correct) or multipass opcodes can be
used for both forward and backward translation. If these opcodes are
needed different tables will be required.
lou_backTranslateString first performs pass4, if
present, then pass3, then pass2, then the
backtranslation, then corrections. Note that this is exactly the
inverse of forward translation.
Liblouis may contain code borrowed from the Linux screen reader BRLTTY, Copyright © 1999-2006 by the BRLTTY Team.
Copyright © 2004-2007 ViewPlus Technologies, Inc. www.viewplus.com.
Copyright © 2007,2009 Abilitiessoft, Inc. www.abilitiessoft.com.
Liblouis is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Liblouis is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License along with Liblouis. If not, see http://www.gnu.org/licenses/.
You use the liblouis library by calling the following functions,
lou_translateString, lou_backTranslateString,
lou_logFile, lou_logPrint,
lou_endLog, lou_getTable,
lou_translate, lou_backTranslate, lou_hyphenate,
lou_charToDots,
lou_dotsToChar,
lou_compileString, lou_readCharFromFile,
lou_version and lou_free.
These are described below. The header file, liblouis.h, also
contains brief descriptions. Liblouis is written in straight C. It has
just three code modules, compileTranslationTable.c,
lou_translateString.c and lou_backTranslateString.c. In
addition, there are two header files, liblouis.h, which defines
the API, and louis.h, used only internally and by liblouisxml.
The latter includes
liblouis.h.
Persons who wish to use liblouis from Python may want to skip ahead to Python bindings.
compileTranslationTable.c keeps track of all translation tables
which an application has used. It is called by the translation,
hyphenation and checking functions when they start. If a table has not
yet been compiled compileTranslationTable.c checks it for
correctness and compiles it into an efficient internal representation.
The main entry point is lou_getTable. Since it is the module
that keeps track of memory usage, it also contains the lou_free
function. In addition, it contains the lou_logFile,
lou_logPrint and lou_endLog functions, plus some utility
functions which are
used by the other modules.
By default, liblouis handles all characters internally as 16-bit unsigned integers. It can be compiled for 32-bit characters as explained below. The meanings of these integers are not hard-coded. Rather they are defined by the character-definition opcodes. However, the standard printable characters, from decimal 32 to 126 are recognized for the purpose of processing the opcodes. Hence, the following definition is included in liblouis.h. It is correct for computers with at least 32-bit processors.
#define widechar unsigned short int
To make liblouis handle 32-bit Unicode simply remove the word
short in the above define. This will cause the translate and
back-translate functions to expect input in 32-bit form and to deliver
their output in this form. The input to the compiler (tables) is
unaffected except that two new escape sequences for 20-bit and 32-bit
characters are recognized.
Here are the definitions of the eleven liblouis functions and their parameters. They are given in terms of 16-bit Unicode. If liblouis has been compiled for 32-bit Unicode simply read 32 instead of 16.
The data structure TranslationTableHeader is defined by a
typedef statement in louis.h. To find the beginning,
search for the word ‘header’. As its name implies, this is
actually the table header. Data are placed in the ruleArea
array, which is the last item defined in this structure. This array is
declared with a length of 1 and is expanded as needed. The table
header consists mostly of arrays of pointers of size HASHNUM.
These pointers are actually offsets into ruleArea and point to
chains of items which have been placed in the same hash bucket by a
simple hashing algorithm. HASHNUM should be a prime and is
currently 1123. The structure of the table was chosen to optimize
speed rather than memory usage.
The first part of the table contains miscellaneous information, such as the number of passes and whether various opcodes have been used. It also contains the amount of memory allocated to the table and the amount actually used.
The next section contains pointers to various braille indicators and
begins with capitalSign. The rules pointed to contain the
dot pattern for the indicator and an opcode which is used by the
back-translator but does not appear in the list of opcodes. The
braille indicators also include various kinds of emphasis, such as
italic and bold and information about the length of emphasized
phrases. The latter is contained directly in the table item instead of
in a rule.
After the braille indicators comes information about when a letter sign should be used.
Next is an array of size HASHNUM which points to character
definitions. These are created by the character-definition opcodes.
Following this is a similar array pointing to definitions of
single-cell dot patterns. This is also created from the
character-definition opcodes. If a character definition contains a
multi-cell dot pattern this is compiled into ordinary forward and
backward rules. If such a multi-cell dot pattern contains a single
cell which has not previously been defined that cell is placed in this
array, but is given the attribute space.
Next come arrays that map characters to single-cell dot patterns and dots to characters. These are created from both character-definition opcodes and display opcodes.
Next is an array of size 256 which maps characters in this range to
dot patterns which may consist of multiple cells. It is used, for
example, to map ‘{’ to dots 456-246. These mappings are created
by the compdots
or the comp6 opcode (see comp6).
Next are two small arrays that held pointers to chains of rules
produced by the swapcd opcode (see swapcd) and the swapdd opcode (see swapdd) and by
some multipass, context and correct opcodes.
Now we get to an array of size HASHNUM which points to chains
of rules for forward translation.
Following this is a similar array for back-translation.
Finally is the ruleArea, an array of variable size to which
various structures are mapped and to which almost everything else
points.
char *lou_version ()
This function returns a pointer to a character string containing the version of liblouis, plus other information, such as the release date and perhaps notable changes.
int lou_translateString (
const char * tableList,
const widechar * inbuf,
int *inlen,
widechar *outbuf,
int *outlen,
char *typeform,
char *spacing,
int mode);
This function takes a string of 16-bit Unicode characters in
inbuf and translates it into a string of 16-bit characters in
outbuf. Each 16-bit character produces a particular dot pattern
in one braille cell when sent to an embosser or braille display or to
a screen type font. Which 16-bit character represents which dot pattern
is indicated by the character-definition and display opcodes in the
translation table.
The tableList parameter points to a list of translation tables
separated by commas. If only one table is given, no comma should be
used after it. It is these tables which control just how the
translation is made, whether in Grade 2, Grade 1, or something else.
liblouis knows where to find all the tables that have been distributed
with it. So you can just give a table name such as en-us-g2.ctb
and liblouis will load it. You can also give a table name which
includes a path. If this is the first table in a list, all the tables
in the list must be on the same path. You can specify a path on which
liblouis will look for table names by setting the environment variable
LOUIS_TABLEPATH. This environment variable can contain one or
more paths separated by commas. On receiving a table name liblouis
first checks to see if it can be found on any of these paths. If not,
it then checks to see if it can be found in the current directory, or,
if the first (or only) name in a table list, if it contains a
path name, can be found on that path. If not, it checks to see if it
can be found on the path where the distributed tables have been
installed. If a table has already been loaded and compiled this
path-checking is skipped.
The tables in a list are all compiled into the same internal table.
The list is then regarded as the name of this table. As explained in
How to Write Translation Tables, each table is a file which may
be plain text, big-endian Unicode or little-endian Unicode. A table
(or list of tables) is compiled into an internal representation the
first time it is used. Liblouis keeps track of which tables have been
compiled. For this reason, it is essential to call the lou_free
function at the end of your application to avoid memory leaks. Do
NOT call lou_free after each translation. This will
force liblouis to compile the translation tables each time they are
used, leading to great inefficiency.
Note that both the *inlen and *outlen parameters are
pointers to integers. When the function is called, these integers
contain the maximum input and output lengths, respectively. When it
returns, they are set to the actual lengths used.
The typeform parameter is used to indicate italic type,
boldface type, computer braille, etc. It is a string of characters
with the same length as the input buffer pointed to by *inbuf.
However, it is used to pass back character-by-character results, so
enough space must be provided to match the *outlen parameter.
Each character indicates the typeform of the corresponding character
in the input buffer. The values are as follows: 0 plain-text; 1
italic; 2 bold; 4 underline; 8 computer braille. These values can be
added for multiple emphasis. If this parameter is NULL, no
checking for type forms is done. In addition, if this parameter is not
NULL, it is set on return to have an 8 at every position
corresponding to a character in outbuf which was defined to
have a dot representation containing dot 7, dot 8 or both, and to 0
otherwise.
The spacing parameter is used to indicate differences in
spacing between the input string and the translated output string. It
is also of the same length as the string pointed to by *inbuf.
If this parameter is NULL, no spacing information is computed.
The mode parameter specifies how the translation should be
done. The valid values of mode are listed in liblouis.h. They
are all powers of 2, so that a combined mode can be specified by
adding up different values.
The function returns 1 if no errors were encountered and 0 if a complete translation could not be done.
int lou_translate (
const char * tableList,
const widechar * const inbuf,
int *inlen,
widechar * outbuf,
int *outlen,
char *typeform,
char *spacing,
int *outputPos,
int *inputPos,
int *cursorPos,
int mode);
This function adds the parameters outputPos, inputPos and
cursorPos, to facilitate use in screen reader programs. The
outputPos parameter must point to an array of integers with at
least inlen elements. On return, this array will contain the
position in outbuf corresponding to each input position.
Similarly, inputPos must point to an array of integers of at
least outlen elements. On return, this array will contain the
position in inbuf corresponding to each position in
outbuf.
cursorPos must point to an integer containing the position of the
cursor in the input. On return, it will contain the cursor position in
the output. Any parameter after outlen may be NULL. In
this case, the actions corresponding to it will not be carried out. The
mode parameter, however, must be present and must be an integer,
not a pointer to an integer. If the compbrlAtCursor bit is set in
the mode parameter the space-bounded characters containing the
cursor will be translated in computer braille. If the
compbrlLeftCursor bit is set only the characters to the left of
the cursor will be in computer braille. This bit overrides
compbrlAtCursor.
When the dotsIO bit is set, during translation, produce output as dot patterns. During
back-translation accept input as dot patterns. Note that the produced
dot patterns are affected if you have any display opcode (see display) defined
in any of your tables.
The ucBrl (Unicode Braille) bit is used by lou_charToDots and lou_translate. It causes the dot
patterns to be Unicode Braille rather than the liblouis representation.
Note that you will not notice any change when setting ucBrl unless dotsIO is also set.
lou_dotsToChar and lou_backTranslate recognize Unicode
braille automatically.
The otherTrans mode needs special description. If it is set
liblouis will attempt to call a wrapper for another translator. These
other translators are usually for Asian languages. The calling sequence
is the same as for liblouis itself except that the trantab
parameter gives the name of the other translator, possibly abbreviated,
followed by a colon, followed by whatever other information the other
translator needs. This is specific for each translator. If no such
information is needed the colon should be omitted. The result of calling
either the translate or back-translate functions with this mode bit set
will be the same as calling without it set. That is, the wrapper for the
other translator simulates a call to liblouis. Note that the wrappers
are not implemented at this time. Setting this mode bit will result in
failure (return value of 0).
int lou_backTranslateString (
const char * tableList,
const widechar * inbuf,
int *inlen,
widechar *outbuf,
int *outlen,
char *typeform,
char *spacing,
int mode);
This is exactly the opposite of lou_translateString.
inbuf is a string of 16-bit Unicode characters representing
braille. outbuf will contain a string of 16–bit Unicode
characters. typeform will indicate any emphasis found in the
input string, while spacing will indicate any differences in
spacing between the input and output strings. The typeform and
spacing parameters may be NULL if this information is
not needed. mode again specifies how the back-translation
should be done.
int lou_backTranslate (
const char * tableList,
const widechar * inbufx,
int *inlen,
widechar * outbuf,
int *outlen,
char *typeform,
char *spacing,
int *outputPos,
int *inputPos,
int *cursorPos,
int mode);
This function is exactly the inverse of lou_translate.
int lou_hyphenate (
const char *tableList,
const widechar *inbuf,
int inlen,
char *hyphens,
int mode);
This function looks at the characters in inbuf and if it finds
a sequence of letters attempts to hyphenate it as a word. Note that
lou_hyphenate operates on single words only, and spaces or punctuation
marks between letters are not allowed. Leading and trailing
punctuation marks are ignored. The table named by the tableList
parameter must contain a hyphenation table. If it does not, the
function does nothing. inlen is the length of the character
string in inbuf. hyphens is an array of characters and
must be of size inlen + 1 (to account for the NULL terminator).
If hyphenation is successful it will have a 1 at the beginning of each
syllable and a 0 elsewhere. If the mode parameter is 0
inbuf is assumed to contain untranslated characters. Any
nonzero value means that inbuf contains a translation. In this
case, it is back-translated, hyphenation is performed, and it is
re-translated so that the hyphens can be placed correctly. The
lou_translate and lou_backTranslate functions are used
in this process. lou_hyphenate returns 1 if hyphenation was
successful and 0 otherwise. In the latter case, the contents of the
hyphens parameter are undefined. This function was provided for
use in liblouisxml.
int lou_compileString (const char *tableList, const char *inString)
This function enables you to compile a table entry on the fly at
run-time. The new entry is added to tableList and remains in force
until lou_free is called. If tableList has not previously
been loaded it is loaded and compiled. inString contains the
table entry to be added. It may be anything valid. Error messages
will be produced if it is invalid. The function returns 1 on success and
0 on failure.
int lou_dotsToChar (const char *tableList, const widechar *inbuf, widechar *outbuf, int length, int)
This function takes a widechar string in inbuf consisting of dot
patterns and converts it to a widechar string in outbuf
consisting of characters according to the specifications in
tableList. length is the length of both inbuf and
outbuf. The dot patterns in inbuf can be in either
liblouis format or Unicode braille. The function returns 1 on success
and 0 on failure.
int lou_charToDots (const char *tableList, const widechar *inbuf, widechar *outbuf, int length, int mode)
This function is the inverse of lou_dotsToChar. It takes a
widechar string in inbuf consisting of characters and converts it
to a widechar string in outbuf consisting of dot patterns
according to the specifications in tableList. length is the
length of both inbuf and outbuf. The dot patterns in
outbufbuf are in liblouis format if the mode bit ucBrl is
not set and in Unicode format if it is set. The function returns 1 on
success and 0 on failure.
void lou_logFile (char *fileName);
This function is used when it is not convenient either to let messages be printed on stderr or to use redirection, as when liblouis is used in a GUI application or in liblouisxml. Any error messages generated will be printed to the file given in this call. The entire path name of the file must be given.
void lou_logPrint (char *format, ...);
This function is called like fprint. It can be used by other
libraries to print messages to the file specified by the call to
lou_logFile. In particular, it is used by the companion
library liblouisxml.
lou_logEnd ();
This function is used at the end of processing a document to close the log file, so that it can be read by the rest of the program.
char * lou_setDataPath (char *path);
This function is used to tell liblouis and liblouisutdml where tables
and files are located. It thus makes them completely relocatable, even
on Linux. The path is the directory where the subdirectories
liblouis/tables and liblouisutdml/lbu_files are rooted or
located. The function returns a pointer to the path.
char * lou_getDataPath ();
This function returns a pointer to the path set by
lou_setDataPath. If no path has been set it returns NULL.
void *lou_getTable (char *tablelist);
tablelist is a list of names of table files separated by
commas, as explained previously
(see tableList parameter in
lou_translateString). If no errors are found this function
returns a pointer to the compiled table. If errors are found messages
are printed to the log file, which is stderr unless a different
filename has been given using the lou_logFile function.
Errors result in a NULL pointer being returned.
int lou_readCharFromFile (const char *fileName, int *mode);
This function is provided for situations where it is necessary to read
a file which may contain little-endian or big-endian 16-bit Unicode
characters or ASCII8 characters. The return value is a little-endian
character, encoded as an integer. The fileName parameter is the
name of the file to be read. The mode parameter is a pointer to
an integer which must be set to 1 on the first call. After that, the
function takes care of it. On end-of-file the function returns
EOF.
void lou_free ();
This function should be called at the end of the application to free
all memory allocated by liblouis. Failure to do so will result in
memory leaks. Do NOT call lou_free after each
translation. This will force liblouis to compile the translation
tables every time they are used, resulting in great inefficiency.
There are Python bindings for lou_translateString,
lou_translate and lou_version. For installation
instructions see the the README file in the python
directory. Usage information is included in the Python module itself.
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| Index Entry | Section | ||
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lou_allround: | lou_allround | ||
lou_checkhyphens: | lou_checkhyphens | ||
lou_checktable: | lou_checktable | ||
lou_debug: | lou_debug | ||
lou_trace: | lou_trace | ||
lou_translate: | lou_translate (program) | ||
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