[BRLTTY] The unknown-character sign (26)

[Dave Mielke]

[quoted lines by Mario Lang on 2008/11/24 at 20:27 +0100]

>What I am looking for is a way for 6-dot contracted braille tables to override 
>the default for invalid characters.

Checking for a definition of \uFFFD in the contraction table would solve this.

>I think we are looking at two different types of unknown characters, aren't we?
>There are those that have no mapping in either the contraction or the
>text-table, and there are those which we fail to map back to a proper unicode
>codepoint because the character wasn't in the font currently being used.

Fortunately, Linux makes this easy because it gives us \uFFFD for characters 
not defined within the currently loaded font.

>To come back to your question, I think it only makes sense to
>represent a unknown character as its escape sequence if we actually know
>what character it was, and, while 6dot mode is active, because only
>in 6dot contracted braille mode will the user assume that character width can
>be unequal to cells used.  So yes, maybe it would be useful to
>have contracted braille print unknown characters with an escape sequence,

So as long as the character isn't \uFFFD.

>However, this still leaves us with the other unknown character
>case, that one where the font couldn't be used to map back to the real
>character value.  In that case, I still think the contraction table
>author should be able to define what should be printed, instead of relying on
>what the text-table ultimately defines.

Explicitly defining \uFFFD should resolve this, although I think a default of 
all eight dots would be sufficient for most cases.

>> What do you think about eliminating the text table as a fallback and
>> insisting that the contraction table define all of its characters?
>I am not sure this is useful, especially since a user might *want* to mix
>different text/contraction tables.  Also, if a contraction table
>really only uses 6dot output it does seem to make sense to be able to
>fall back to the characters defined in a text-table that use dot7 or dot8.
>This way, excessive duplication of character definitions can be avoided.

Yes, but it could also cause confusion, especially if the text table 
representation of the character uses only the upper six dots.

Also, unless we change the way text tables work, it's not possible to tell from 
the outside that a character isn't defined. Once we fall back to the text 
table, we're going to get a representation even if it's the "unknown" one.

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