Re: [A12n-Collab] Re: 5 categories of African orthographies (Latin)

[David Rowe <David_Rowe@xxxxxxx>]

> The fact that the question keeps getting raised (I hear it
> from others occasionally) is sign enough that there is a need to 
> either clarify
> the support issues and how those are being addressed, or clarify how 
> the system
> doesn't work. Continued doubts about dynamic composition either need to be
> addressed with better explanations (and real support) or alternatively 
> - again from
> a NPOV - with a real proposal that makes the justification and proposes
> specific precomposed characters. This so that we can move forward one 
> way or
> another rather than recycling debates.
In order for a Unicode font to support dynamic composition, the font 
must contain the necessary information and there must be software that 
can use that information to correctly position the diacritic with the 
base character. Under Windows, the Uniscribe processor provides the 
software support necessary for an application to render combining 
diacritics (provided the application knows to use the Uniscribe 
processor). For example, Word 2003 running on Windows XP can render 
U+025B U+0303 U+0300 (epsilon with the tilde combining diacritic on top 
of the epsilon and the grave combining diacritic stacked on top of the 
tilde) provided that a font (such as Doulos SIL) is used which contains 
the information for placing these diacritics.
It is also possible to create a font that provides support for a subset 
of base character / diacritic combinations by including all the needed 
combinations as precomposed characters in the Private Use Area, and then 
adding substitution information (using Microsoft's VOLT) that will, for 
example, substitute the precomposed epsilon+tilde+grave character which 
I put at U+E04F when the combination U+025B U+0303 U+0300 is found in 
the text. I have created such fonts by using SIL's Encore Font system 
(originally designed for creating custom 8-bit fonts, but also capable 
of creating Unicode fonts) then adding the substitution information with 
VOLT. The Microsoft Uniscribe processor is still needed to do the 
substitution, so this solution should work with Word 2003 on Windows XP.
My need for a sans-serif font and a mono-spaced font was the motivation 
for creating fonts that support only a subset of base character / 
diacritic combinations. (SIL Doulos is a serif, variable width font.) 
When a sans-serif Unicode font that supports all the needed characters 
becomes available, then it can be used instead of my custom font with no 
change in the text of the document, just a change of font.
If the text were to contain the sequence U+025B U+0300 U+0303 (epsilon 
with grave and tilde), then the Doulos SIL font will render it correctly 
(with the tilde on top of the grave on top of the epsilon) whereas the 
fonts I created will not since that particular combination doesn't occur 
as a precomposed character (because none of the languages that this font 
was intended to support uses that combination).
I don't know what the situation is for Mac. For Linux, there exist 
already, or are in development, ways to render Unicode text which uses 
combining diacritics. The font needs to include the necessary 
information and the application and the operating system need to provide 
the software support for the rendering.
David Rowe
SIL Togo-Benin