> > Why do you think that it is bad design?
>
> Let's say you have 40 characters that all need to change glyph when
> they are preceded by any one of 30 other characters. This makes
> 30*40 substitution rules! If instead you could express the
> "preceded by one of those 30 characters" as a single context
> definition, then you would just need one context definition and 40
> rules, one for each character to tell the alternate glyph to use
> under this context.
This is GSUB either format 5 (context substitution) or 6 (context
substitution with ability to look back and look forward). Exactly one
substitution, provided it comes from a single lookup, which is rather
likely.
> > However, it works reasonably well, and noone asks you to use more than
> > a single lookup.
>
> What do you mean by this "single lookup"?
This is a technical term of the OpenType specification. As mentioned
earlier, a `feature' consists of an arbitrary number of lookups
(normally, there is only one lookup table in a feature), and a single
lookup means that you apply its substitution rule to a string of
glyphs. Please check the GSUB table information in the OpenType
specification for more details.
> BTW another issue of the substitution rules is that, as far as I can
> tell, they can delete or insert extra glyphs arbitrarily.
Of course. How would you handle a ligature? `f' + `l' = `fl' -- this
means that a character has been deleted.
> From a character cell perspective that's very bad, since it makes it
> possible that the font represents things that cannot be displayed
> consistently in the cells.
The font designer has to be careful to get this right.
> > And there is still the question who is going to implement this.
>
> Anyone can since the spec is trivial to implement. With all but the
> context-matcher implemented so far, my implementation compiles to 528
> bytes of i386 code.
I'm *really* interested to see this :-)
> > Mongolian can be and is written horizontally as well.
>
> I didn't find any better references searching google than you would.
> It seems to be a new invention, and the glyphs are rotated 90
> degrees from their vertical presentation in order to combine nicely.
I rather think that this is an invention to overcome the complications
with computers. I'll ask a friend who is an expert for Mongolian.
> In terms of what I "want" (for my own use): Latin, Tibetan, Japanese,
> and mathematical notations.
Hmm. Mathematical notation is two-dimensional by its very nature.
Please elaborate.
Werner
