[omega] Question about the paper published in EuroTeX 2005

[Yannis Haralambous]

Benjamin Bayart answered by the following:

> Si on dit que "k-" est un glyphe different pour "c", ca sonne faux. Par
> contre, si on dit que "k-k" est une chaine de glyphes differente, dans
> cette langue, pour la chaine de caracteres "ck", alors ca sonne juste.
> La notion de "variantes de glyphe" devrait donc etre comprise dans un
> contexte, et pas de maniere absolue. Le cas des "d" de Zapfino est
> different parce que le contexte ne jour que graphiquement et pas au
> niveau linguistique, mais il me semble que ca rentre dans le meme 
> cadre.

(in English: if we say that "k-" is a different glyph for "c", that 
sounds wrong. But, on the contrary, if we say that "k-k" is a different 
glyph string, in this language, for the character string "ck", that 
sounds OK. The notion of "glyph variant " should be understood in a 
context, and not in absolute way. The case of Zapfino "d" glyphs is 
different  because the context is only graphical and not linguistic, 
but it seems that the frame is the same)

My answer:

we need an atomic unit for algorithms (like OTPs, paragraph builder, 
etc.) to operate. Let us take again the example of backen: I agree that 
saying that "k" is a variant glyph of "c" is absurd. But suppose now 
that in our font we have an `ak' ligature. Then, when the word is 
hyphenated "bak-ken", I would indeed like to have that ligature in the 
part "bak-". Which means that the algorithm must detect a "k" texteme 
to be able to apply the ligature. So even if it sounds absurd to have a 
"c" with a "k" glyph, maybe this is how the engine sees it.

My argument about "pseudo" is that we weren't very precise yet on what 
we mean when talk of a "glyph". A TeX charnode contains a number which 
is the position of a glyph in the current font. So in some sense this 
is both concrete and abstract. Concrete because we obtain one and only 
image (when I say glyph 97 of font CMR10 at size 10 points, this is a 
unique image, modulo the version changes by Knuth, and this no matter 
how I get there, be it bitmap or vector outlines). Abstract because 
being only a table position we can change fonts, and provided we have 
the same glyph encoding we can get something else.

So the question is: how abstract must a "texteme glyph reference" be?

How many levels of specification do we have? When we say that we want 
glyph=a, do we mean just any a (default font) at any size (default 
size)? a in a font named foo? a in a font named foo version 1.01? in a 
given size? using a specific outline?

In TeX this problem was avoided by using *only* TFM. TeX knows only 
about TFM and if we want to combine TFM with real-world fonts then all 
the ambiguity of identifying a font is outside the scope of TeX. Omega 
will soon read OpenType fonts directly. In a TTF-flavored OpenType font 
you must go through GSUB/GPOS to access certain glyphs (not accessible 
from cmap), otherwise the only way is through glyph indexes which are 
not reliable, so the problem of identifying a glyph in a texteme is a 
hot topic.

But first of all, we should be very clear about what we are talking 
about when we say "glyph". (Vocabulary, terminology, taxonomy: sounds 
like a talk by Chris, Joachim and Christina???)

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