boundary char and implicit kerns

[Hilmar Schlegel]

Han The Thanh wrote:
> 
> > Perhaps I misunderstand something here: isn't it exactly the other way
> > round that hanging characters (mid-line and/or begin/end-line) work in
> > Tex only with the above described configuration?
> > I.e. industry-standard fonts have kerns with respect to space (if at
> > all) to allow for optical kerning with resepct to word-boundaries. Using
> > space as a placeholder and translating into kerns with the Tex
> > boundarychar is an obviuos way to make use of this concept in Tex
> > despite it doesn't have a space character.
> >
> > This works fine as far as I could see. Possibly you should consider how
> > you achieve this effect in another way. In any case are the kerns with
> > respect to the boundary character the first/last things which Tex sees
> > at both ends of a line. It is essential for hanging characters that
> > these kerns are not discarded.
> >
> > Perhaps it is necessary to take those corrections in the AFM into
> > account when defining hanging in an alternative approach to see the
> > correct spacing at the end?
> 
> I think that kerns with respect to the space in some font are not intended
> for hanging certain characters, but simply to make the interword space not
> to look so large when the adjacent char is a period or a comma. 

So we've to make the definitions more clear:
You understand by hanging begin/end of line spacing adjustments as
described in The Book which must be achieved from within Tex by special
handling of the characters in question since Tex itself doesn't have a
mecahnism to detect if a character is placed at the line boundaries.
The boundarychar mechanism is working on the font-level and can be used
for various effects like initial/final character variants (via LIGs),
interword spacing (via KRNs) (more exact begin/end word spacing since it
applies the same way also to begin/end line spacings).
Finally hanging hyphenation (to make e.g. hanging puntuation complete)
can only be achieved on the font-level with the definition of an
alternative hyphenchar.

>I am not
> sure about the real intention of the boundary char mechanism, but I think
> it's similar, ie for optical adjustment of interword spaces.

In case you observe that hanging characters within Tex (based on
discarding repeated positve/negative spacings) doesn't work for
characters having kerns with the boundarychar means that instead of
discarding the space, which cancels the hangamount within a line, this
space is *not* discarded after a kern with the boundarychar. Is this
what you observe? 
This indicates that discarding stops here for some reason - is it
possible there is something else slipping in the way from the macros for
the characters in question which tells Tex not to discard the following
items?

Best regards,
Hilmar Schlegel
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