boundary char and implicit kerns

[Han The Thanh]

> > 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?

I do marginal kerning in pdftex. The mechanism detects whether the ending
object of a line is a character, and inserts a kern if needed. Now it stops
to work when kerns with respect to interword space are used via the
boundarychar, because when eg the comma ends up at the end of a line, there
is still an implicit kern after it. Thus the comma is not "protruded"
as it should be at all.

Regards,
Thanh