[XeTeX] Line-breaking algorithms in XeTeX

John Was john.was at ntlworld.com
Mon Apr 27 09:37:32 CEST 2009

Hello Jonathan

I couldn't think of a way of providing a test case - I could obviously show 
a PDF with the overfull rule when \looseness is -1 and without the overfull 
rule (with the paragraph a line longer) when \looseness is set to 0 or 1, 
but that wouldn't be helpful.  The font is MinionPro, and in all sizes I 
have the \spaceskip set thus:

\spaceskip 0.25em plus 0.25em minus 0.075em

This seems to produce satisfactory results in the \hsize and typesizes that 
I work with.  But it shouldn't generate this problem, I think, and certainly 
not my situation (2), where the sole difference is whether there is a \vfill 
\eject or \vskip between items.



----- Original Message ----- 
From: "Jonathan Kew" <jfkthame at googlemail.com>
To: "Unicode-based TeX for Mac OS X and other platforms" <xetex at tug.org>
Sent: Sunday, April 26, 2009 11:50 PM
Subject: Re: [XeTeX] Line-breaking algorithms in XeTeX

> On 26 Apr 2009, at 14:57, John Was wrote:
>> Dear All
>> Since starting to use (plain) XeTeX I've noticed something strange
>> with the paragraphing/line-breaking mechanism which has never
>> happened during the ten years or so during which I have used
>> traditional TeX.  It is cropping up in the fourth issue of a
>> periodical that I have set with XeTeX, so I'm pretty sure that it's
>> not a random fluke.
>> (1) I sometimes get an overfull rule (i.e. rectangular box) at the
>> right-hand side which will disappear when I either (a) attach the
>> word causing the problem to the next word with ~, forcing it over (I
>> sometimes have to put the word in an \hbox{} as well); or (b) when I
>> increase the line-count by giving \looseness1 for the paragraph.  In
>> the past, plain TeX would always make such decisions for itself and
>> never generate an overfull rule when it could find a way to justify
>> the paragraph without doing so.  This happens most frequently in the
>> reviews section of the periodical, where  \looseness is set to -1 by
>> default to save as much space as possible:  but until I started to
>> use XeTeX, it was always the case that if the paragraph could not
>> lose a line, then the negative looseness was ignored and the
>> paragraph was set successfully with normal looseness  (i.e.
>> \looseness = 0).  It was never (I think) the case that a tight
>> looseness which generated an overfull box would get through and need
>> manual intervention from me.  So has something altered in the way
>> XeTeX is handling the line-breaks, giving priority to the looseness
>> command even at the expense of generating an overfull rule, and even
>> when zero looseness would cause that error to disappear?
>> (2) This is even more puzzling (and more of an nuisance).  For the
>> purpose of sending contributors proofs of their reviews I start each
>> review on a new page so that they don't also receive the tops and
>> tails of adjacent reviews, but while initially typesetting I have
>> the reviews running on consecutively, as they will do in the final
>> published version.  There is a switch at the end of each review
>> which generates a \vfill \eject when \ifseparatereviews is true,
>> otherwise it just produces a \vskip: there is no other difference.
>> Yet I sometimes get overfull rules showing up (at random points)
>> when the reviews are separated out, even though the same paragraph
>> typeset without error while the reviews were set to run on
>> continuously.  The problem almost (but not entirely) disappears if I
>> double the \hfuzz when the \ifseparatereviews switch is true, but
>> that is no more than a quick fix to prevent authors receiving proofs
>> with worrying blobs at the right-hand side.  This seems
>> incomprehensible, but as it has happened with four out of four
>> periodical issues I can't be imagining it - and the commands are
>> precisely the same as the ones I used when the periodical was
>> typeset using traditional plain TeX, with no new parameters such as
>> alteration to \spaceskip or anything else that might cause this to
>> happen.
>> (1) and (2) seem likely to be part of the same problem (though not
>> necessarily so).  Any ideas, or at least insight into what XeTeX is
>> doing that old plain TeX didn't?
> No obvious ideas come to mind; any chance you could come up with a
> small test-case that illustrates the problem(s), and doesn't require
> proprietary fonts, etc.?
> JK
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