[XeTeX] Line-breaking algorithms in XeTeX

[Jonathan Kew]

On 28 Apr 2009, at 09:16, John Was wrote:

> One thing I've learnt from your sample, though, is that (Xe)TeX  
> selects
> different hyphenation and line-breaking points when the \looseness is
> reduced to -1, even though that doesn't in fact produce a smaller  
> number of
> lines.  I had always assumed that if the line-count ends up the same  
> as if
> \loosness were 0, then the \looseness-1 would be ignored without  
> causing any
> effect at all.

Yes, this can seem surprising at times; actually, it's even possible  
for \looseness=-1 to result in the paragraph being set slightly  
*looser* (with more text on the last line), if it can't fit into fewer  
lines.

One reason this happens, I believe, is that without \looseness,  
(Xe)TeX may have been setting the paragraph successfully on its first- 
pass attempt, without doing any hyphenation, whereas adding the  
\looseness command triggers the hyphenation pass, and then (even if  
the line count ends up the same) a different set of breaks may be  
chosen, depending on the badness of the resulting lines, compatibility  
of adjacent lines, etc.

JK