Re: Vertical spacing regression !?

[Boris Shingarov]

Hi Arno,

Here are some design remarks about what I understood from my work on the 
breaker.  Hopefully a sum of a few [dozen?] of mails like this will 
together constitute something to transform into a chapter in the CG...
> > Thanks, I've reverted the patch in the meantime. However, the
> > information that you see in annotate-spacing is actually computed after
> > line-breaking,
>
> ok, so why can't it then be taken into account for page-breaking?
The problem is that page-breaking does not come *after* page-breaking.  
Well, conceptually, it sort of does.  But the optimization of breaking 
is global over the product space of all line breaking possibilities by 
all page breaking possibilities.  It is done in the hope to get more 
optimal breaks.   The downside is that we are forced to use "height 
estimations" instead of real heights of things; otherwise we would need 
to do page layout for every possible break configuration, which would be 
prohibitively expensive.

This was discussed on this list only a few weeks ago.  I think we are on 
our way to get rid of global page*line breaking.  Height estimation is 
just too inaccurate, so you either have music running off the page at 
the bottom, or (no less ugly) empty space.  We did fix a bunch of height 
estimation issues, but I do not know how many more are left.  This one 
we are discussing right now, is another example of problem caused by 
height estimation.

> Since it more than doubles the computation time (often by far) there 
> must be something redundant, no?
No, because the height used in page breaking, is not the actual height; 
it is just an estimate (often wrong).  This is what Joe means when he 
says, "...and they aren't actually correct to begin with".

Boris