Re: Lyrics break estimation of vertical spacing

[Boris Shingarov]

Here are the two illustrative examples.

On Fri, 19 Mar 2010 11:05:30 -0400, Boris Shingarov  wrote:
On Wed, 13 Jan 2010 19:37:14  1100, Joe Neeman  wrote:
 >
 >  > It's nice to see someone else touching the page-breaking code
 >   
 >  The more I dig into it, the more questions I have.
 >  Back in January, we talked about vertical space estimation in the
case of Prob (i.e. markup); now I am investigating vertical space
estimation for the case of Grob (i.e. score).  The problem is that
when the user tries to fit more systems on the page by decreasing the
spacing/padding, the final page layout does take it into account, but
it has no effect on the estimations at the page-breaking stage.  The
visual result is that the lines do get closer together, but the number
of lines per page still remains the same, leaving a big empty gap at
the bottom of the page.
 >   
 >  This problem is caused by the calculation of ext_len in the "for
(i=...)" cycle in Page_breaking::min_page_count().
 >  For illustration, I have attached two examples:
simple-nolyrics.ly, and the same score with added lyrics,
simple-lyrics.ly.  When there are no lyrics, the estimation works as
expected.  The ext_len in this case is 7.947, and the specified
padding of 1.2 causes the systems to be nicely squeezed together.  Now
if we add lyrics, the final spacing and the estimation start to diverge
dramatically.  In the final layout of the page, the lyrics do not
cause a significant change to the distance between the systems: the
lyrics just sit in the gap which was already there.  But the
estimator, during the page breaking, get confused: for each line, the
rod is lengthened by ext_len padding, and ext_len in this case is as
much as 10.203.  So the page breaker thinks that the systems are much
taller than they will finally prove to be when the final spacing is
done.
 >   
 >  Trying to do something about this ext_len problem, I traced the
origin of the ext-len back to the Y-ext of the Grob
(fill_line_details() calls sys->pure_height(), which calls
get_property_data("Y-extent")).  Bien sûr, the Y-extent includes the
whole height of the lyrics.
 >   
 >  I am not sure what can be done to reconcile the estimator with the
actual spacing, so that users can get tight spacing.  My understanding
is that the final spacing actually tries to avoid skyline collisions;
the page breaker ignores the actual shape of the skyline and treats the
systems as solid rectangular blocks.  So the only possible workarond
would be to manually indicate the ext_len to the page breaker
(something like, a block variable in the score layout) -- extremely
ugly, and I would only opt for that if I knew for sure that all other
options were exhausted.
 >  
 > Boris
 >  
 >  
 >  
 >
 >
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 >
 >

simple-nolyrics.ly
Description: Text Data

simple-nolyrics.ly
Description: Text Data