is there a callback for length-by-duration? (possible sponsorhip)

[Kieren MacMillan]

Hi all,

Some time ago, I asked Han-Wen about sponsoring a feature where  
lengths (widths, really) could be set by duration (e.g., 16th note)  
instead of staff spaces. At the time, I couldn't think of a good  
example to back up my intuition of its importance, so we let it drop...

However, the recent thread with Ralph (about TrillSpan lengths) gave  
me the example I now need to ask the question(s) correctly!  =)

Is there a callback function that can be used to get the width, at  
the current point in the current measure, of a given musical duration?
If not, can such a thing be developed?

In the following snippet, the TrillSpan is shortened (using the  
#'right #'padding setting) by a HARD-CODED (i.e., duration- 
independent) length in staff-spaces:

%%%  BEGIN SNIPPET
\version "2.11.32"
theExample =
{
        \clef bass
        \once \override TrillSpanner #'bound-details #'right #'padding = #10
        f2.\startTrillSpan e4\stopTrillSpan
}
\score { \theExample }
\score { \theExample \layout { line-width = 5\in } }
\score { \theExample \layout { line-width = 3\in } }
%%%  END SNIPPET

The problem is, the shorter the measure is, the shorter the DURATION  
of the TrillSpan becomes (i.e., appears to be); consequently, if you  
truly want a TrillSpan that (visually) lasts three beats, you need to  
adjust the #'padding tweak every time there's a change in the page  
layout.  =(

If, on the other hand, one could use
	\once \override TrillSpanner #'bound-details #'right #'padding = # 
(duration-to-spaces . 4)
or something like that, then Lilypond could adjust on-the-fly, and  
always trim the right-edge by (in this example) a quarter's width.

In my dream world, Hairpin (etc.) would implement the #'bound-details  
#'right/left #'padding interface, and you could have "floating"  
hairpins -- e.g., starting one eighth after the note, and ending  
after a half-note duration -- without the need for extra voices or  
skip notes, etc. [!!!!]

What say all of ye?

Thanks,
Kieren.