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Re: Close or document the stretchability=0 backdoor?

 From: Joe Neeman Subject: Re: Close or document the stretchability=0 backdoor? Date: Tue, 31 May 2011 11:14:45 +0300

On Fri, May 27, 2011 at 10:44 PM, Keith OHara wrote:
One of the special-cases in the LilyPond code is confusing me.
( This special-case code is in Page_layout_Problem::get_fixed_spacing(). )

If the flexible-vertical-spacing variable 'stretchability is defined, but has
value 0.0, such as:
*-*-spacing = #'((basic-distance . 16)
(stretchability . 0)
(minimum-distance . 0))
then
a) the staves will not stretch to fill extra space
(documented and sensible)
b) the staves will not compress, making basic-distance serve as minimum-distance
(not documented, and I don't know if this it is done consistently)
c) some people expect padding to be ignored, per Carl's comment on issue 1654
(not documented and I can't find it in the code history)

It seems to me that one could achieve (b) by setting minimum-distance equal to
basic-distance.  It is less cumbersome, however, to say
\override VerticalAxisGroup #'staff-staff-spacing #'stretchability = #0.0
than to look up whatever basic-distance happens to be.

This backdoor really confuses me, however, in cases like the definition of
Lyrics in egraver-init.ly.
\override VerticalAxisGroup #'nonstaff-nonstaff-spacing =
#'((basic-distance . 0)
(minimum-distance . 2.8)
(stretchability . 0))
Why is basic-distance smaller than minimum-distance?
What distance should I expect in tight spacing (and a single-line parent staff
like a drum staff) ?

What is stretchability = 0 supposed to do ?

I don't think it was originally "supposed" to do anything, but it turned out to be a useful way to force a fixed spacing. With this application in mind, I think (b) and (c) are both appropriate, and that the distance should be the maximum of basic-distance and minimum-distance. So in the definition above, basic-distance could be anything less than 2.8 and the effect would be the same.

I'm not 100% attached to this, though, so if you want to change something specific then don't be shy...

Cheers,
Joe