rubber in spacelinestext

[eroux]

For cases where the baselineskip (or lineskip) have big rubbers (and looking at Solesmes's old books, they certainly should), it would be wise to introduce the possibility of a rubber in spacelinestext.

To take a concrete example, see page 602/603 of the Graduale Triplex: the Stabat Mater score has a much smaller inter line spacing on page 602 than on page 603 or 604. This shrinking also affects (the equivalent of) spacelinestext, which is 5mm on page 602 and 6mm on page 603/604. This allows the score to avoid the problems discussed in #1182 (comment)

Doing this is currently impossible in gregorio due to the "box model" in which the notes are typeset with something similar to \raise\spacelinestext\hbox{notes}. I don't think changing this model is a good idea and it would be way too much work.

But there is still a possibility to do that in Lua post processing. I see two ways of doing this:

First is by registering a function in the pre_output_filter (I think), it would be possible to see the shrinking/stretching TeX has set for a particular page, and change the spacelinestext in accordance by moving the boxes in Lua. This would not be easy and would require proper testing. Also, this would not really be 100% satisfactory, as the lines would have to keep the same height (not to disrupt the spaces and have the page be too short) and baseline (in order for the page to be bottom-aligned on the baseline of the text of the last line), but at least it would be better than what we have, and by fine tuning the spaces, one should be able to achieve anything.

Second is by registering a function in post_linebreak_filter that would extract the text of all score lines, remove it and and paste it in a new hbox beneath the line, keeping all the horizontal spaces (I'm quite sure it's not easy), and adding a vertical rubber between the two lines. This has the advantage of making TeX completely aware of the rubber when it makes its vertical spacing decisions and thus will give a better output...

Obviously I'm not expecting any of it for 5.0... What do you think?