Clarifying line spacing

[davidweichiang]

I've been trying to understand the various line spacing options and it seems I'm not the only one to find them challenging (#1375, #1528, https://groups.google.com/g/gregorio-users/c/_K5qfmGVunI). I had to draw a picture:

The height of the line is the maximum of the three heights (1), (2), and (3). Spaces that default to less than 1pt have been increased to 1pt for visibility.

I'd like to suggest adding this picture to the documentation, but want to check that it's correct, and also whether this was really how it was meant to be. Specifically:

1. I'm surprised that abovelinestextheight is in (1), where it doesn't actually reach as high as the abovelinestext. Would it make more sense to move it to (2) instead, starting from the baseline of the abovelinestext and creating a little bit of whitespace at the top?

2. The height (2) includes 4 copies of interstafflinespace, which looks like an off-by-one error to me (as noted here). Can the extra interstafflinespace be deleted, and the default abovelinestextraise be increased to compensate?

3. The documentation for noteadditionalspacelinestextthreshold says that it's "the number of low notes which will add space between the lines and the lyrics (additionalbottomspace)." To me that sounds like a higher threshold leads to less additional space, but it's actually the opposite (higher threshold leads to more additional space).

4. Because the default additionaltopspacenabcthreshold of 4 is so high, high notes collide with nabc neumes.

[davidweichiang]

Illustration of the 4th point above:

[rpspringuel]

We definitely need some additional clarity here, but it's something that I won't have time to look at until after the release is finished.

[davidweichiang]

Understood! As a result of digging into this, I've found a few bugs (besides the one above, which I'm thinking there might be an explanation for?). I'll make separate issues for those.