Poor page breaking with figures

[davidchisnall]

Using the figure environment, I am seeing two rendering issues:

First, the caption begins too low, overlapping the end of the page. Second, the caption itself is split across two pages. I believe part of this is an artefact of the fact that figures are not floats (which is sad, given that SILE was specifically designed to make floats easier to support with good placement than TeX), but I think the rest of it is that the figure is not being treated as a complete box for the purpose of layout and so line breaking is allowed in the middle.

[Omikhleia]

On the underlying SILE issues:

• Regarding overfull pages (causing overlaps with the folio or footnote frames) = Images sometimes overflow under strict skip values sile-typesetter/sile#552, pagebuilder prefers an overfull frame with worse badness over an underfull one sile-typesetter/sile#1404 (and probably a few other similar/related issues)
• Regarding floats: Support for floating figures sile-typesetter/sile#458 (no comment...)
• Another option here would be "grouped" lines (kept together): see notably How to avoid page breaks inside a paragraph? sile-typesetter/sile#1843 for a similar need in another context...

... given that SILE was specifically designed to make floats easier to support with good placement than TeX

I don't think this was ever true, unfortunately... A mere claim not backed up by any serious attempt. ^^

Regarding our options, well, it's not that obvious. One could rewrite these environments to use some grouping logic as done in my ptable package (or, as a mere workaround, wrap them in a 1x1 ptable - it's an overkill but it somewhat works). That's not fully satisfying though, as the overfull page issue can still ticks in and lead to even worse issues.

My feelings are that, in order to make any progress, lots of internal things have to change. It's opinionated, and I am pessimistic by nature, but IMHO, the typesetter is still highly problematic, the pagebuilder is an utter mess, the frame logic is totally broken (ouch! another claim that does not live to the expectations...), the footnote and "insertion" logic is just insane (and likely wrong, sile-typesetter/sile#619)....

So I could be tempted to say that this is not an issue for re·sil·ient...

But at the same time, I made so many attempts in 2023 at experimenting with a new typesetter and pagebuilder (I briefly alluded to it here: sile-typesetter/sile#2166 (comment)), that... well, who knows, perhaps something might came out of it... It's a deep rewriting, however, compatibility-breaking with many core packages,¹ which I might end up throwing away: Such a "fork" (there's no better word...) is inherently hard, and a single developer cannot do much in these areas...

Footnotes

1. I don't understand why SILE just keeps those broken packages, hardly maintained and impending on the capability to move forward, such as "linespacing", "gutenberg", "grid" (ouch! Another one bites the dust... but it never worked https://github.com/sile-typesetter/sile/issues/1720), etc. Sometimes one has to cut old branches of a tree. ↩

[davidchisnall]

Thanks for the detailed response. I think my requirements for figures are simpler than the general case for floats for two reasons:

• I'm using a single-column layout with full-width figures, so the only degrees of freedom are move-down and move-up.
• I am following the conventional rule that figures must appear after the first reference to them, which means that the only degree of freedom for the figure is to move it down.

I think this should allow a simple greedy LaTeX-like approach to get good(ish) results, where figures appear in one of two places:

• Their original location.
• The top of the next page, if the current location would result in an overfull page, with paragraphs moved up to fill in the space.

Ideally, floats would not be reordered, so if moving a figure down required moving it past another float then they'd move together and, as a last resort, produce a page of figures ( / listings / tables).

I am not familiar with the internals of SILE's page layout engine though, so I don't have a good intuition of how hard this would be to implement: is it a simple 'move and redraw' call or a complete rewrite of some core logic?

[Omikhleia]

The top of the next page, if the current location would result in an overfull page, with paragraphs moved up to fill in the space.

But what if the "moved up" content is a table, which is page-split, and with headers repeated? The devil is always in the details ;)

(Not all tables can be considered as float, especially those spanning multiple pages anyway)

[davidchisnall]

But what if the "moved up" content is a table, which is page-split, and with headers repeated?

I consider tables that need to be split across pages bad style for anything that isn't a datasheet, so this would not be a problem for me. If I have tables that need splitting, they get split into something more concise.

(Not all tables can be considered as float, especially those spanning multiple pages anyway)

This is why LaTeX has \table and \tabular, one for the table that has a caption and floats, the other for the table data itself, which may be used outside of \table and split.