Fixed distance from top of the page / SILE internals documentation?

[raphCode]

Hi, I would like to typeset the title page of my thesis, like this:

But I fail to add the vertical space at the beginning of the page.

I looked at the source and found this:

sile/typesetters/base.lua

Lines 571 to 574 in 1a86520

	-- Annoyingly, explicit glue *should* disappear at the top of a page.
	-- if you don't want that, add an empty vbox or something.
	if not self.frame.state.totals.pastTop and not line.discardable and not line.explicit then
	self.frame.state.totals.pastTop = true

I then tried:

\begin{document}
\vbox{}
\skip[height=30pt]
\center{hello world}
\end{document}

Also putting the \skip into the \vbox did not work.

I read through the source trying to grasp how the data / content flows through the typesetter (and where exactly my whitespace is dropped), but could not understand it.
Is there a high-level overview how the typesetter / data model in SILE works? The in-source comments were not sufficient for me.

Also, why does \vbox{content goes here} not produce visible output?

[Omikhleia]

Quick answer:
Using an \hbox{} instead of a \vbox{} does the trick (more or less).

Longer delails:
\vbox{} doesn't output anything (it collates it potential content, wraps it via a temporary typesetter state in a vbox node (actually a vbox list), and returns it for consumption by other routines (... which might make use of it in Lua - e.g. it's used in the footnotes logic -, but it won't be of any use in a SIL document). These things are really arcane low-level constructs.

[Omikhleia]

BTW the in-code comment mentioning an empty vbox is not wrong per se. At this point, the typesetter is processing a list of vboxes (lines are vboxes, possibly containing hboxes and other horizontal contents).

[raphCode]

Thank you, \hbox works!

In generally I am interested in the inner workings of the tools I use, and thus my desire to understand SILE better still stands.
I feel a glossary of the terms used in the code (like you just wrote about vboxes) and some explanation for important functions would go a long way.
Especially on a typesetting system that is designed to be hacked.

If I can figure out enough of it I may write something down eventually.

[Omikhleia]

Your are welcome!
The problem would be maintaining such a documentation - lots of things may still change / evolve. In-code comments are also lacking in many places in the code (IMHO, the maintainers could disagree - but sometimes the intent behind some tricky code is far from obvious and could be better explained).