4

Compare this

\nonstopmode
\tracingonline=1

\showboxdepth=\maxdimen
\showboxbreadth=\maxdimen

\output={\shipout\vbox{
  \unvbox255
  \showlists
}}

\nopagenumbers
\noindent A
\bye

and this

\nonstopmode
\tracingonline=1

\showboxdepth=\maxdimen
\showboxbreadth=\maxdimen

\newbox\pagebox
\let\oldshipout\shipout
\def\shipout{\global\let\shipout\oldshipout\afterassignment\myboat\setbox\pagebox=}
\def\myboat{\aftergroup\myship}
\def\myship{\setbox\pagebox=\vbox{
  \unvbox\pagebox
  \showlists
}\shipout\box\pagebox}

\nopagenumbers
\noindent A
\bye

In the second trace we have some additional output compared to that of the first example, namely:

  • Additional output of the second example before identical output of both examples:
\vbox(0.0+0.0)x469.75499, glue set 14.0fil
.\glue -22.5
.\hbox(8.5+0.0)x469.75499, glue set 469.75499fil
..\vbox(8.5+0.0)x0.0
..\glue 0.0 plus 1.0fil
.\glue 0.0 plus 1.0fil minus 1.0fil
\vbox(643.20255+0.0)x469.75499, glue set 633.20255fill
  • Additional output of the second example after identical output of both examples:
\glue(\baselineskip) 24.0
\hbox(0.0+0.0)x469.75499, glue set 469.75499fil
.\glue 0.0 plus 1.0fil

where 643.20255 equals 8.9in (i.e., 11 - 1 - 1 - 0.1) and 469.75499 equals 6.5in (i.e., 8.5 - 1 - 1)

The question is about this additional output - what does it mean and why does it appear in the second example?

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1 Answer 1

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4

In the first case you are just seeing box255, in the second case you are using the plain tex output routine and hooking into \shipout so you are seeing

\shipout\vbox{\makeheadline\pagebody\makefootline}%

and the extra boxes you see are the headline and footline which are white because of \nopagenumbers but the box and glue structure is still there.

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6
  • What do all that glues in the headline mean?
    – Igor Liferenko
    Aug 27, 2015 at 21:25
  • @IgorLiferenko it's directly the output of plain.tex, no computation is involved, for example the headline is defined by \def\makeheadline{\vbox to\z@{\vskip-22.5\p@ \line{\vbox to8.5\p@{}\the\headline}\vss}\nointerlineskip} and \the\headline is empty in this case, so you get exactly the boxes and glue that you show.
    – David Carlisle
    Aug 27, 2015 at 21:27
  • Can't be a box (in this case the footline box) just empty, without the need for glue in it?
    – Igor Liferenko
    Aug 27, 2015 at 21:31
  • @IgorLiferenko you can make such a box but plain tex doesn't: the footline is \def\makefootline{\baselineskip24\p@\lineskiplimit\z@\line{\the\footline}} it doesn't not make the box just because \the\footline is empty
    – David Carlisle
    Aug 27, 2015 at 21:32
  • If we want to remove footline completely, \nopagenumbers is not enough. What else should be done to suppress that \glue(\baselineskip) 24.0?
    – Igor Liferenko
    Aug 28, 2015 at 0:35

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