ConTeXt: Calculating remaining space on page like testpage

The pagechecker mechanism

The \testpage command uses a mechanism called pagechecker to compute the remaining space on the page.

You can use the mechanism to place content on the page with the inbetween key. With the lines key you specifiy how many lines should be remain on the page to place the content.

The result of the calculated number of lines depend on the method key which accepts a number between 0 and 3.

\startsetups [fillpage]
    \vfill
    \midaligned{\externalfigure[dummy][lines=4]}
    \vfill
\stopsetups

\definepagechecker [fillpage] [method=2,lines=8,inbetween=\directsetup{fillpage}]

\showgrid

\setuppapersize[A7]

\starttext

\input jojomayer

\checkpage [fillpage] [lines=9]

\page

\input jojomayer

\checkpage [fillpage]

\stoptext

Manual calculation of the remanining space

When you want more control about the measurement of the remaining space you can use the two registers \pagegoal and \pagetotal.

\pagegoal

Height of the text block which forces a page break when reached.

\pagetotal

Height of the current material on the page.

---

To get the height of the remaining space you substract \pagetotal from \pagegoal.

\showgrid

\setuppapersize[A7]

\starttext

\input jojomayer

\the\dimexpr\pagegoal-\pagetotal\relax

\stoptext

To get he number of lines for the calculated value you can use the \getnoflines or \getroundednoflines commands. The resulting number is stored in the \noflines register.

Result with \getnoflines

\showgrid

\setuppapersize[A7]

\starttext

\input jojomayer

\getnoflines{\dimexpr\pagegoal-\pagetotal\relax}
\number\noflines

\stoptext

Result with \getroundednoflines

\showgrid

\setuppapersize[A7]

\starttext

\input jojomayer

\getroundednoflines{\dimexpr\pagegoal-\pagetotal\relax}
\number\noflines

\stoptext

Although Wolfgang answered the crux of the question by explaining how page space is counted, it took me a couple more Google searches to figure out how to actually use the noflines register in an if statement. The following code seems to produce what I was looking for:

\setuppapersize[A5]

\define[1]\FillPage{
\par
\getnoflines{\the\dimexpr\pagegoal-\pagetotal\relax}
\ifnum\number\noflines>#1\relax
  \vfill
  \midaligned{\externalfigure[dummy][height=0.3\textheight]}
  \vfill
\fi
\page[yes]
}

\starttext

\input knuth

\FillPage{10}

\input knuth
\input knuth

\FillPage{10}

\input knuth

\stoptext