How to achieve line above right-aligned caption in floating figure in ConTeXt?

Question

I would like to achieve, what in Adobe InDesign is called a paragraph rule over a figure's caption, i.e a line on top of the caption, while keeping the caption itself right-aligned (actually aligned to the inner edge).

Here's an MWE (note that this MWE is similar to what I posted for a previous question and will soon be reworked to incorporate the suggestions I received there):

\setuppapersize[A4][A4]
\setuplayout
[
  backspace=11.67mm,        width=131.25mm,
  topspace=21.21mm,         height=254.57mm,
  headerdistance=13pt,      header=13pt,
  footerdistance=13pt,      footer=13pt,
  rightmargindistance=13pt, rightmargin=39.5mm,
]
\setuppagenumbering[alternative=doublesided, location=]
\setuphead[section][
  page=right,
  continue=no,
  style=\ssb\bf,
]
\definefloat[pagefloat][pagefloats][graphic]
\setupfloat[pagefloat][
  default={page,header,footer},
]
\setupcaption[pagefloat][
  width=max,
  location=top,
  topframe=on,
  toffset=\dimexpr 0.25\lineheight \relax,
  headstyle=\ss\bf,
  style=\tfx,
  align=inner,
  spaceinbetween=nowhite,
]
\def\PageFigure#1#2%
  {
  \placepagefloat[]{#2}{
    \offset[
      leftoffset=\dimexpr -\rightmarginwidth - \lineheight \relax,
    ]{
      \externalfigure[#1][
        width=\dimexpr \textwidth + \rightmarginwidth + \lineheight \relax,
        height=\textheight,
        factor=fit,
      ]
    }
  }
}
\starttext
\chapter{Testing Ruled Captions}
\dorecurse{3}{\input{knuth}}
\PageFigure{https://via.placeholder.com/1600x900.png?text=Wide+Test+Image}{First line\Second line of the caption}
\section{First Section}
\dorecurse{4}{\input{knuth}}
\stoptext

This is what it looks like, when I remove the line topframe=on,: This is how I would like to align the text.

But here's, what it looks like with the topframe on:

Why does merely adding the top frame change the alignment of the text? Can this be prevented or, better even, adjusted separately?

Any help would be much appreciated.

Answer 1

Not sure that I understand what you want, but maybe something like this? Note the location={top,right}. In your definition of PageFigure you had spurious spaces (try the example with your definition, and you will see how they show). Also, setting the size of the paper to A4 is not necessary, that is the default.

I first forgot. The topframe=on makes your code go into a different path (with a frame), and that is responsible for the effect you see.

\setuppapersize[A4][A4]
\setuplayout
[
  backspace=11.67mm,        width=131.25mm,
  topspace=21.21mm,         height=254.57mm,
  headerdistance=13pt,      header=13pt,
  footerdistance=13pt,      footer=13pt,
  rightmargindistance=13pt, rightmargin=39.5mm,
]
\setuppagenumbering[alternative=doublesided, location=]
\setuphead[section][
  page=right,
  continue=no,
  style=\ssb\bf,
]
\definefloat[pagefloat][pagefloats][graphic]
\setupfloat[pagefloat][
  default={page,header,footer},
]
\setupcaption[pagefloat][
   location={top,right},
   topframe=on,
   toffset=0.25\lineheight,
   headstyle=\ss\bf,
   style=\tfx,
   align=inner,
   spaceinbetween=nowhite,
]
\starttexdefinition PageFigure #1#2
  \placepagefloat
    {#2}
    {\offset
      [leftoffset=\dimexpr - \rightmarginwidth - \lineheight \relax] 
      {\externalfigure
         [#1]
         [width=\dimexpr \textwidth + \rightmarginwidth + \lineheight \relax,
          height=\textheight,
          factor=fit]}}
\stoptexdefinition
\showframe
\starttext
\chapter{Testing Ruled Captions}
\dorecurse{3}{\input{knuth}}
\PageFigure{https://via.placeholder.com/1600x900.png?text=Wide+Test+Image}{First line\par Second line of the caption}
\section{First Section}
\dorecurse{4}{\input{knuth}}
\stoptext

Update Hans has now opened up the possibilities a bit. Note the loffset that will take care of the extra piece of the bar.

In addition, I have done a few modifications:

• Added location=inner to the float, and also removed the offset (I did not understand the \lineheight you had there).
• Changed to \start...\stop syntax.

If we (you) are lucky, there will be a new release soon where this is included.

\useMPlibrary[dum]
\setuplayout
  [backspace=11.67mm,        width=131.25mm,
   topspace=21.21mm,         height=254.57mm,
   headerdistance=13pt,      header=13pt,
   footerdistance=13pt,      footer=13pt,
   rightmargindistance=13pt, rightmargin=39.5mm]
\setuppagenumbering
  [alternative=doublesided,location=]
\setuphead
  [section]
  [page=right,
   continue=no,
   style=\ssb\bf]
\defineframed
  [MyCaptionFramed]
  [align={lohi,flushleft},
   width=fit,
   frame=off,
   strut=no,
   topframe=on,
   loffset=3cm,
   toffset=0.25\lineheight]
\starttexdefinition protected MyCaption #1
  \MyCaptionFramed {
    % we ignore #1 as we want to unpack
    \unvbox\floatcaptionbox
  }
\stoptexdefinition
\definefloat[pagefloat][pagefloats][graphic]
\setupfloat
  [pagefloat]
  [default={page,header,footer},
   location=inner]
\setupcaption
  [pagefloat]
  [location={top,inner},
   align=inner,
   command=\MyCaption,
   spaceinbetween=nowhite]
\starttexdefinition PageFigure #1#2
  \startplacefloat
    [pagefloat]
    [title={#2}]
      \externalfigure
        [#1]
        [width=\dimexpr \textwidth + \rightmargintotal \relax,
         height=\textheight,
         factor=fit]
  \stopplacefloat
\stoptexdefinition
\showframe
\starttext
\startchapter[title={Testing Ruled Captions}]
\dorecurse{3}{\input{knuth}}
\PageFigure
    {some dummy}
    {First line\Second line of the caption}
\startsection[title={First Section}]
\dorecurse{4}{\input{knuth}}


\stopsection
\stopchapter
\stoptext

Result: