How to position a figure with a colored page background on the opposite page of a section head?

Question

After having tried pretty much every idea that I could think of in the last two days, I would like to ask for help here. In a nutshell, I would like to achieve the following in ConTeXt:

• Position an external figure on the opposite (left) page of a section head.
• Colorize that whole left page.

Here's an MWE:

\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=]
\definecolor[warmwhite] [r=0.94, g=0.93, b=0.90]
\definehead[PageSection][section]
\setuphead[PageSection][
  page=right,
  continue=no,
  style=\ssb\bf,
]
\definelayer[emphpage][
  x=0mm,
  y=0mm,
  width=\paperwidth,
  height=\paperheight,
  repeat=yes,
]
\setlayer[emphpage][]{
  \framed[
    frame=off,
    width=210mm,
    height=297mm,
    background=color,
    backgroundcolor=warmwhite,
  ]{}
}
\definefloat[leftpagefloat][leftpagefloats][graphic]
\setupfloat[leftpagefloat][
  default={leftpage,header,footer},
]
\def\LeftPageFigure#1%
  {
  \setupbackgrounds[page][
    background=emphpage,
  ]
  \placeleftpagefloat[]{}{
    \offset[
      leftoffset=\dimexpr -\rightmarginwidth - \lineheight \relax,
    ]{
      \externalfigure[#1][
        width=\dimexpr \textwidth + \rightmarginwidth + \lineheight \relax,
        height=\textheight,
        factor=fit,
      ]
    }
  }
}
\starttext
\chapter{Testing Full Page Figures}
\dorecurse{3}{\input{knuth}}
\PageSection{First Section: Image should be on opposite page}
\LeftPageFigure{https://via.placeholder.com/1600x900.png?text=Wide+Test+Image}
\dorecurse{3}{\input{knuth}}
\PageSection{Second section}
\dorecurse{3}{\input{knuth}}
\stoptext

And here's what this looks like:

As you can see, there's two problems:

• The figure is positioned on the next left page, although the page to the left of the current section is totally free. Why is that the case?
• How can I colorize the page under the figure – and only that page? (By the way: Removing the line repeat: yes, results in the background not appearing at all.)

Any help would be much appreciated.

Answer 1

This can be one way. The left page is done in MetaFun. The figure is called via a user variable.

\setuppagenumbering
  [alternative=doublesided,
   location=]
\setupexternalfigures
  [location={local,global,default}]
\definecolor
  [warmwhite]
  [r=0.94, g=0.93, b=0.90]
\startuseMPgraphic{section}
  StartPage ;
    fill Page withcolor "warmwhite" ;
    picture img ;
    img := image(
      draw externalfigure "\namedstructureuservariable{section:+}{figure}" 
    ) xsized 0.8PaperWidth ;
    img := img shifted -(center img)
               shifted 0.5(PaperWidth,PaperHeight) ;
    draw img ;
  StopPage ;
\stopuseMPgraphic
\definelayer
  [section]
  [width=\paperwidth,
   height=\paperheight]
\defineoverlay
  [pagebackground]
  [\directsetup{pagebackground}]
\setupbackgrounds
  [page]
  [background=pagebackground]
\startsetups pagebackground
  \doifelsemode {sectionpage} {
    \setlayer
      [section]
      [preset=lefttop]
      {\useMPgraphic{section}}
      \globaldisablemode[sectionpage]
     } {
     }
     \placelayer[section]
\stopsetups
\startsetups section:before
  \page[even]
  \globalenablemode[sectionpage]
  \null
  \page[yes]
\stopsetups
\setuphead
  [section]
  [before=\setup{section:before}]
\starttext
  \startsection[title=First section][figure={hacker}]
    \dorecurse{10}{\samplefile{douglas} }
  \stopsection
  \startsection[title=Second section][figure={cow}]
    \dorecurse{10}{\samplefile{knuth} }
  \stopsection
\stoptext

Here is how a section page comes out:

Here is an overview over more pages: