refined structure fullsize image macros

structure_images.tex

% macros=mkvi
%
% pretty dummy graphics
\useMPlibrary
  [dum]

% fake text
\usemodule
  [visual]

\setuppagenumbering
  [alternative=doublesided]

\setupbackgrounds
  [page]
  [background=chapterimages]

\definelayer
  [chapterimages]

\setuphead
  [chapter]
  [before=\setups{s:chapter}]

\definemakeup
  [chapterimage]
  [page]

\definelayout
  [chapterimage] %% invoked automagically by the makeup
  [page]

\setupmakeup
  [chapterimage]
  [page=left,]    %% images on verso

\unprotect

\newcount \image_counter
\newdimen\image_ht
\newdimen\image_wd

\def\defineimagelist{\dodoubleempty\do_define_image_list}

\def\do_define_image_list[#structurename][#lst]{%
  \edef\current_structure{#structurename}%
  \scratchcounter0
  \ifsecondargument
    \processcommalist[#lst]\define_structure_image
  \fi%
}

%% We-F¢ll do this in an old fashioned manner with macros for each image.
\def\define_structure_image#img{%
  \advance\scratchcounter\plusone
  \expandafter\def%
    \csname\current_structure _image_\the\scratchcounter\endcsname{%
    \externalfigure[#img]%
  }%
}

\unexpanded\def\pickimage{\dosingleempty\do_pick_image}

\def\do_pick_image[#structurename]{%
  \global\advance\image_counter\plusone
  \ifcsname#{structurename}_image_\the\image_counter\endcsname
    \expandafter\let\expandafter%
      \current_structure_image\csname#{structurename}_image_\the\image_counter\endcsname
  \else
    \let\current_structure_image\structure_image_dummy
  \fi%
  \setbox\scratchbox\vbox{\current_structure_image}%
  \image_ht\ht\scratchbox
  \image_wd\wd\scratchbox
  \ctxcommand{test_image_dimensions()}
    {\current_structure_image[width=\paperwidth]}
    {\current_structure_image[height=\paperheight]}%
}

\def\structure_image_dummy{\externalfigure[dummy]}%

%% Since \externalfigure[foo][factor=max] doesn¢t work anymore, we¢ll
%% have to bypass TEX¢s inadequate integer arithmetic through Lua.
\startluacode
  local td = tex.dimen
  commands.test_image_dimensions = function ()
    local pht, pwd = td.paperheight, td.paperwidth
    local iht, iwd = td.image_ht,    td.image_wd
    local result = iht / iwd > pht / pwd
    -- print(iht, iwd, pht, pwd, result)
    commands.doifelse(result)
  end
\stopluacode

\protect

%% This gets shortened to setting the makeup and calling the image
%% selector.
\startsetups s:chapter
  \startchapterimagemakeup
    \pickimage[chapter]
  \stopchapterimagemakeup
\stopsetups

\starttext

\defineimagelist[chapter][cow,mill,hacker]

\dorecurse{1}{
  \startchapter [title=First Chapter]
    \startsection [title=Some section]
      \fakewords{500}{1000}
    \stopsection
  \stopchapter

  \startchapter [title=Second Chapter]
    \startsection [title=Some other section]
      \fakewords{500}{1000}
    \stopsection
  \stopchapter

  \startchapter [title=Third Chapter]
    \startsection [title=Some new section]
      \fakewords{500}{1000}
    \stopsection
  \stopchapter
}

\stoptext