%D \module
%D   [       file=pack-lyr,
%D        version=2000.10.20,
%D          title=\CONTEXT\ Packaging Macros,
%D       subtitle=Layers,
%D         author=Hans Hagen,
%D           date=\currentdate,
%D      copyright={PRAGMA / Hans Hagen \& Ton Otten}]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See mreadme.pdf for
%C details.

\writestatus{loading}{ConTeXt Packaging Macros / Layers}

% todo : first / last / next / +... => page key
%        test on left/right box when no doublesided option given
%        use \ifcsname instead of doifvalue

\unprotect

% When being backgrounds layers get the background offset
% displacement. Should be an option, on by default
% (compatibility).

% positie=forceer == ja maar dan ook in status=herhaal

%D The layering mechanism implemented here is independent of
%D the output routine, but future extensions may depend on a
%D more close cooperation.

%D First we overload a macro from \type {core-rul}. From now on
%D we accept a (optional) argument: the specific layer it
%D will go in. This means that we can move an overlay from one
%D background to the other using the dimensions of the parent.

\ifx\undefined\defineoverlay \message{loaded to early} \wait \fi

\unexpanded\def\defineoverlay
  {\dotripleempty\dodefineoverlay}

\def\dodefineoverlay[#1][#2][#3]% overlay [layer] content
  {\ifthirdargument
    %\writestatus{BEWARE}{This (overlay definition) has changed!}% temp
     \def\docommand##1{\setvalue{\??ov##1}{\setlayer[#2]{\executedefinedoverlay{##1}{#3}}}}
   \else
     \def\docommand##1{\setvalue{\??ov##1}{\executedefinedoverlay{##1}{#2}}}%
   \fi
   \processcommalist[#1]\docommand}

%D When tracing is turned on, a couple of boxes will
%D show up as well as the reference point.

\newif\iftracelayers % \tracelayerstrue

%D This handy constant saved some string memory.

\def\@@layerbox{@@layerbox}

%D \macros
%D   {definelayer}
%D
%D Each layer gets its own (global) box. This also means that
%D the data that goes into a layer, is typeset immediately.
%D Each layer automatically gets an associated overlay,
%D which can be used in any background assignment.

% todo : links/rechts

\unexpanded\def\definelayer
  {\dodoubleargument\dodefinelayer}

\def\dodefinelayer[#1][#2]% \zeropoint ipv \!!zeropoint
  {\setuplayer
     [#1]
     [\c!doublesided=,\c!preset=,
      \c!state=\v!start,\c!direction=\v!normal,\c!option=,
      \c!x=\zeropoint,\c!y=\zeropoint,\c!position=\v!no,
      \c!line=0,\c!column=0,
      \c!width=\nextboxwd,\c!height=\nextboxht,
      \c!offset=\zeropoint,\c!rotation=, % geen 0 !
      \c!hoffset=\zeropoint,\c!voffset=\zeropoint,
      \c!dx=\zeropoint,\c!dy=\zeropoint,
      \c!location=rb,\c!position=\v!no,\c!page=,
      \c!method=\v!overlay,
      \c!sx=1,\c!sy=1,\c!corner=,#2]%
   \doifvalue{\??ll#1\c!doublesided}\v!yes
     {\dopresetlayerbox{\v!left #1}%
      \dopresetlayerbox{\v!right#1}}%
   \dopresetlayerbox{#1}%
   \defineoverlay[#1][\composedlayer{#1}]}

\def\dopresetlayerbox#1%
  {\ifcsname\@@layerbox#1\endcsname
     \resetlayer[#1]%
   \else
     \expandafter\newbox\csname\@@layerbox#1\endcsname
   \fi}

%D \macros
%D   {setuplayer}
%D
%D After a layer is defined, you can change its
%D characteristics.

\unexpanded\def\setuplayer
  {\dodoubleargument\dosetuplayer}

\def\dosetuplayer[#1][#2]%
  {\def\docommand##1{\getparameters[\??ll##1][#2]}%
   \processcommalist[#1]\docommand}

%D \macros
%D   {setlayer}
%D
%D Data is moved into a layer with the following macro. When
%D \type {position} is set, relative positioning is used, with
%D the current point as reference point. Otherwise the topleft
%D corner is used as reference point.
%D
%D \starttyping
%D \setlayer [identifier] [optional parameters] {data}
%D \stoptyping

\newcount\currentlayerdata

\let\currentlayerwidth \!!zeropoint
\let\currentlayerheight\!!zeropoint

\def\setcurrentlayerdimensions
  {\dodoubleempty\dosetcurrentlayerdimensions}

\def\dosetcurrentlayerdimensions[#1][#2]% name left|right
  {\edef\currentlayerwidth {\thelayerwidth {#2#1}}%
   \edef\currentlayerheight{\thelayerheight{#2#1}}}

\def\thelayerwidth #1{\the\wd\executeifdefined{\@@layerbox#1}\emptybox}
\def\thelayerheight#1{\the\ht\executeifdefined{\@@layerbox#1}\emptybox}

\def\setlayer
  {\dotripleempty\dosetlayer}

% \def\dosetlayer[#1][#2][#3]% #4 == box do \fi is ok
%   {\doifelsevalue{\??ll#1\c!state}\v!stop
%      {\dowithnextbox\donothing\hbox}
%      {\ifthirdargument
%         \dodosetlayer[#1][#2][#3]%
%       \else
%         \dodosetlayer[#1][][#2]%
%       \fi}}

\def\dosetlayer[#1][#2][#3]% #4 == box do \fi is ok
  {\doifelsevalue{\??ll#1\c!state}\v!stop
     {\dowithnextbox\donothing\hbox}
     {\ifthirdargument
        \dodosetlayer[#1][#2][#3]%
      \else
        \doifassignmentelse{#2}
          {\dodosetlayer[#1][][#2]}%
          {\dodosetlayer[#1][#2][]}%
      \fi}}

\def\dodosetlayer[#1][#2][#3]% #2 = links/rechts
  {\bgroup
   \recalculatebackgrounds % brrr
   \global\advance\currentlayerdata\plusone
   \forgetall
   \dontcomplain
   \doifvalue{\??ll#1\c!option}\v!test\tracelayerstrue
   \iftracelayers\traceboxplacementtrue\fi
   \dowithnextbox{\dodosetlayerindeed{#1}{#2}{#3}\egroup}\hbox}

\def\dodosetlayerindeed#1#2#3% #2 = links/rechts
  {\ifcsname\@@layerbox#1\endcsname % nb: odd/even discard, left/right not
     \edef\@@layerloc{#2}%
     \ifx\@@layerloc\v!even
       \ifodd\realpageno
       %  discard nextbox
       \else
         \dododosetlayer[#1][\v!left][#3]%
       \fi
     \else\ifx\@@layerloc\v!odd
       \ifodd\realpageno
         \dododosetlayer[#1][\v!right][#3]%
      %\else
       %  discard nextbox
       \fi
     \else
       \dododosetlayer[#1][#2][#3]%
     \fi\fi
   \else
     \writestatus{layer}{unknown layer #1}%
   \fi}

\newbox\layerbox

\newdimen\@@layerxsiz
\newdimen\@@layerysiz
\newdimen\@@layerxoff
\newdimen\@@layeryoff
\newdimen\@@layerxpos
\newdimen\@@layerypos

\let\lastlayerxpos\!!zeropoint
\let\lastlayerypos\!!zeropoint
\let\lastlayerwd  \!!zeropoint
\let\lastlayerht  \!!zeropoint
\let\lastlayerdp  \!!zeropoint

% todo left/right

\def\setlastlayerpos#1%
  {\edef\layerpage{\MPp{lyr:\the\currentlayerdata}}%
   \xdef\lastlayerxpos{\the\dimexpr-\MPx{lyr:#1:\layerpage}+\MPx{lyr:\the\currentlayerdata}\relax}%
   \xdef\lastlayerypos{\the\dimexpr \MPy{lyr:#1:\layerpage}-\MPy{lyr:\the\currentlayerdata}\relax}}

\unexpanded\def\definelayerpreset
 {\dodoubleargument\dodefinelayerpreset}

% \def\dodefinelayerpreset[#1][#2]%
%   {\setvalue{\??ll\??ll#1}{\dopresetlayer{#2}}}
%
% more fun: \definelayerpreset[whatever][lefttop]

\def\dodefinelayerpreset[#1][#2]%
  {\doifassignmentelse{#2}
     {\setvalue{\??ll\??ll#1}{\dopresetlayer{#2}}}
     {\setvalue{\??ll\??ll#1}{\csname\??ll\??ll#2\endcsname}}}

\def\dopresetlayer#1#2#3% #1=list #2=tag #3=list
  {\getparameters[\??ll#2][#1,#3]}

\letempty\currentlayer

\def\layerparameter#1{\csname\??ll\currentlayer#1\endcsname}

\newdimen\layerwidth
\newdimen\layerheight

\chardef\@@lacome=1 % LAyerCOnstructionMEthod / temp, will be default

\def\dododosetlayer[#1][#2][#3]% will be sped up
  {% we use the global width, never change this
   \def\currentlayer{#1}%
   \@@layerxsiz\layerparameter\c!width
   \@@layerysiz\layerparameter\c!height
   \layerwidth \@@layerxsiz
   \layerheight\@@layerysiz
   % preroll
   \getparameters[\??ll\currentlayer][#3]%
   %
   % \executeifdefined{\??ll\??ll\layerparameter\c!preset}\gobbletwoarguments\currentlayer{#3}%
   %
   \edef\@@currentlayerpreset{\layerparameter\c!preset}%
   \ifcsname\??ll\??ll\@@currentlayerpreset\endcsname\csname\??ll\??ll\@@currentlayerpreset\endcsname\currentlayer{#3}\fi
   %
   \doif{\layerparameter\c!position}\v!overlay  % slow, use \dosetvalue instead
     {\getparameters[\??ll\currentlayer][\c!width=\zeropoint,\c!height=\zeropoint,\c!position=\v!yes]}%
   \doifsomething{\layerparameter\c!rotation}% todo: use direct lowlevel call
     {\setbox\nextbox\hbox{\rotate[\c!location=\v!high,\c!rotation=\layerparameter\c!rotation]{\flushnextbox}}}%
   % no, not local
   % \@@layerxsiz\layerparameter\c!width
   % \@@layerysiz\layerparameter\c!height
   % never change that
   \@@layerxpos\layerparameter\c!x
   \@@layerypos\layerparameter\c!y
   \doifelse{\layerparameter\c!hoffset}\v!max{\@@layerxoff\@@layerxsiz}{\@@layerxoff\layerparameter\c!hoffset}%
   \doifelse{\layerparameter\c!voffset}\v!max{\@@layeryoff\@@layerysiz}{\@@layeryoff\layerparameter\c!voffset}%
   % dx/dy are internal context ones and can be used in preset
   \advance\@@layerxoff\dimexpr\layerparameter\c!offset+\layerparameter\c!dx\relax
   \advance\@@layeryoff\dimexpr\layerparameter\c!offset+\layerparameter\c!dy\relax
   \@@layerxpos\layerparameter\c!sx\@@layerxpos
   \@@layerypos\layerparameter\c!sy\@@layerypos
   \@@layerxoff\layerparameter\c!sx\@@layerxoff
   \@@layeryoff\layerparameter\c!sy\@@layeryoff
   \edef\@@currentlayerposition{\layerparameter\c!position}%
   \ifx\@@currentlayerposition\v!yes   % combine ^
     \setlastlayerpos{#2\currentlayer}% sets \layerpage; todo l/r %%%%%%%%%%%%
     \@@layerxpos\lastlayerxpos
     \@@layerypos\lastlayerypos
     \letgvalue{\??ll\currentlayer\layerpage\c!position}\v!yes
     \letgvalue{\??ll\currentlayer\c!state}\v!start % needed ?
     \setbox\layerbox\vbox to \@@layerysiz{\hbox to \@@layerxsiz{\xypos{lyr:\the\currentlayerdata}\hss}\vss}%
   \else
     \setbox\layerbox\emptybox
     \globallet\lastlayerxpos\!!zeropoint
     \globallet\lastlayerypos\!!zeropoint
     \normalexpanded{\noexpand\doifinset{\v!bottom}{\layerparameter\c!corner}}\dosetlayerbottompositions
     \normalexpanded{\noexpand\doifinset{\v!right }{\layerparameter\c!corner}}\dosetlayerrightpositions
     \normalexpanded{\noexpand\doifinset{\v!middle}{\layerparameter\c!corner}}\dosetlayermiddlepositions
     \edef\layerpage{\layerparameter\c!page}%
   \fi
   \ifx\layerpage\empty \else % is expanded
     \edef\layerpage{:\layerpage}%
     \ifcsname\@@layerbox#2\currentlayer\layerpage\endcsname \else
       \expandafter\newbox\csname\@@layerbox#2\currentlayer\layerpage\endcsname
     \fi
   \fi
   \chardef\layerpagebox\csname\@@layerbox#2\currentlayer\layerpage\endcsname
   \ifvoid\layerpagebox
     \gsetboxllx\layerpagebox\zeropoint
     \gsetboxlly\layerpagebox\zeropoint
   \fi
   \global\setbox\layerpagebox\vbox %to \layerparameter\c!height % new, otherwise no negative y possible
     {\offinterlineskip
     %postpone, to after nextboxwd correction % \hsize\layerparameter\c!width % new, keep box small
     %\ifvoid\csname\@@layerbox\currentlayer\layerpage\endcsname\else % why not #2#1
      \ifvoid\layerpagebox
        \let\lastlayerwidth \zeropoint
        \let\lastlayerheight\zeropoint
      \else
        \edef\lastlayerwidth {\the\wd\layerpagebox}%
        \edef\lastlayerheight{\the\ht\layerpagebox}%
        \ht\layerpagebox\zeropoint
        \dp\layerpagebox\zeropoint
        \wd\layerpagebox\zeropoint
        \doifnot{\layerparameter\c!direction}\v!reverse{\box\layerpagebox}%
      \fi
      % don't move
      \xdef\lastlayerwd{\the\nextboxwd}%
      \xdef\lastlayerht{\the\nextboxht}% % not entirely ok when grid !
      \xdef\lastlayerdp{\the\nextboxdp}% % not entirely ok when grid !
      % this code
      \doifelse{\layerparameter\c!location}\v!grid\donetrue\donefalse
      \ifdone
        \nextboxht\strutheight
        \nextboxdp\strutdepth
      \else
        \setbox\nextbox\hbox{\alignedbox[\layerparameter\c!location]\vbox{\flushnextbox}}%
      \fi
      \ifnum\layerparameter\c!line=\zerocount\else % no \ifcase, can be negative
        \advance\@@layerypos\dimexpr\layerparameter\c!line\lineheight+\topskip-\lineheight-\nextboxht\relax
      \fi
      \ifnum\layerparameter\c!column=\zerocount\else % no \ifcase, can be negative
        \advance\@@layerxpos\layoutcolumnoffset{\layerparameter\c!column}%
      \fi
      \ifdone
        \setbox\nextbox\hbox{\alignedbox[rb]\vbox{\flushnextbox}}%
      \fi
      % ll registration
      \scratchdimen\@@layerxpos
      \advance\scratchdimen\@@layerxoff
      \ifdim\scratchdimen<\getboxllx\layerpagebox
        \gsetboxllx\layerpagebox\scratchdimen
      \fi
      \ifcase\@@lacome\or % this test will become obsolete
        \advance\scratchdimen\nextboxwd
        \nextboxwd\ifdim\scratchdimen>\lastlayerwidth \scratchdimen \else \lastlayerwidth \fi
      \fi
      \scratchdimen\dimexpr\@@layerypos+\@@layeryoff\relax
      \ifdim\scratchdimen<\getboxlly\layerpagebox
        \gsetboxlly\layerpagebox\scratchdimen
      \fi
      % ll compensation
      \ifcase\@@lacome\or % this test will become obsolete
        \advance\scratchdimen\dimexpr\nextboxht+\nextboxdp\relax
        \nextboxht\ifdim\scratchdimen>\lastlayerheight \scratchdimen \else \lastlayerheight \fi
        \nextboxdp\zeropoint
      \fi
      % placement
      \hsize\layerparameter\c!width % new, keep box small
      \vbox to \layerparameter\c!height \bgroup
        \smashbox\nextbox
        \vskip\dimexpr\@@layerypos+\@@layeryoff\relax
        \hskip\dimexpr\@@layerxpos+\@@layerxoff\relax
        \flushnextbox
        \ifvoid\layerpagebox
          % already flushed
        \else
          % the reverse case % check !
          \vskip-\dimexpr\@@layerypos+\@@layeryoff\relax
          \box\layerpagebox
        \fi
      \egroup}%
   % when position is true, the layerbox holds the compensation and needs
   % to be placed; never change this !
   \ifvoid\layerbox\else\box\layerbox\fi}

\def\dosetlayerbottompositions
  {\ifnum\layerparameter\c!line=\zerocount\else % can be < 0
     \setevalue{\??ll\currentlayer\c!line}{\the\numexpr-\layerparameter\c!line+\layoutlines+\plusone\relax}%
   \fi
   \ifdim\@@layerysiz>\zeropoint
     \advance\@@layerypos-\@@layerysiz
     \@@layerypos-\@@layerypos
     \@@layeryoff-\@@layeryoff
   \fi}

\def\dosetlayerrightpositions
  {\ifnum\layerparameter\c!column=\zerocount\else % can be < 0
     \setevalue{\??ll\currentlayer\c!column}{\the\numexpr-\layerparameter\c!column+\layoutcolumns+\plusone\relax}%
   \fi
   \ifdim\@@layerxsiz>\zeropoint
     \advance\@@layerxpos-\@@layerxsiz
     \@@layerxpos-\@@layerxpos
     \@@layerxoff-\@@layerxoff
   \fi}

\def\dosetlayermiddlepositions
  {\ifdim\@@layerxsiz>\zeropoint \advance\@@layerxpos.5\@@layerxsiz \fi
   \ifdim\@@layerysiz>\zeropoint \advance\@@layerypos.5\@@layerysiz \fi}

%D Given the task to be accomplished, the previous macro is
%D not even that complicated. It mainly comes down to skipping
%D to the right place and placing a box on top of or below the
%D existing content. In the case of position tracking, another
%D reference point is chosen.

%D \macros
%D  {doifelselayerdata}

\def\doifelselayerdata#1%
  {\ifcsname\@@layerbox#1\endcsname
     \ifvoid\csname\@@layerbox#1\endcsname
       \@EAEAEA\secondoftwoarguments
     \else
       \@EAEAEA\firstoftwoarguments
     \fi
   \else
     \@EA\secondoftwoarguments
   \fi}

%D \macros
%D  {flushlayer}
%D
%D When we flush a layer, we flush both the main one and the
%D page dependent one (when defined). This feature is more
%D efficient in \ETEX\ since there testing for an undefined
%D macro does not takes hash space.

% todo: setups before flush, handy hook

% \unexpanded\def\flushlayer[#1]%
%   {\doifelsevalue{\??ll#1\c!state}\v!next
%      {\global\letvalue{\??ll#1\c!state}\v!start} % dangerous, stack-built-up
%      {\doifelsevalue{\??ll#1\c!state}\v!continue
%         {\global\letvalue{\??ll#1\c!state}\v!repeat} % dangerous, stack-built-up
%         {\doifelsevalue{\??ll#1\c!doublesided}\v!yes
%            {\doifundefinedelse{\@@layerbox#1}%
%               {\dodoflushlayerA[#1]}
%               {\doifbothsidesoverruled
%                  {\dodoflushlayerB\v!left [#1]}%  left
%                  {\dodoflushlayerB\v!right[#1]}% right
%                  {\dodoflushlayerB\v!left [#1]}}}%  left
%            {\dodoflushlayerA[#1]}}}}

\unexpanded\def\flushlayer[#1]% quite core, so optimized
  {\begingroup
   \forgetall
   \edef\currentlayer{#1}%
   \edef\@@currentlayerstate{\csname\??ll\currentlayer\c!state\endcsname}%
   \ifx\@@currentlayerstate\v!stop
     % nothing
   \else\ifx\@@currentlayerstate\v!next
     \global\expandafter\let\csname\??ll\currentlayer\c!state\endcsname\v!start  % dangerous, stack-built-up
   \else\ifx\@@currentlayerstate\v!continue
     \global\expandafter\let\csname\??ll\currentlayer\c!state\endcsname\v!repeat % dangerous, stack-built-up
   \else
     \edef\@@currentlayerdoublesided{\csname\??ll\currentlayer\c!doublesided\endcsname}%
     \ifx\@@currentlayerdoublesided\v!yes
       \ifcsname\@@layerbox#1\endcsname
         % we can make a dedicated one for this
         \doifbothsidesoverruled{\dodoflushlayerB\v!left}{\dodoflushlayerB\v!right}{\dodoflushlayerB\v!left}%
       \else
         \dodoflushlayerA
       \fi
     \else
        \dodoflushlayerA
     \fi
   \fi\fi\fi
   \endgroup}

%  \ifcase#1\else\writestatus{layer}{unknown layer #3}\fi

\def\dodoflushlayerA
  {\startoverlay
     {\ifcsname\@@layerbox\currentlayer           \endcsname\dodoflushlayer\plusone   \currentlayer            \fi}%
     {\ifcsname\@@layerbox\currentlayer:\realfolio\endcsname\dodoflushlayer\zerocount{\currentlayer:\realfolio}\fi}%
   \stopoverlay}

\def\dodoflushlayerB#1%
  {\startoverlay
     {\ifcsname\@@layerbox  \currentlayer           \endcsname\dodoflushlayer\plusone     \currentlayer            \fi}%
     {\ifcsname\@@layerbox  \currentlayer:\realfolio\endcsname\dodoflushlayer\zerocount  {\currentlayer:\realfolio}\fi}%
     {\ifcsname\@@layerbox#1\currentlayer           \endcsname\dodoflushlayer\plusone  {#1\currentlayer           }\fi}%
     {\ifcsname\@@layerbox#1\currentlayer:\realfolio\endcsname\dodoflushlayer\zerocount{#1\currentlayer:\realfolio}\fi}%
   \stopoverlay}

\def\dodoflushlayer#1#2% quite core, so optimized
  {\begingroup % already grouped
   \offinterlineskip
   \edef\@@currentlayermethod{\csname\??ll\currentlayer\c!method\endcsname}%
   \edef\@@currentlayeroption{\csname\??ll\currentlayer\c!option\endcsname}%
   % needed because we need to handle method but we should find a way to
   % speed this up
   \edef\@@currentlayerpreset{\csname\??ll\currentlayer\c!preset\endcsname}%
   \ifcsname\??ll\??ll\@@currentlayerpreset\endcsname\csname\??ll\??ll\@@currentlayerpreset\endcsname\currentlayer{}\fi
   %
   \ifx\@@currentlayeroption\v!test
     \tracelayerstrue
   \fi
   \iftracelayers
     \traceboxplacementtrue
   \fi
   \!!doneafalse
   \!!donebfalse
   \ifx\@@currentlayermethod\v!overlay\!!doneatrue\fi
   \ifx\@@currentlayermethod\v!fit    \!!donebtrue\fi
   \!!donectrue
   \ifcase#1\else
     \edef\@@currentlayerposition{\csname\??ll\currentlayer\c!position\endcsname}%
     \ifx\@@currentlayerposition\v!yes \else
       \edef\@@currentlayerrepeat{\csname\??ll\currentlayer\c!repeat\endcsname}%
     % \edef\@@currentlayerstate {\csname\??ll\currentlayer\c!state\endcsname}% actually this is already set
       \ifx\@@currentlayerrepeat\v!yes
         \!!donecfalse
       \else\ifx\@@currentlayerstate\v!repeat
         \!!donecfalse
       \fi\fi
     \fi
   \fi
   \chardef\layerbox\csname\@@layerbox#2\endcsname % \@@layerbox\currentlayer
   % we need to copy in order to retain the negative offsets for a next
   % stage of additions, i.e. llx/lly accumulate in repeat mode and the
   % compensation may differ each flush depending on added content
   \setbox\nextbox
     \if!!doneb
       \therepositionededlayerbox
     \else
       \if!!donec\box\else\copy\fi\layerbox % sorry for the delay due to copying
     \fi
   % todo: method=offset => overlayoffset right/down (handy for backgrounds with offset)
   \doifoverlayelse{#2}{\setlayoutcomponentattribute\v!layer{#2}}\resetlayoutcomponentattribute
   \iftracelayers \ruledvbox \else \vbox \fi \if!!donea to \overlayheight \fi \layoutcomponentboxattribute
     {\hbox \if!!donea to \overlaywidth \fi
        {\edef\@@currentlayerpageposition{\csname\??ll#2\realfolio\c!position\endcsname}%
         \ifx\@@currentlayerpageposition\v!yes\xypos{lyr:#2:\realfolio}\fi
         \box\nextbox
         \hss}%
      \vss}%
   \if!!donec
     \gsetboxllx\layerbox\zeropoint
     \gsetboxlly\layerbox\zeropoint
   \fi
   \endgroup}

\def\therepositionededlayerbox % assumes that \if!!donec is set (todo: use dedicated flags)
  {\vbox
     {\vskip-\getboxlly\layerbox
      \hskip-\getboxllx\layerbox
      \hsize-\dimexpr\getboxllx\layerbox-\wd\layerbox\relax
      \if!!donec\box\else\copy\fi\layerbox}}

% \definelayer[test][method=fit] \setupcolors[state=start] \tracelayerstrue
%
% \framed[framecolor=red,offset=overlay]{\setlayer[test]{aa}\setlayer[test][x=10pt]{g}\flushlayer[test]}
% \framed[framecolor=red,offset=overlay]{\setlayer[test]{aa}\setlayer[test][x=-10pt]{bb}\flushlayer[test]}
% \framed[framecolor=red,offset=overlay]{\setlayer[test][x=-20pt]{cccccc}\flushlayer[test]}
% \framed[framecolor=red,offset=overlay]{\setlayer[test]{dd}\setlayer[test][x=-20pt,y=-3pt]{eeeeee}\flushlayer[test]}

%D \macros
%D  {composedlayer,placelayer,tightlayer}
%D
%D This is a handy shortcut, which saves a couple of braces
%D when we use it as parameter. This name also suits better
%D to other layering commands.

\def\composedlayer#1{\flushlayer[#1]}

\let\placelayer\flushlayer

\def\tightlayer[#1]%
  {\hbox
     {\def\currentlayer{#1}% todo: left/right
      \setbox\nextbox\emptybox       % hoogte/breedte are \wd\nextbox/\ht\nextbox
      \hsize\layerparameter\c!width  % \overlaywidth   = \hsize
      \vsize\layerparameter\c!height % \overlaywheight = \vsize
      \composedlayer{#1}}}

%D \macros
%D  {resetlayer}
%D
%D This macro hardly needs an explanation (and is seldom
%D needed as well).

\def\doresetlayer#1%
  {\ifcsname\@@layerbox#1\endcsname
     \global\setbox\csname\@@layerbox#1\endcsname\emptybox
   \fi}

\def\resetlayer[#1]%
  {\doresetlayer{#1}%
   \doifvalue{\??ll#1\c!doublesided}\v!yes % kind of redundant test
     {\doresetlayer{\v!left #1}%
      \doresetlayer{\v!right#1}}%
   \doresetlayer{#1:\realfolio}}

%D \macros
%D  {setMPlayer}
%D
%D The following layer macro uses the positions that are
%D registered by \METAPOST.
%D
%D \starttyping
%D \definelayer[test]
%D
%D \setMPlayer [test] [somepos-1] {Whatever we want here!}
%D \setMPlayer [test] [somepos-2] {Whatever we need there!}
%D \setMPlayer [test] [somepos-3] {\externalfigure[cow.mps][width=2cm]}
%D
%D \startuseMPgraphic{oeps}
%D   draw fullcircle scaled 10cm withcolor red ;
%D   register ("somepos-1",2cm,3cm,center currentpicture) ;
%D   register ("somepos-2",8cm,5cm,(-1cm,-2cm)) ;
%D   register ("somepos-3",0cm,0cm,(-2cm,2cm)) ;
%D \stopuseMPgraphic
%D
%D \getMPlayer[test]{\useMPgraphic{oeps}}
%D \stoptyping
%D
%D The last line is equivalent to
%D
%D \starttyping
%D \framed
%D   [background={foreground,test},offset=overlay]
%D   {\useMPgraphic{oeps}}
%D \stoptyping

\def\setMPlayer
  {\dotripleempty\dosetMPlayer}

\def\MPlayerwidth {\hsize}
\def\MPlayerheight{\vsize}

\def\dosetMPlayer[#1][#2][#3]%
  {\edef\MPlayerwidth {\MPw{#2}}%
   \edef\MPlayerheight{\MPh{#2}}%
   \setlayer[#1][\c!x=\MPx{#2},\c!y=\MPy{#2},\c!position=\v!no,#3]}

\def\getMPlayer
  {\dodoubleempty\dogetMPlayer}

\def\dogetMPlayer[#1][#2]%
  {\framed
     [\c!background={\v!foreground,#1},
      \c!frame=\v!off,
      \c!offset=\v!overlay,#2]}

% The next mechanism is obsolete and will be removed in \MKIV\ (or move to
% the compatibility module.

\newskip\xposition  \newskip\yposition
\newskip\xdimension \newskip\ydimension
\newskip\xoffset    \newskip\yoffset

% already defined \newbox\positionbox

\unexpanded\def\startpositioning
  {\bgroup
   \xposition \zeropoint     \yposition \zeropoint
   \xdimension\zeropoint     \ydimension\zeropoint
   \xoffset   \zeropoint     \yoffset   \zeropoint
   \hfuzz     \paperwidth \vfuzz     \paperheight
   \setbox\positionbox\hbox\bgroup}

\unexpanded\def\stoppositioning
  {\doifnot\@@psoffset\v!yes
     {\global\xoffset\zeropoint
      \global\yoffset\zeropoint}%
   \global\advance\xdimension \xoffset
   \global\advance\ydimension \yoffset
   \egroup
   \vbox to \ydimension
     {\vskip\yoffset
      \hbox to \xdimension
        {\hskip\xoffset
         \box\positionbox
         \hfill}
      \vfill}%
   \egroup}

\def\resetpositioning
  {\getparameters[\??ps]
     [\c!state=\v!start,%
      \c!unit=\s!cm,%
      \c!factor=1,%
      \c!scale=1,%
      \c!xfactor=\@@psfactor,%
      \c!yfactor=\@@psfactor,%
      \c!xscale=\@@psscale,%
      \c!yscale=\@@psscale,%
      \c!xstep=\v!absolute,%
      \c!ystep=\v!absolute,%
      \c!xoffset=\!!zeropoint,%
      \c!yoffset=\!!zeropoint]}

\unexpanded\def\setuppositioning
  {\resetpositioning
   \dodoubleargument\getparameters[\??ps]}

\def\calculateposition#1#2#3#4#5#6#7#8#9%
  {\setdimensionwithunit\scratchskip{#1}\@@psunit
   \scratchskip#8\scratchskip
   \scratchskip#9\scratchskip
   \advance\scratchskip #4\relax
   \doif{#2}\v!relative
     {\advance\scratchskip #3%
      \let#4\!!zeropoint}%
   #3\scratchskip\relax
   \doifnot\@@psstate\v!overlay
     {\scratchskip#5\relax
      \advance\scratchskip #3\relax
      \ifdim#3<-#7\relax          \global#7-#3\relax          \fi
      \ifdim\scratchskip>#6\relax \global#6\scratchskip\relax \fi}}

\def\position
  {\dosingleempty\doposition}

\def\doposition[#1]#2(#3,#4)%
  {\dowithnextbox
     {\bgroup
      \getparameters[\??ps][#1]%
      \dontcomplain
      \calculateposition{#3}\@@psxstep\xposition
        \@@psxoffset{\nextboxwd}\xdimension\xoffset
        \@@psxscale\@@psxfactor
      \scratchdimen\nextboxht \advance\scratchdimen \nextboxdp
      \calculateposition{#4}\@@psystep\yposition
        \@@psyoffset\scratchdimen\ydimension\yoffset
        \@@psyscale\@@psyfactor
      \vbox to \zeropoint % kan beter.
        {\vskip\yposition
         \hbox to \zeropoint
           {\hskip\xposition
            \flushnextbox
            \hss}
         \vss}%
      \xdef\dopoppositioning
        {\xposition\the\xposition
         \yposition\the\yposition
         \noexpand\def\noexpand\@@psxoffset{\@@psxoffset}%
         \noexpand\def\noexpand\@@psyoffset{\@@psyoffset}}%
      \egroup
      \dopoppositioning
      \ignorespaces}
   \hbox}

\resetpositioning

\setuppositioning
  [\c!unit=\s!cm,
   \c!factor=1,
   \c!scale=1,
   \c!xstep=\v!absolute,
   \c!ystep=\v!absolute,
   \c!offset=\v!yes,
   \c!xoffset=\!!zeropoint,
   \c!yoffset=\!!zeropoint]

%D Watch out, a redefinition:

\ifx\settextpagecontent\undefined \writestatus\m!systems{error in page-lyr.tex} \wait \fi

\let\normalsettextpagecontent\settextpagecontent

\definelayer
  [OTRTEXT]

\setuplayer
  [OTRTEXT]
  [\c!width=\innermakeupwidth,
   \c!height=\textheight]

% will be overloaded in page-spr

\def\settextpagecontent#1#2#3% #2 and #3 will disappear
  {\doifelselayerdata{OTRTEXT}
     {\setbox#1\hbox to \makeupwidth
        {\startoverlay
           {\tightlayer[OTRTEXT]} % first, otherwise problems with toc
           {\normalsettextpagecontent{#1}{#2}{#3}\box#1}
         \stopoverlay}%
      \dp#1\zeropoint}%
     {\normalsettextpagecontent{#1}{#2}{#3}}}

\protect \endinput