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%D \module
%D   [       file=meta-imp-grid,
%D        version=2016.01.08,
%D          title=\METAPOST\ Graphics,
%D       subtitle=Grid Tracing,
%D         author=Hans Hagen,
%D           date=\currentdate,
%D      copyright={PRAGMA ADE \& \CONTEXT\ Development Team}]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See mreadme.pdf for
%C details.

\startMPextensions
    color mfun_grid_lgc[] ;
    numeric mfun_grid_lgi ;
    mfun_grid_lgc[1] := red ;
    mfun_grid_lgc[2] := green ;
    mfun_grid_lgc[3] := blue ;
    mfun_grid_lgi := 1 ;
\stopMPextensions

\startuseMPgraphic{linegrid}
  % if unknown mfun_grid_lgi :
  %     color mfun_grid_lgc[] ;
  %     numeric mfun_grid_lgi ;
  %     mfun_grid_lgc[1] := red ;
  %     mfun_grid_lgc[2] := green ;
  %     mfun_grid_lgc[3] := blue ;
  %     mfun_grid_lgi := 1 ;
  % elseif mfun_grid_lgi = 3 :
    if mfun_grid_lgi = 3 :
        mfun_grid_lgi := 1 ;
    else :
        mfun_grid_lgi := mfun_grid_lgi + 1 ;
    fi ;
    draw image (
        save f, t ;
        pair f, t ;
        f := (0,-2*LineHeight) ;
        t := (0, 2*LineHeight) ;
        for i=ypart f - eps step LineHeight/5 until ypart t + eps:
            draw (-EmWidth,i) -- (0,i) ;
        endfor ;
        draw f -- t ;
        draw (-EmWidth,0) -- (EmWidth,0) ;
  % ) shifted (-mfun_grid_lgi*EmWidth/3,0) withcolor mfun_grid_lgc[mfun_grid_lgi] ;
    ) shifted (-EmWidth/2,0) withcolor mfun_grid_lgc[mfun_grid_lgi] ;
    setbounds currentpicture to origin -- cycle ;
\stopuseMPgraphic

\unexpanded\def\showlinegrid{\dontleavehmode\smash{\useMPgraphic{linegrid}}}

\continueifinputfile{meta-imp-grid.mkiv}

\starttext

\showlinegrid \input tufte \par
\showlinegrid \input tufte \par
\showlinegrid \input ward \par
\showlinegrid \input ward \par

\stoptext